The “Science” behind “Bonking”

It has long been “common knowledge” that endurance exercise is fuel-limited. This is most familiar to general populations as a commonly observed limitation on foot races that says that you will run out of energy, “hit the wall,” or “bonk” at around 20 miles. For common race distances, the effect is most frequently observed in the marathon (26.2 miles). The standard explanation is that you can only store a limited amount of carbohydrate fuel in the form of muscle and liver glycogen, and that you will bonk when the supplies run out. Management of the “problem” is variously accomplished by “carb-loading” before the race to super-fill (“super-compensate”) the glycogen stores, and by various refueling strategies during the race (which support a large industry that provides special drinks, gels, bars, and the like).

At the same time, there is a growing body of evidence that there is no reason to depend on stored carbohydrate fuel which is limited in quantity if you can mobilize your fat stores as a source of fuel. The problem seems to be that anyone who is accustomed (adapted) to eating a high-carbohydrate diet tends to preferentially burn carbohydrates as fuel and does not utilize the energy available from fat in a way that spares carbohydrate use during endurance exercise. These are the so-called “sugar burners” as compared to “fat burners.”

There are a few possible strategies to enhance fat burning. The most commonly discussed strategy is to train at slower, more aerobic paces, to train the body to optimize using fat as fuel. This approach was discussed by Alan Couzens. Although carbohydrate was also restricted, the main emphasis was on low intensity training, with pretty amazing results from the combined approach.

A second strategy, perhaps even simpler, is to change to a low-carbohydrate diet. Maintaining glycogen supplies in a partially depleted state encourages the development of aerobic apparatus via the action of AMP Kinase (increased mitochondria and associated fat burning enzymes such as carnitine palmitoyltransferase, necessary to help transport fatty acids across the mitochondrial membrane). People with deficiencies of carnitine palmitoyltransferase are unable to utilize fat for fuel efficiently, and can even suffer damaged muscles and rhabdomyelosis resulting in kidney failure. There is also evidence that people suffering from obesity and diabetes may have impaired mitochondrial function, which inhibits their fat burning and at least partially explains the origins of their condition. Similarly, a deficiency of carnitine can result in impaired mitochondrial function and even lead to symptoms of metabolic syndrome.

There is an adaptation period of a few weeks to lowering carbohydrate intake during which the appropriate metabolic pathways and aerobic enzymes and cofactors are up-regulated to better utilize fat as a substrate, allowing blood glucose levels during exercise to be more easily maintained. As Alan Couzens explains, lower carb intake should result in a 20% increase in fat burning in a few weeks. Numerous scientific studies bear this out. A recent study, for example, concludes that a high fat diet increased the rate of whole-body and muscle fat oxidation while reducing the rate of muscle glycogenolysis during submaximal exercise, even after restoring high carbohydrate intake. In this study, a high fat diet was followed for up to two weeks (which we know is just long enough to see significant adaptation), during which usual training was followed. A high carbohydrate diet was restored for the last few days along with a taper before racing. The athletes were able to maintain the increased fat oxidation even after reinstating high carbohydrate intake.

There is at least one supplement (Vespa) which is alleged to improve fat-burning during exercise. There is also some evidence that, all else being equal, the athlete that has the most efficient fat-burning ability can consistently beat the athlete with the higher raw power output (higher VO_{2 max}) in endurance competition.

An article in Running Times last year discussed the strategy of carb cutting to increase endurance, though the article suggests mainly forgoing carb supplementation on training runs, not going low carb in the training diet. The Ethiopian runners again are cited as evidence that a high carb diet is necessary to fuel runs, and there is speculation that they are somehow training themselves not to use glycogen as the predominant fuel by not consuming carbs while actually training. This conclusion seems unlikely to us, since it is well known that exercise causes an increase in blood sugar (due to action of epinephrine and glucagon), effectively mimicking the result of taking in supplemental carbohydrate while exercising. A marathoner does not “hit the wall” until they have run ~20 miles at race pace anyway, a condition not likely encountered during routine training runs for most of us.

Despite all that is known about “the wall,” people persist in believing that a high carbohydrate diet is necessary if you hope to be a good runner. A high carbohydrate diet may work well for Ethiopian runners, who are known for running prodigious mileage and training three times per day. At this training intensity, we seriously doubt their glycogen stores are ever completely full, and they may effectively be training in a perpetually glycogen depleted state, which would upregulate the same enzyme systems that occurs in the low carbohydrate condition. Perhaps what they are mainly training is the rapid replenishment of glycogen stores. However, for most of us amateur or older runners, we’re only fooling ourselves if we think we need to fuel exercise with a high carb diet. David does quite well on very minimal carbs (less than 50 g/day), while Cynthia prefers closer to 100 g/day. Since we generally only run once per day and even then usually less than 10 miles, there is no reason to consume any more carbs to fuel our exercise habits.

David has been a low-carb runner for more than two years now, keeping carbohydrate consumption low before, during, and after endurance exercise. While he won’t brag about his speed and has no plans to challenge any course records, he consistently finds that he has even energy levels and rapid post-event recovery with minimal muscle aches and pains. Fat fueling works for him! However, until very recently, while he refueled less often than is typical, he did routinely refuel using protein and fat for any event of more than 3–4 hours.

He was prompted to revisit the issue when he heard about a recent study (“Metabolic Factors Limiting Performance in Marathon Runners”) by Ben Rapoport when he had his fifteen minutes of fame on NPR in October 2010. Ben was attempting to model bonking in the context of marathon runners. David subsequently exchanged e-mail with Ben whose initial reaction was to interpret David’s personal experience as simply confirming Ben’s model, arguing that fat-fueling works, and that you don’t bonk as long as you run sufficiently slowly. David pointed out some flaws in Ben’s interpretation of the scientific data he analyzed and challenged him to define a performance that David might personally be able to achieve that he might consider sufficient to disprove his model, but he never responded to the challenge. We summarize here both a couple of the criticisms of Ben’s model and analysis and some more recent personal experience and experiments.

A key part of the experimental data that Ben used in building his model came from Romijn AJ, Coyle EF, Sidossis LS, Gastaldelli A, Horowitz JF, et al. (1993) “Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration,” Am J Physiol 265: E380–E391. That paper shows (in Fig. 8) the relative amount of particular substrates which are used in short-term exercise at three different intensities. The subject’s dietary habits were not identified, and any changes that might take place over more than a few minutes were ignored. The data show that more intense efforts use primarily muscle glycogen as fuel. They do not show the pathways that feed the muscle glycogen. Glycogen and glucose are rapidly interconverted, and over longer periods of exercise, new supplies of glucose are made by the liver. The liver, in turn, is capable of using any available fuel source via gluconeogenesis to keep blood glucose levels up. These fuel sources include ingested carbohydrates, protein, and fat as well as lactate, glycerol from liver and adipose tissue fat stores, and even body protein stores such as in muscle tissue. Which source predominates depends on availability. Some of the necessary up- and down-regulation of different metabolic pathways can take place on a short time scale to respond to instantaneous demand for fuel from the muscles and other bodily functions. However, there are also some key adjustments that respond more to average demand and supply to deal with changing diet, exercise demand, starvation, and the like, and these are events that tend to have time constants measured in days or weeks, not minutes or hours. The adaptation processes may be numerous and complex (or at least, we fully admit that we don’t understand all of them) and likely include adaptations in the skeletal muscles as well as in the liver. However, we think it is clear that the data extracted from Romijn do not actually show the underlying consumed fuel source for various levels of endurance exercise, and the data cannot reasonably be extrapolated to model and predict bonking.

One factor which Ben cites in favor of the importance of carbohydrate and fueling is the relative stoichiometry of oxygen consumption for each substrate. He states that carbohydrate oxidation typically generates approximately 120 kcal per mole of respired oxygen, whereas fatty acid oxidation typically generates only approximately 100 kcal per mole of oxygen. We would argue that this, while probably correct, is irrelevant in that the difference is relatively small, and the availability of oxygen is never a limiting factor at a typical aerobic marathon pace. There is always an excess of oxygen available. In any case, regardless of the underlying fuel used to generate blood glucose/glycogen, the immediate muscle activity (except for very short term bursts of power) is fueled by glucose.

Notwithstanding the possible flaws, Ben’s model does make predictions as to whether one is likely to bonk in a marathon. In particular, he has provided a calculator for the marathon. Putting in the best estimates David has for his personal situation, the calculator predicts that he should not bonk for finishing times over about 3 hrs. Since he can only sustain a 3-hour marathon pace for about 1 mile or so, he may never be able to really challenge the model experimentally using himself as a test subject.

However, David was inspired to test his need for refueling during running, and now has a significant number of test results that, if not sufficient to explicitly challenge Ben’s model, are at least startling to many people. For some time now, David routinely runs training runs of up to 4–5 hrs without refueling (water alone). This is still true when he runs with his son at faster speeds who typically chooses to refuel using at least a couple of energy bars or gels. David first pushed the envelope a bit on what was supposed to be a planned 18-mile run that stretched to about 26 miles when the route was blocked by an impassable rain-swollen river crossing. That run extended over about 7 hours including some significant rest breaks, so it was not at a strenuous pace, although it did include some significant elevation change. David consumed only water. The next experience was a 50K (31-mile), mostly flat run, again at a very modest pace, finishing in 6:50, again on water alone. Two weeks later, he completed a much more strenuous 50K on rough rocky trails with about 6000 ft of elevation gain (and loss) in 7:28 on water alone. In all three of these runs, David started with a breakfast of a two-egg cheese omelet and bacon about two hours beforehand. He felt some minor hunger around normal lunch time, but otherwise did not experience any significant fluctuations in energy level, and he certainly did not bonk. His overall weight was down about 3–4 lbs at the end of the day compared to the beginning, but looking at beginning-of-day weights across several days, any weight change was pretty much lost in the noise. Of course, it only takes about a pound of fat to fuel a 50K run, anyway. Another week later David ran yet another moderately hilly 50K event which was slower (8:04) due to snowy trail conditions. Due to a late start (10 am), he chose to eat a little at around the 6-hour mark, mostly because it was a long time since breakfast. However, the ability to do three 50K runs in four weekends, certainly confirms that post-event recovery was rapid.

So what’s the take-away message from all this? It’s interesting to compare our reaction and the experiments that David decided to try to others. Greg Crowther also blogged on Ben’s article. Greg’s big concern was inter- and intra-personal variability, and he questioned the validity of extrapolating from some sort of average measurement of performance to the needs of one individual on one particular day. He didn’t ask (as we did) if too many runners are needlessly afraid of bonking but rather, “if ingesting extra carbohydrates before and during a marathon might help you avoid ‘hitting the wall,’ why wouldn’t you do it? Especially after investing all of that time and effort in training, traveling to the race, etc.?” Hey, we’ll even go further: if ingesting fuel (whether carbohydrate, protein, or fat) during a race might help you maintain a faster pace for whatever reason and for whatever limited portion of the race, even it’s just a placebo effect (or tricking your Central Governor), then by all means go for it! But we also stand by our basic conclusions that (1) carbohydrates are simply not necessary before, during, or after endurance exercise, (2) they’re probably not even particularly beneficial, at least if you are suitably adapted to efficient fat-burning (and may be harmful to your long-term health) and not logging very high miles at high intensity, and (3) unless you’re able and motivated to run, say, an entire marathon at a high percentage of your VO_{2 max}, consuming much of any food immediately before, during, or after the event is entirely optional. Moreover, there may be some benefit to having the stomach completely empty while racing, in order to avoid stomach upset, intestinal discomfort/diarrhea or nausea when running at high intensity. (David does continue to believe in significant protein consumption sometime in the 12 hours or so following strenuous exercise as an aid to muscle repair and recovery, but the timing of even that refueling does not appear to be nearly as critical as some authors suggest.) David will probably resume modest consumption of fat and protein on longer events, just because he’s not big on skipping meals entirely, anyway, but it’s reassuring to know that he can easily keep going without any fuel if for any reason he needs to.

Weight Loss Followup

In previous posts, we discussed the first several months of weight loss for Cynthia and David. We’re now about two and a half years into our new eating lifestyle, and it seems like a followup post is overdue.

We have been taking near daily measurements through the period with some more occasional measurements of other parameters, so we have lots of data to misinterpret according to whatever bias or slant you might want to apply. As usual, life is complicated, and the data are subject to a lot of coarse- and fine-grained hypotheses that can be postulated to explain various features. We’ll offer an assortment of hypotheses, some of which are more strongly supported by the data than others. Since it’s all basically post-hoc analysis based on two subjects, none of the hypotheses can really be considered confirmed.

The most striking observation, perhaps, is that the change to lower-carbohydrate consumption continues to be working. (Let’s not call it a “diet” since that seems to mean something that people try to use as a temporary measure that ultimately fails when they revert to “normal” eating habits.) While our weight loss has inevitably slowed and had various “plateaus” and “reversals” or “setbacks,” our weight loss is holding and we are both at or near our lows of recent years some 30 months into our new eating habits. This bodes well for the long term. We are emphatically not counting calories, calorie restricting to the point of gnawing hunger, or otherwise depriving ourselves of the enjoyment of eating. Sure, we are emphasizing some different foods and limiting consumption of sugars of all sorts and simple starches, but, for the most part, we don't crave them and can satisfy what cravings we have with either small portions or satisfactory substitutes. Fortunately, we don’t often share meals with the high-carb/low fat crowd, so we don’t face a lot of peer pressure to “cheat,” and we stay away from the French bakeries. There are some differences between our preferred diets as well. David tends more towards the “Optimal Diet” (lower carb and high in butter and cream) while Cynthia gives in more to carb cravings (more fruit and indulgences such as Chinese dumplings), to which she attributes her various weight stalls and reversals!

And, of course, at least among those who are open-minded enough to have actually investigated the current state of the science, acceptance of the low-carb lifestyle has been increasing steadily. While advertising by the food industry continues to be abysmally misleading, we’ve noticed an increasing number of more positive references to low-carb nutrition in everything from a steadily increasing number of blogs posts, books, and scientific papers to passing references in recent movies (e.g., “She eats CARBS” from The Devil Wears Prada). The mainstream government agencies and medical societies are generally still not recognizing the error of the low fat diet, as seen in the 2010 USDA guidelines. (Quoting from Question 5 in Appendix E-1, “Conclusions”: “No optimal macronutrient proportion was identified for enhancing weight loss or weight maintenance. However, decreasing caloric intake led to increased weight loss and improved weight maintenance. Therefore, diets that are reduced in calories and have macronutrient proportions that are within the ranges recommended in the Dietary References Intakes (IOM, 2002/2005) (protein: 10%-35%; carbohydrate: 45%-65%; fat: 20%-35%) are appropriate for individuals who desire to lose weight or maintain weight loss. Diets that are less than 45 percent carbohydrate or more than 35 percent protein are difficult to adhere to, are not more effective than other calorie-controlled diets for weight loss and weight maintenance, and may pose health risk, and are therefore not recommended for weight loss or maintenance.”) There are, of course, huge entrenched economic interests that will continue to fight the status quo tooth and nail. There is some evidence that they may start to crack in the foreseeable future—for example, the American Diabetes Association now recognizes that a low carb diet may be useful for weight loss in diabetics—but progress continues to be slow.

Then there’s the exercise wild card. We all “know” that increased exercise is a “necessary” part of any “reputable” weight loss program. And yes, we have increased our level of exercise. We were never serious couch potatoes, but we weren’t serious athletes, either. Very roughly, we were running 20–30 mi/wk two and a half years ago, and increased to 30–40 mi/wk, and have often done closer to 50 mi/wk. We also started running ultramarathons of 30–50 mi in one day, averaging more than one such event per month at times. So what did all that exercise do for us? Well, we certainly got stronger and faster. We generally feel good and energetic (aside from the inevitable sore muscles and minor injuries). We also continue to find that more often than not, increased exercise correlates with weight gain, not weight loss! This happens both over the short term
(water weight of up to a few pounds the day after an ultra-marathon that may take a few days to lose), and over the longer term (our weight loss trends reversed for about 4 months after we started doing frequent day runs in excess of about 15 mi). We also tended to see a stalling of any downward trend whenever we increased our weekly mileage significantly. Partly, this is because moderate mileage increases such as this are easily compensated for by eating more. There are exceptions too, for example, during August 2009, Cynthia upped her mileage considerably (>50 mi/wk) and found she could not eat enough to keep her weight stable. However, this amount of mileage was not sustainable (due to an injury in May 2009 that began to cause pain), and eventually she gained it back.

There are, of course, several competing things going on when you exercise a lot. Over the period of the exercise itself, the dominant effect is usually level of hydration, and body weight is often used to monitor endurance athletes for dehydration and/or over-hydration. If you exercise hard enough and long enough, you can also deplete your glycogen stores to account for another pound or so of temporary weight loss (including the accompanying water of hydration). So generally speaking, you usually finish a long, hard bout of exercise down a few pounds. But then, of course, you eat and drink. Your appetite increases, so you may eat more than normal, and there are various reasons why you might retain extra fluid. That’s why we frequently saw a net increase in weight the day after. Interestingly, the size of this effect has generally decreased over time. Probably, as our bodies have become better adapted to the rigors of a long, hard day of exercise, they no longer see it as stressful. This is supported by the evidence of less muscle soreness and edema, as well. In David’s case, there is probably also a nutritional effect. He ran earlier events consuming more than normal carbs during and immediately after the event, and then, in later events, switched to more strict low-carb fueling before, during, and after. Some amount of fluid retention would be expected to be correlated with a temporary increase in carb consumption, and eliminating the carbs apparently eliminates that source of fluid retention. Cynthia tends to push harder into her non-aerobic zone, especially when trying to keep up with David, and chooses to consume more carbs during and after long runs, but she’s been showing smaller post-event weight spikes more recently, too. The effect is a sensitive measure of training, because it is more pronounced when less prepared or perhaps as a response to heat stress. The water retention is most likely due to a complex interplay of hormones that signal the kidneys to retain salt and fluid. Such a response is understandable after the stimulus of long and/or hot conditions.

The increased appetite can generally overcome any predicted weight loss from a purely thermodynamic point of view. The problem is that you have to run on the order of 30 mi or so to burn enough calories to consume a pound of body fat (assuming that you’re actually burning fat for fuel). If you do that over a week, it’s pretty easy to unknowingly increase your daily food Calorie consumption enough to more than offset that burn. It’s not much more than an extra couple of “servings” of something tasty per day. Be careful about rewarding yourself with too many bowls of ice cream or extra double cheesburgers!

An increased level of exercise, if done systematically and with adequate general nutritional support (enough protein, for example), often results in muscle building. This can result in body “recomposition”: loss of body fat and increase in lean muscle mass with no net change in weight. That may explain some of the apparent plateauing of our weights. Running doesn’t build bulky muscles the way, say, weightlifting does, but we have seen some measurable changes in body measurements.



Nevertheless, with a very blurry-eyed look at the weight loss data over 30 months, a simple-minded interpretation and hypothesis is that we both generally lost weight more or less linearly for 8 months until we started seriously increasing our level of exercise and then stalled out, remaining at a more or less constant weight for the following 22 months. (Click on figures to show larger.)



With slightly less blurry eyes, one immediately notices that longer trending period tends to follow more of an exponential curve rather than a straight line. A simple model which can be made to fit the data pretty well is to assume that you are always approaching an asymptote (target weight) exponentially so that your rate of weight loss (gain) gets steadily slower as you approach your target. Fitting such exponentials to the various regions on our graphs gives a pretty good fit (i.e., the data looks like it fits a set of straight line segments on a semi-log plot where an estimated target weight is subtracted out). Measured time constants vary from about half a year to two years. And while our weight loss is now hard to see from day to day or even week to week, we are still losing at an average net rate of about a pound every 2–3 months. We both feel like we should be able to lose another 10–15 pounds, but that could take a few years.


It is interesting to treat the data using some of the technical indicators typically used on financial charts. For example, one can draw upper("resistance") and lower ("support") levels and trend lines. Weight can bounce off or break through these lines as you can see on Cynthia's chart from August 2008 through February 2010. You can also see a downward trending channel or notice triangular patterns with converging oscillations, double bottoms, retracement levels, all very similar to observed price behavior on financial charts.

There are other secondary effects that may also be present in the data. While we have so far explained the weight gain last fall as due to increased exercise, it could also be due, at least in part, to a normal seasonal effect. Some weight gain through the fall and into mid-winter is perhaps genetically programmed to store fuel for the winter. Some of it may also be just increased fluid retention in cooler weather (or just reduced dehydration?—the body probably undergoes larger hydration cycles in hot weather as one sweats and eventually replaces lost fluid—but note that a drop in core body temperature actually has the reverse effect as anyone who dives in cold water can attest: the body naturally sheds excess water when cold).

Cynthia's data also show a pronounced oscillation with an amplitude of 4–6 pounds and a period of 1.5–3 months. We have, so far, been unable to correlate this oscillation with any obvious body cycles, lifestyle cycles, eating, or exercise habits. Being female, one might suspect menstrual cycle effects, but the period is too long and the amplitude is too large. (Menstrual cycles generally result in monthly weight variations with an amplitude of about 2 pounds. In order to see it, you typically have to average several months of data [with the end dates of cycles carefully lined up if the length of the cycle is at all irregular] since the amplitude is comparable to normal day to day fluctuations. It's actually more noticeable as a cyclic change in waist circumference.)

David's data show periods of unexpectedly rapid weight loss (July/August 2008, January/February 2009, May/June 2010). Again, we have not been able to clearly explain these periods, although similar “success” periods seem to be commonly reported anecdotally. Perhaps the body suddenly decides to adjust its natural setpoint in some important way. Fat storage and loss is driven more by hormonal signals than by daily calorie balance anyway.

If you want to keep losing weight, you may need to keep reducing your calorie consumption as well. In principle, this should happen automatically if you basically eat to satiety, but eating habits can often be somewhat independent of satiety if you are in the habit of eating particular portion sizes (e.g., 2 eggs and 2 slices of cheese, etc.). Presumably with a little conscious effort, you should be able to readjust your habits to your new needs as you lose weight, but some portions are a little hard to adjust. (It’s not convenient to cook 1.8 eggs for breakfast…)


Another interesting comparison is to plot David’s weight vs. Cynthia’s weight. This plot is noisier in that David’s and Cynthia’s weight gain and loss have not always been tightly correlated despite similar diet and exercise schedules. Overall, David’s weight is approximately 30% higher than Cynthia’s at any given time, but he has been losing about 1.2 pounds for each pound that Cynthia loses. We're still not sure how much more weight we can (or should) realistically lose. The corresponding weight loss rates are consistent with our college-age weights of about 167/125 pounds respectively, but a more realistic goal may be more like 175/133 pounds.

There are other measures of body composition that are often used to determine “ideal” weights. The most commonly used is the Body Mass Index or BMI. This is based purely on height and weight and does not take skeletal build or musculature into account at all. David is still classed as borderline “overweight” by standard BMI guidelines; Cynthia is “normal” at a BMI of ~23.

However, David is relatively well-muscled and big-boned. At least by current American on-the-street standards, most people would not say he was overweight at this point. Another approach to determining ideal body composition is to estimate percent fat. There are many ways to make this measurement—all approximations based on indirect measurements of one sort or another.
Underwater weighing is the current standard against which other measurements are typically evaluated, but it is imperfect, too. Skin-fold thickness is also popular, because it’s easy, but it can be unreliable. See Lyle McDonald’s post for more in-depth discussion. The two methods that are most readily available to most people (including us) are Bioelectric Impedance Analysis (BIA), a measurement built-in to some digital bathroom scales, and various formulas based on using additional body dimensions such as waist, hip, and neck circumference. Our favorite of these right now is a set of formulas derived by the US Navy based on height, waist, weight, neck, and hip (women only): %Fat=495/(1.0324 - 0.19077(log(waist - neck)) + 0.15456(log(height))) - 450 for men or %Fat=495/(1.29579 - 0.35004(log(waist + hip - neck)) + 0.22100(log(height))) - 450 for women. These give current values of 28.5% fat for Cynthia and 18.5% fat for David. Not surprisingly, these calculations put both of us solidly in the “acceptable” range, but still significantly above the upper end of the target ranges for athletes (presumably based on a young military test population: 20% for women, 13% for men). Just to give you some idea of the uncertainty in these measurements, the BIA method as implemented by a Weight Watchers bathroom scale gives 25.4% for Cynthia and 22.4% for David, showing discrepancies on the order of 3% and in opposite directions for Cynthia and for David.

And finally, just a quick observation about the Weight Watcher brand scale implementation of the BIA measurement: while we have been unable to locate any information on the algorithms implemented in the scale, it is clear that there is more than a little something amiss. First, it tends to report %Water in the mid-50s, while the human body is generally estimated to be closer to 70% water. While the instructions warn you that the data may be inaccurate if you take measurements immediately after heavy exercise or large fluid consumption, it is nevertheless disconcerting that it actually tends to report higher %Water (and lower %Fat) after losing a lot of fluid due to exercise and sweating, and lower %Water after a large drink! Clearly the algorithms and/or measurements fail to accurately account for variations in the distribution of fluid throughout the body. Another anomaly we have noticed is that the scale has reported a slight decrease in %Bone for both of us as we have lost weight, something that is very unlikely to be valid.

So all in all, our way of eating is pretty easy to sustain and requires no weighing or measuring. If we really wanted to lose weight faster, I'm sure it could be done using more discipline, but then we'd have to worry about regaining once the discipline slips. This way the changes are more gradual, and perhaps, more sustainable.

Primal Potatoes—a Contrary View

In a recent blog entry “Primal Potatoes, Part 2” the author tries to make the case that humans are evolutionarily adapted to include starchy foods such as tubers in their diets, and that there would be survival advantage in keeping glycogen stores higher using these sources of starch. We don’t disagree with all of the conclusions he ends up with, but we think that a lot of the “evidence” used is factually incorrect or misleading. Here’s our take:

There is no doubt that the human digestive system has broad adaptability. The fact that humans are “omnivores” who can digest most everything that any animals eat except straight cellulosic materials (grasses and dietary fiber) clearly provides a survival advantage in that it allows humans to survive for at least short periods on whatever food source is available. It does not follow, however, that the ability of humans to digest starches means that they provided a real performance advantage in hunting and/or survival activities and would have been a required part of our diet. Rather, it seems to us that humans are well-adapted to depend predominantly on aerobic exercise (fat-burning) which can be sustained for many hours even in a fasting state if necessary, using the generous fat storage capacity available. Humans have an auxiliary system able to use the much more limited glycogen (carbohydrate) energy stores for primarily short bursts of intense exercise (30 seconds or less at a time) or to supplement for higher intensities than can be sustained by fat burning alone. This auxiliary system is further adapted to replenish/restore itself fairly quickly, again, even without the aid of carbohydrate consumption. While it is possible to create situations in athletic training and competition where the rate of depletion can be faster than can be replenished in steady-state (especially for athletes who normally depend on carbohydrates for much of their calories), it does not follow that such rapid depletion followed by rapid replenishment using dietary carbohydrate sources, was ever important or necessary in evolutionary terms. Note, for example, that athletes in many sports routinely use “reps” or “intervals” of intense activity separated by recovery periods, typically with no consumption of food or drink during the recovery periods. To the extent that fuel stores are being restored during these recovery periods, the process does not depend on any particular external source of calories.

In effect, the body is so effective at conserving and recycling its limited stores of carbohydrate fuels that large dietary replenishment would not have been required to satisfy the needs of hunting and survival activities.

An article by Fournier was cited in “Primal Potatoes, Part 2” to support statements that “whereas typical glycogen stores will support an intense aerobic exercise for a few hours, a single maximal sprint effort will deplete one-third to one-half of glycogen stores,” and “humans can replenish glycogen stores without dietary carbohydrate, and even while fasting.” However, the article link did not work. A PubMed search found this article which appears to match the cited reference. The article describes experiments on rats which were made to engage in moderate exercise (swimming for 30 min with a weight attached to their tails), followed by a 3 min “sprint” (when a much heavier weight was attached to their tails). The main point of the article was to determine if the lactate produced during the sprint was the predominant source of carbon incorporated into newly synthesized glycogen. If you look at Fig. 1, you’ll see that the preceding “moderate” exercise depleted glycogen by at least 50% before the “sprint,” which then further depleted glycogen (down to ~25%). The researchers concluded that the lactate only provided ~50% of the newly synthesized glycogen, which actually is not surprising given that the previous glycogen depleting activity had already reduced the amount of glycogen available, and presumably any lactate that was or could have been produced and therefore recycled into new glycogen had already been oxidized completely and was no longer available. In short, the cited article does not support the first statement, which appears to be an exaggeration. There was no study showing that a single maximal sprint would deplete one third to one half of stored glycogen. Rather, the already ~50% depleted glycogen stores were further depleted by the “sprint,” showing only that the higher intensity the effort, the more rapidly the glycogen was depleted. A “single maximal sprint effort” as we understand it would most certainly not deplete a large fraction of glycogen stores. It may feel like it, but that feeling is not due to glycogen depletion per se, but rather due to acidification due to lactate and carbonic acid accumulation.

There is some confusion as to the meaning of the word “sprint” and how different available fuel sources are used by humans in sprint versus endurance events. True “sprints” (short bursts of maximal effort) do not even depend on glycolysis, which is too slow. There is a third energy storage and release system based on local stores of ATP and phosphocreatine. These are typically sufficient for about 7 sec of peak power output, and are rapidly regenerated during any rest periods using energy from aerobic glycolysis or fat metabolism. (See, for example, Noakes, Lore of Running, p 154.) But even a 100 m sprint by a world-class runner takes longer than 7 sec, and some energy must then come from glycolysis. Perhaps a single clean-and-jerk or other 1 rep maximum weight lift can be completed using primarily this sort of burst of maximal energy, but even a typical set of 10–15 reps in weight training takes longer.

Most people probably think of a “sprint” as something roughly equivalent to a 100 m or maybe as much as a 200 m maximum-speed run. Such efforts typically take about 10–30 sec to complete and, while not possible using only the local ATP and phosphocreatine stores, the effort can be completed mostly anaerobically, fueled by local glycogen stores. The 30-second maximum for this kind of effort is probably limited by the acidification resulting from the rapid production of lactate. What glycogen has been depleted can regenerate rapidly when the maximal effort ends. “Recovery” for a repeat of a similar effort largely consists of clearing enough of the accumulated lactate to relieve the sensation of “burning” in the muscles. Fuel depletion is not a major issue.

Three minutes of a “sprint swim” (or an 800 m run or a flight from a predator) is a much more complex sort of “maximal effort.” It is not a sprint as usually defined for humans. It is a “middle distance” that cannot be completed purely anaerobically. Even an 800 m run (2 min of effort for good runners) is considered an endurance event (requiring stamina), and much of the speed can be developed using aerobic training, though a maximal effort of this length will utilize primarily glycolysis and generate a lot of lactate temporarily. (See http://www.lydiardfoundation.org/news/pdfs/Snellcourirenglish.pdf.)

Clearly the maximum power output that can be maintained for a few seconds cannot be maintained even for 30 sec. A several minute effort is still in the range where a human is capable of power output that exceeds levels that can be sustained for much longer periods of time. The relative contributions of the available metabolic paths to energy generation for an maximal effort of a few minutes can vary widely depending on the individual, the level of normal activity, training or conditioning, dietary habits and adaptations, and the level of actual effort relative to the individual’s maximum capabilities over that distance or time. Anaerobic metabolism of glucose from glycogen is certainly one of the possible contributors. In fact, the limit to how fast any one individual can go for, say 3 min of “maximal effort” is probably still set by acidification caused by lactate accumulation (and carbonic acid from CO₂ produced) due to anaerobic glucose metabolism. As the anaerobic metabolism of glucose is very inefficient and would rapidly deplete stores, additional more efficient metabolic pathways will be tapped, most notably aerobic metabolism of fat and glucose. Another important contribution comes from the aerobic metabolism of the lactate (produced from anaerobic metabolism of glucose mentioned above). If lactate is being overproduced, some of its carbon will be further metabolized in the heart and muscles and converted to CO₂ through the citric acid cycle, and eventually breathed out, with most of the potential ATP from the glucose being realized eventually, even if not in the muscle of origin. At modest production rates, much of the lactate can be used aerobically and directly as fuel by the muscles, and is a preferred fuel of slow twitch and heart muscle. In addition to possibly being used by other muscles, this lactate is also taken up by the liver and converted back into glucose and then glycogen, and thus recycled. In short, it takes a lot more than a 3-minute burst of effort to substantially deplete glycogen stores. In fact, calculations show that the glycogen stores of a well-trained runner are sufficient to last for approximately 2 hours if glycogen is the exclusive fuel (and, incidentally, stored fat would last for about 59 hours [estimates based on glycogen and fat stores present in lean elite athletes]).

Thus, it is simply not true that there would be an urgent need to fully replenish severely depleted glycogen stores after a single episode of high-energy activity. The glycogen stores would not be significantly depleted (e.g., >50%); any of several available replenishment mechanisms would be sufficient to provide necessary restoration; and there is, in any case, no urgent need to provide complete restoration within 24 hours anyway.

This renders meaningless all of speculation in the post about the large amount of protein that would need to be consumed to replace the allegedly depleted glycogen. Let’s consider first, how the body actually uses its glycogen stores. Many authors tend to focus on how much total glycogen can be stored in muscles, how much activity that amount will support, and how much time is required to replace and completely refill the muscle glycogen stores. This way of thinking may be relevant when trying to achieve peak athletic performance for a particular competitive event, but this artificial effort would not be relevant to “normal” life and evolutionary pressures. In "normal" life, maximal effort may be required on occasion, but would be punctuated with adequate rest periods to allow recovery and maintenance of energy stores. Glycogen available to any given muscle for short intense effort is only that stored locally in that muscle—you can’t steal from other muscles. Similarly, once stored in the muscles, it remains there until used—it is not depleted beyond a certain level that is protected and maintained even after extensive exercise.

Even after an exhausting race, a person can still increase their efforts and sprint to the finish. A critical detail to remember is that lactate produced from glycolysis can be used to regenerate glucose and then glycogen, as well as enter the citric acid cycle and be used aerobically. Hence lactate is recyclable—glucose can be used anaerobically for a brief intense effort, then rapidly regenerated from lactate once there is a rest period. Under conditions of carbohydrate restriction or partial glycogen depletion, the body simply intensifies the recycling effort and favors use of fat for most energy needs. Only during periods of long and repeated exhaustive glycogen depletion would glycogen levels be dangerously low, and then the body would attempt to reserve them for emergencies, decreasing intensity of activity to levels supported by fat metabolism.

That said, the amount of glycogen storage for any given muscle is also “trainable” in that the amount of glycogen stored increases with increased use. A well-trained athlete may be able to top out his muscle glycogen stores at as much as three to four times that of a sedentary individual. And presumably, a paleolithic hunter is more similar to a modern athlete than to a couch potato. Further, the “normal” steady state condition for an active individual is probably not with glycogen stores full, but more like half full. This seems to be the condition measured for endurance athletes in steady state (i.e., several hours into a many-hour event or in everyday training. See Noakes, Lore of Running pages 101–102 and references therein). It may take a day or more of resting and relatively high carb eating to fully replenish glycogen stores to maximum capacity, but the half-full steady state can be maintained more or less indefinitely, and is fully capable of supporting most any activity that is needed. Further, at least for individuals adapted to fuel their activity primarily on fat, this steady state can be maintained with little or no carbohydrate consumption, and at levels of protein consumption that are modest compared to any levels that might overwhelm the kidneys.

A further point is that it is possible that this “half full” condition is optimal for health, in that muscles that are not topped out with glycogen are still hungry for more glucose, that is, they still express glucose transporters on their surface that are actively scavenging for glucose. This constant glucose uptake by hungry muscles would tend to keep blood glucose levels low, optimize insulin sensitivity and thereby keep insulin levels low, compared to the condition of the over-fed over-carbed SAD consumers. (See http://ajpendo.physiology.org/cgi/reprint/285/4/E729.)

As soon as liver glycogen starts to decrease, gluconeogenesis kicks in, and if adapted to fat burning, gluconeogenesis enzymes may be up-regulated. One can sustain aerobic activity (presumably using glycogen stores in addition to fat stores) for many hours and still have no measurable depletion of blood glucose levels! In fact, we routinely observe the opposite (elevated blood glucose after hours of running). Glycogen stores can thus be regularly replenished (at least partially- enough to call on in emergencies) as needed even during prolonged aerobic exercise, even when fasting, to support the needs of occasional anaerobic activity. As already noted, data indicates that trained athletes can maintain a steady state level of average glycogen stores that are approximately 50% of their maximum capacity. Put another way, during prolonged periods of inactivity, a trained athlete can store ahead approximately twice the “normal” levels of glycogen stores. (Note that even the “normal” levels are about twice those measured in sedentary humans.)

In the example given of hauling a buffalo carcass out of a ravine, this activity may involve some anaerobic activity, but it will necessarily stretch over an extended period of time and be completed primarily using aerobic metabolism. There may be brief bursts of high intensity effort as needed, and there may even be bursts of extreme effort for particular heavy lifting tasks, but on average the task will necessarily be completed with levels of effort that can be sustained over hours not minutes. We suggest that hauling out a buffalo carcass would not necessarily require a lot of glycogen, and even if it did, would not necessitate gorging on potatoes or other carb food to replenish glycogen stores. Perhaps the one situation outside of athletic competition that could force someone to put out maximum effort for as long as possible (and thus seriously deplete glycogen stores) is a fight (or flight) for life. These events presumably don’t occur in close succession, so the primary evolutionary adaptation would be to provide the capacity to sustain the necessary fight or flight long enough to survive the immediate crisis. An ability to fully recharge is not necessary and would not confer much less of a survival advantage.

In fact, it is often argued that the key characteristic of humans that makes them surprisingly competitive in the predator vs. prey world compared to animals that are nominally bigger, stronger, and faster is that humans don’t depend on their peak power output capabilities but instead on their ability to maintain lesser levels of output for very long times (as, for example, in a “persistence” hunt, where they literally outlast and outrun their nominally faster prey).

Other purported advantages of eating tubers cited in “Primal Potatoes, Part 2”:

1. Lower dietary protein/meat requirement, reducing the pressure for success in hunting large animals, and making it possible to feed more people (offspring) with each kill.

This seems to be a common misconception! Eating less carbohydrate means eating more fat, not more protein. And the hunting of large animals provides increased fat relative to smaller animal sources of protein. It is difficult to eat large amounts of protein, and most people find it almost impossible to eat too much protein. It is true, however, that tubers are easier to store for extended periods than meat (and meat fat) which must be more carefully prepared for long term storage, especially in warmer climates. Agriculture does enable more concentrated population centers and was probably a major driving force for the increasing urbanization of the world. However, it is not at all clear that the sort of monoculture version of agriculture that has come to dominate how we feed large populations is a positive step. In fact it is becoming increasingly recognized that we may be destroying the planet faster with mass agriculture than we ever did by overhunting.

2. Less burden on the liver for ammonia detoxification.

This is nonsense. Again, protein consumption tends to be self-limiting at levels well below anything that would present any significant burden to the liver (or kidneys).

3. Easier to avoid protein poisoning while at the same time maintaining greater glycogen stores.

Again nonsense. Protein poisoning is just not a serious risk. And it is not difficult to maintain more than adequate glycogen stores with very low carbohydrate consumption because glycogen stores do not need to be 100% full in order to provide adequate auxilliary anaerobic energy production.

4. Easier to maintain and increase lean mass in response to the stresses of high intensity activity, with a lower dietary protein requirement.

False! As anyone who has seriously tried a low-carbohydrate diet knows, it is much easier to maintain lean body mass without increasing excess fat storage if carbohydrates are minimized in favor of fats. And carbohydrate consumption always causes blood insulin levels to spike, which has a whole series of negative consequences. Arguably, from a public health point of view, the widespread adoption of higher-carbohydrate diets was the single worst event in human history that is the major cause of most of the so-called “diseases of civilization.” Building lean mass (muscle) is usually easier with adequate protein consumption. The key to maintaining it is to (1) make sure that you maintain sufficient nutrition so as not to catabolize too much of your own protein (which the body will do if other fuel sources are limited) and (2) to consume enough protein for muscle building and rebuilding/repair. (See for example http://www.bodybuilding.com/fun/md92.htm and references cited therein, which provides evidence that excessive carbohydrate consumption post-exercise actually inhibits optimal muscle growth and repair.)

5. Reduced pressure to hunt only the fattest animals by use of carbohydrate instead of fat to dilute the protein content of the diet; which greatly enlarges the pool of potential prey, increasing dramatically the amount of energy available for harvest.

Fat is good anyway! You should always be hunting for your fat needs as well as your protein needs. You just don’t need that much total protein. But you do need some protein, and high quality protein (i.e., the so called “essential” amino acids) is hard to get in sufficient quantity from non-animal sources.

In summary, while we certainly believe that humans likely ate tubers and other starchy vegetables (and eventually the New World potato) when they could be found, we see no evidence that that behavior conveyed any sort of evolutionary advantage beyond survival in times of limited food availability.

Consumer Reports “Secrets of Thin People”

The February 2009 issue of Consumer Reports features an article called “Dieting on a budget, plus the secrets of thin people, based on our survey of 21,000 readers.” The feature includes some equipment reviews, plus articles on “healthful eating on a budget” and their analysis of their reader survey results.

I often find that I disagree with articles and reviews in Consumer Reports, more so whenever I actually have strong personal knowledge of a product or subject area. Their articles are not, of course, peer-reviewed scientific reports. Nevertheless, and despite the fact that they carefully avoid accepting commercial advertising and sponsorship, they typically conform to current industry norms, assuming, I guess, that their readers are responding to the advertising and product offerings they are exposed to and are merely wanting to choose which of the available highly-promoted products to buy. The same can be said of their financial and medical advice in that their advisors (and authors??) seem to be drawn from the mainstream establishment.

There was a time, a few decades ago, when product reviews often used a home-made recipe or similar non-commercial product as a reference for comparison. Quite often, the commercial products did not compete favorably with the non-commercial version. Such comparisons are very rare nowadays. I suppose it, at least partly, reflects the fact that fewer and fewer people are inclined to make their own anything, even when the results are superior. (Though I will admit that there are at least some real examples where modern manufacturing methods make commercial products that are superior…)

Anyway, I digress. In typical fashion, the results of the survey are presented in a way that contains almost no real data and certainly no statistical analysis. Here's what I think are the real “data”:

Survey date: 2007; 21,632 respondents; 66% “overweight” including 22% “obese”

• 16% “never overweight”
• 15% “successful losers” (defined as weighing at least 10% less than at their heaviest and keeping the weight off for at least 3 years)
• 42% “failed dieters”
• 27% other

“more than half” of successful losers did so without the aid of a commercial diet program (defined as a particular medical treatment, book, or pills)

The data get pretty thin after that! The report claims that “through statistical analyses [not presented], we were able to identify six key behaviors that correlated most strongly with having a healthy body mass index.” The standard threshold of 25 is used for defining “overweight,” even though such arbitrary BMI thresholds are well-known to be misleading due to variability in skeletal frame size and muscle development.

The basic conclusion of the study was that successful losers generally embraced the allegedly good behaviors naturally practiced by the always-thin, typically slightly more so, and “significantly” more so than did the failed dieters. Therefore, the authors conclude, all you need to do to be a successful loser is to “quite literally, live like a thin person.”

The six behaviors (“secrets of the slim”) recommended turn out to be current mainstream party-line recommendations:

	Succ. losers	Always thin	failed dieters
watch portions	62%	57%	42%
limit fat	53	47	35
eat fruits and vegetables	49	49	38
eat whole grains	“consistently opted for”		?
eat at home	weight correlated with number of meals out
“exercise, exercise, exercise”*	32	31	23

*while the authors suggested that vigorous exercise that increased breathing and heart rate for 30 minutes or longer was strongly linked to lower BMI, the survey numbers are for strength training at least once per week

The authors note that going low carb is conspicuously absent from the list and that limiting carbohydrates correlated with higher BMI in the survey, though they did note that such a correlation could be an artifact of the fact that those with higher starting BMI are more likely to try a low carb diet. Despite this admission, they still claim that “the findings do suggest that cutting carbs alone, without other healthful behaviors such as exercise and portion control, might not lead to great results.”

They also reported that three other strategies did not show significant effects: eating small meals, never eating between meals. and including lean protein with most meals.

Surveys like this are notoriously unreliable for establishing any true cause-and-effect relationships. It is easy to bias the questions to support the conclusion you want to reach. The conclusion about the ineffectiveness of low carb diets is a good example. If only a small percentage of the survey population had even tried it, then, of course, such a diet would show low significance overall, and therefore no conclusion whatsoever can really be drawn as to the effectiveness of a low carb diet.

Another aspect of the report is the definition of “successful” dieters as those who’ve achieved a 10% weight loss (and kept it off for at least 3 years). For the 22% of respondents characterized as “obese,” 10% is barely a good starting point. At the end of the article, the authors, in a section labeled “Realistic goals are one key to weight loss,” state, “A 10 percent loss might not sound like much, but it can significantly improve overall health and reduce risk of disease.” True perhaps, but pretty discouraging from a public health point of view, if that’s the best we can hope for! And we clearly now know better. There are countless examples of people who have successfully lost much more weight and kept it off (for example, by carbohydrate restriction for those who have demonstrated sensitivity to carbohydrate consumption), and they’ve done it without resorting to extreme methods such as bariatric surgery or starvation diets.

At best, one can use survey data like this to construct hypotheses that might be worth testing. To base public health recommendations on the results is ridiculous!

I look at the data and draw a rather different set of conclusions:

1. We have a serious weight problem if 5 out of 6 people cannot be identified as always thin!
2. Current recommendations don’t work!
3. Even those mainstream recommendations that maybe seem to help don’t help very much.

And I would tend to characterize most of the six “secrets of the slim” as not secrets at all, but just things that are sometimes associated with the habits of people who successfully control weight, and not things that can fix it. Clearly, you can do all six “right” and not lose weight. Yes, overeating (oversized portions, eating out a lot) can make you fat, but it’s much more important what you eat and how you manage what you eat: control of portion size is realistically about controlling hunger and satiety and not about counting calories.

The reported survey benefits of lower fat, higher fruits, vegetables, and whole grains probably just correlate with respondents who are generally attempting to pursue healthful habits, a population who are more likely to exercise self control at earlier signs of a weight problem anyway. Also, the statistics presented do not make a very convincing case that these particular food choices are significantly beneficial; only about half of each survey group practiced any particular allegedly beneficial choice anyway. Hard evidence of benefits of such dietary recommendations are actually thin to non-existent. Exercise is good for overall health, but it's a poor strategy for weight control. Being completely sedentary is correlated with obesity, but it is not a cause. If anything, it’s the other way around: obesity causes people to lead a sedentary lifestyle.

And indeed, one could make a similar argument about causation with respect to the whole survey: always-thin people tend to eat less and exercise more because their body condition makes them so inclined. They are sated with less food and find exercise easier and more enjoyable. Trying to emulate the effect (eating less and exercising more) to achieve the cause (weighing less) doesn't work very well! The authors’ conclusion that all you need to do to be a successful loser is to “quite literally, live like a thin person” gets cause and effect backwards.

Carbohydrate Needs of the Endurance Athlete

There is increasing recognition among endurance athletes that at least some fat and protein consumption is beneficial for athletic activity extending beyond about 3 hours. Further, most athletes understand that increased protein consumption post-event is valuable for speeding recovery and reducing discomfort in the following day or two. But most still insist that you
absolutely need plenty of carbohydrates before, during, and after the activity to achieve peak performance, to avoid “crashing” or “bonking,” and to recover quickly. Aid stations are typically stocked almost exclusively with carbohydrates, especially sugars (jelly beans, cookies, M&Ms) and simple starches (potatoes, potato chips, pretzels, gels), with the addition of some limited fats and protein only in the longest events. I’d come to increasingly suspect that this advice is probably wrong, and especially wrong for anyone who has adopted a low-carbohydrate diet and made the physiological adaptation to fat-burning to fuel exercise. I can now add my own anecdotal experience.

I have been restricting my average carbohydrate intake to an estimated 20% or so of calories for about 10 months now. At the same time I have been increasing my exercise level and capability substantially, to the point where I have now completed several 50K running events and one 50-mile event. At first, running seemed harder without the usual high level of carbohydrate intake, but over a few weeks it got steadily easier until fairly suddenly it was much easier. I was able to run uphill again (which I hadn't been able to do for a while); I found my routine breathing rate during comfortable sustained running had slowed by at least a third; I could easily go longer without any food intake besides water.

Nevertheless, for my first 50K event, I consumed more like a 50% carbohydrate meal the night before, and consumed the usual assortment of provided aid station food, avoiding only the simple sugars. The running went well enough, though my weight jumped a few pounds the next day and took three or four days to return to “normal.”

Since then I have been steadily cutting back on carbohydrate supplementation during the run. I did the 50-miler on mostly protein and fat with only a modest amount of carbs thrown in. Then I tried a strict low-carb 50K run, using only a “protein shake” (my own brew of whey protein, soy protein, milk, cocoa, almond meal, and walnut oil) for the entire distance. It worked great! I had steady energy throughout the event (no ups and downs) and was still feeling strong at the end to the point where I did not participate in the post-event food and was happily running around on the beach where the event ended. (The day-after weight gain still occurred; apparently that's not a carbohydrate effect.) So my conclusion is that all this carbohydrate is not needed for successful endurance!

There’s still a question of peak performance. Do you need the carbs and the associated blood glucose spike to do your absolute best? Can you go faster with carbs than without? Does it matter if you’re planning short events (sprints, strength events, etc.) rather than longer endurance events? I don’t know, but I’m increasingly favoring the hypothesis that you don’t actually need much carbohydrate at all once your body is adapted to using fat as its primary fuel.

There are at least a handful of papers related to this topic. These papers all support the idea that only limited amounts of carbohydrates are necessary or even desirable for endurance athletes. See, for example:

Larson-Meyer et al., “Effect of dietary fat on serum and intramyocellular lipids and running performance,” Med Sci Sports Exerc. 2008 May;40(5):892–902 reports 3-day crossover trials of endurance trained runners with low fat (10% fat LFAT) or medium fat (35% fat MFAT), and concludes that “despite approximately 30% lower IMCL [intramyocellular lipids] 0.220±0.032% LFAT, 0.316±0.049% MFAT; P = 0.045) and approximately 22% higher muscle glycogen stores at the start of performance testing (P = 0.10), 10-km performance time was not significantly different following the two diet treatments.” Lipid profiles suggestive of cardiovascular disease were associated with the high-carbohydrate-low-fat diet, and the authors concluded that “even short-term consumption of a low-fat diet may unfavorably alter serum lipids, even in healthy, endurance-trained runners.” (Note that this study did not allow time for adaptation to the different diets, so glycogen and lipid levels in the muscle cells are due only to the acute diets tested);

Vogt et al., “Effects of dietary fat on muscle substrates, metabolism, and performance in athletes,” Med Sci Sports Exerc. 2003 Jun;35(6):952–60, which studied the effect on trained athetes of a high-fat (53% fat) or high-carbohydrate diets (17% fat) for 5 weeks in a randomized crossover design, found that maximal power and vO₂-max during an incremental exercise test to exhaustion were not different between the two diet periods, total work output during a 20-min all-out time trial (298±6 vs 297±7 W) on a bicycle ergometer as well as half-marathon running time (80 min 12 s ± 86 s vs 80 min 24 s ± 82 s) were not different between HF and LF. Blood lactate concentrations and respiratory exchange ratios (RER) were significantly lower after HF than after LF at rest and during all submaximal exercise loads. The authors concluded that “muscle glycogen stores were maintained after a 5-wk high-fat diet period whereas IMCL content was more than doubled. Endurance performance capacity was maintained at moderate to high-exercise intensities with a significantly larger contribution of lipids to total energy turnover”;

Lambert et al., “High-fat diet versus habitual diet prior to carbohydrate loading: effects of exercise metabolism and cycling performance,” Int J Sport Nutr Exerc Metab. 2001 Jun;11(2):209–25. No changes were observed in circulating glucose, lactate, free fatty acid (FFA), and b-hydroxybutyrate concentrations during exercise. However, mean serum glycerol concentrations were significantly higher [indicating mobilization of fat stores with glycerol release] in the HFD-CHO trial. The HFD-CHO diet increased total fat oxidation and reduced total CHO oxidation but did not alter plasma glucose oxidation during exercise. By contrast, the estimated rates of muscle glycogen and lactate oxidation were lower after the HFD-CHO diet. The HFD-CHO treatment was also associated with improved time trial times (29.5±2.9 min vs. 30.9±3.4 min)[150 min cycling at 70% vO₂-max followed by a 20 km time trial] for HFD-CHO and CTL-CHO. They conclude that “high-fat feeding for 10 days prior to CHO-loading was associated with an increased reliance on fat, a decreased reliance on muscle glycogen, and improved time trial performance after prolonged exercise”;

Leddy et al., “Effect of a high or a low fat diet on cardiovascular risk factors in male and female runners,” Med Sci Sports Exerc. 1997 Jan;29(1):17-25, which notes that “restricting fat intake may compromise endurance performance and that increasing fat intake may improve endurance performance,” and concludes that “a 42% fat diet maintained favorable CHD risk factors in female and male runners whereas a 16% fat diet lowered Apo A1 and HDL-C and raised the TC/HDL-C ratio”;

Horvath et al., “The effects of varying dietary fat on performance and metabolism in trained male and female runners,” J Am Coll Nutr. 2000 Feb;19(1):52–60, which concludes that “runners on a low fat diet consume fewer calories and have reduced endurance performance than on a medium or high fat diet [and] a high fat diet, providing sufficient total calories, does not compromise anaerobic power.”

Practical Low-Carb Diets

There’s a lot of hype on all sides (and there are many more than two) of the low-carb diet wars. Unfortunately, many of the arguments and recommendations are based on bad science or even no science at all. Too many dietary recommendations have been based on “expert” pronouncements rather than sound scientific data and rigorously tested hypotheses.

We’ve been reading about nutrition and making our own dietary decisions based on that reading and our personal experience for about nine months now—hardly long enough to become real experts, but long enough to have developed our own best guesses as to what seems to be right, at least for us.

So here is “David and Cynthia’s Guide to Low-Carb Eating.” Basic principle: cut back drastically on the simple starches and sugars that have become ubiquitous in first world diets (of all ethnicities and nationalities). Period. End of story. Beyond that, the details are secondary and serve mostly to provide guidance as to how to achieve that goal. Some details also have secondary health effects that are important if still secondary.

First, of course, you need to recognize what the major dietary sources of simple starches and sugars are. Sugar would seem to be obvious, but there is a large misinformation campaign out there on the part of advocates of various sugar substitutes that can trip up the unwary. A simple rule of thumb is that if it tastes sweet and isn’t a non-nutritive sweetener (sucralose, saccharin, aspartame, etc.), it’s sugar and needs to be minimized in your diet. Calling it “evaporated cane juice” is just a silly marketing ploy. Honey, rice syrup, maple syrup, etc. may have additional valuable trace nutrients, but the macronutrient is still sugar. Even maltodextrin, which is a long-chain sweet-tasting molecule, that gets classified as a starch instead of a sugar, is still readily broken down by the body to simple sugars (that’s why it is added to sports drinks and “gels”). Don’t be fooled by “100% juice” products and similar products with added fruit juice that use apple, pear, or white grape juices to provide sweetness. These are just alternative sources of sugar; apple and pear are particular bad in that they contain a high percentage of fructose. If you must sweeten your food, use whichever of the artificial sweeteners that you tolerate the best. We don’t really like any of those that we have ready access to and generally opt to simply make do with a lot less sweet taste in most of what we eat, but we do use Splenda (maltodextrin and sucralose) when we feel we need a sweetener. (There are at least a couple of potentially better non-nutritive sweeteners with less objectionable taste and aftertaste: Stevia-based products like Truvia and Acesulfame-Potassium-based products like Sweet-One. These sound promising in written descriptions, but we haven’t yet tried them and don’t presently know where to buy them affordably in bulk. Ordinary table sugar and high-fructose corn syrup remain the sweeteners that are commonly sold at much the lowest net price [price per unit of sweetness].) You also need to limit your intake of fruit, especially juices and dried fruits which tend to encourage large servings. All fruits contain significant amounts of sugar in addition to all the nominally healthy micronutrients (vitamins and minerals) that they are often advertised to contain.

Simple starches include starches from grains and tubers. Most ubiquitous in the American diet is wheat starch (flour, bread, etc., etc.). Rice, oats, barley, quinoa, rye, and other grains aren’t much better (despite the advertised heart-healthy characteristics of oats, for example). Potatoes are the most common tuber with a lot of simple starch. These starches are quickly broken down by the body into simple sugars.

Other foods also contain carbohydrates. Many vegetables contain significant amounts of carbohydrates as either starches or sugars or both. Root vegetables such as carrots and legumes such as peas and beans tend to contain more carbohydrates than leafy green vegetables (spinach, kale, lettuce, etc.) and cruciferous vegetables (broccoli, cauliflower). However, for the most part, you are much less likely to overeat starches from these vegetables, and you get plenty of other good nutrients. On balance, we don’t think you need to particularly avoid the higher-carb vegetables, although some recommendations such as those for the “induction phase” of the Atkins diet, have you minimize their consumption as well.

Dairy products contain carbohydrates too. Lactose is a disaccharide of glucose and galactose, and a typical 8 oz serving of milk or yogurt contains 9 to 17 g sugars (depending on how much dry milk was added or how much fermentation occurred). Hard cheeses generally contain much less carbohydrates. Casein is reputed to induce insulin release, which of course defeats the purpose of the low carb diet: to minimize blood insulin as well as glucose, so that the fat cells are releasing fat for use as fuel, rather than sequestering it safely away for long term storage (making us fatter). But that doesn’t stop us from using dairy products which also contain a lot of protein and fat; we aren’t interested achieving zero carb intake or in seeing how fast we can lose weight, but rather in eating a diet that we can sustain indefinitely to reach our target weights and maintain it, without giving up in screaming frustration and deprivation. We like our dairy products! Curiously, the “paleo diet,” which is a fairly low carbohydrate diet, proscribes dairy products but encourages consumption of a lot of fresh fruit. We remain skeptical of such a recommendation unless you happen to be lactose intolerant.

You probably shouldn’t try to cut out all carbohydrates. How low you need to go to be practicing a “low-carb” diet is something that is not well understood. Our own feeling (read “guess”) is that there is no magic threshold, and the answer may vary widely from individual to individual. Certain individuals seem to be able to thrive on a high-carbohydrate diet for a lifetime, while others clearly do much better on much lower levels of carbohydrate consumption. It probably depends on both your genetics and on how active a lifestyle you lead. For us, cutting carbohydrate consumption to under about 20% of calories seems to work well. Some studies have shown significant benefits of lesser reductions. Others advocate more extreme reductions in carbohydrates. If your goal is to get under 20%, then you don’t need to worry too much about starches in vegetables, and it’s even OK to “cheat” a bit. Go ahead and eat some sweet dessert occasionally or even eat that piece of pizza at the party where no low-carb alternatives are offered. Just try to keep the averages down.

The Atkins organization recommends starting out a low-carb diet with an ultra-low-carb induction phase (less than 20 g per day) for a couple of weeks followed by a low-carb phase which is then further relaxed after target weight levels are reached. We don’t see any clear evidence that such a phased approach makes any particular sense or has any particular benefit other than shock value and initial rapid weight loss to keep motivation high. It may be true that the abrupt change induces fat adaptations more quickly, but we know from studies that it takes at least 2 weeks to biochemically adapt (see Phinney (2004)), and a more gradual adaptation may be less stressful for some people. It seems simpler, and probably just as effective, to simply change to a new long-term diet and stick with it. Too many people treat the Atkins diet as a temporary weight loss diet that can be abandoned after the weight loss is achieved. These people regain much or all of the weight lost. It’s not clear that repeated Atkins inductions are as effective either.

A couple of further remarks on types of carbohydrates are worth making. First, we reiterate the important difference between fructose metabolism and glucose metabolism (see our earlier blog entry on fruit). While fructose produces a lower glycemic response, it’s probably a lot worse than glucose from a weight management point of view. Most sweet foods, including most fruits, actually have about equal amounts of fructose and glucose, so there’s not a lot you can do to minimize fructose in favor of glucose, but if you do have a choice, don’t go for high-fructose foods.

Fructose is metabolized by the liver into triglycerides which are then easily stored as fat. The path from glucose to fat is at least a little more convoluted, but it’s clear that triglycerides track carb intake, a fact that is shocking to most people since we’ve been taught that fat intake is what drives triglyceride levels high. However, this is not so: ingested fat is transported in chylomicrons and is preferentially absorbed by the body tissues, and so disappears from the blood quickly (within an hour typically). Triglycerides, in contrast, are produced by the liver in response to fructose and carb ingestion and released into the circulation for hours after each carbohydrate containing meal. Hence high carb meals result in all-day (and all-night) elevated triglycerides. That’s why people on low carb diets have very low triglyceride levels compared with people eating “normal” high carb diets, despite proportionately higher fat intakes (see, for example, Table 3 of Gardner et al., 2007 and Parks and Hellerstein 2000).

Second, certain starches have come to be known as “resistant” starches that are metabolized more slowly and generate less of a glycemic response (blood glucose spike). For the most part, these are the vegetable starches, and perhaps most importantly, the legumes, in particular. This provides a justification for the standard practice in low-carb cooking of replacing grain flours with legume flours such as soy flour or chickpea flour. These bean flours have less carbohydrate than wheat flour, but still have significant amounts. The fact that the starches in beans are digested more slowly seems to make them less prone to producing a blood glucose spike and less prone to being converted to stored fat. Speaking from personal experience, this substitution works quite well, at least from a nutritional point of view. You can do a one-for-one substitution of soy flour for wheat flour in many recipes. Those foods (like bread) which depend on wheat gluten to provide mechanical strength (elasticity) to a dough don’t work too well with a 100% substitution. In these cases, we use part soy flour and part gluten flour (wheat gluten separated from the wheat flour—use about ¼ to ⅓ wheat gluten by volume). Wheat gluten is mostly protein and is safe to use in low-carb cooking (unless, of course, you happen to be sensitive or allergic to it). You can also effectively substitute fiber such as wheat or oat bran and almond meal or other grated nuts for part of the flour.

What about “whole” grains and fiber? Don’t we need a lot of fiber in a healthy diet? Aren’t whole grains good for you? Not as far as we can tell! And we have no personal bias against whole grains. We like to eat them. Our current best hypotheses regarding whole grains and fibers:
1. To the extent that fiber is good for you it is in the context of high-carbohydrate diets only. If nothing else, adding fiber tends to fill you up with more non-nutritive filler, and you eat less simple starch.
2. Fiber probably also promotes intestinal health in the presence of a large carbohydrate load in the gut. Without the large carbohydrate load, it is less clear that you actually need a lot of fiber. Populations that eat no carbohydrates manage just fine with little or no fiber.
3. The alleged binding of starch to fiber to slow starch metabolism seems to be a myth. The measured glycemic response to whole grains is identical to that of equivalent amounts of refined grains. Perhaps if you swallowed genuinely “whole” grains that are still fully encapsulated in fiber (don’t chew!), you might slow the starch metabolism, but otherwise the starch you do consume is still processed quickly.
4. You may get more trace nutrients from whole grain than from refined grain, but there are better sources for those trace nutrients.
5. The same goes for fruit! Fruit contains a lot of fiber and significant trace nutrients, but it comes with too large a dose of sugar.
6. Fiber is not completely non-nutritive. It is at least partially digested by bacteria in the intestines, and seems to lead to production of short chain fatty acids, which are absorbed to varying extent. What that means nutritionally, I’m not sure. There’s probably not enough net usable calories in fiber to be of much significance. On the other hand, the short chain fatty acids are elongated in the liver to increase the net triglycerides.
7. If you want to eat fiber, go ahead! Focus on vegetable sources which come with a lot higher doses of trace nutrients anyway. It’s probably OK to add wheat or oat bran to your baked goods, but do it because you like it, not because it makes you feel virtuous. But be aware that sources of phytic acid such as grains are known to reduce absorption of minerals and may contribute to osteoporesis and other deficiencies.
8. There aren’t any significant nutritional differences between soluble and insoluble fiber that we’ve been able to identify.
9. When you read nutritional labels, “total carbohydrates” generally include any fiber in the food. It’s probably OK to subtract out the fiber as being largely non-nutritive. But beware of claims of “zero net carbs” or similar for foods that obviously contain simple starch. You can only subtract the fiber content from the total carbohydrate content to the extent that the total includes the fiber in the first place. You can’t add fiber to offset the presence of simple starches.

What about fuel for exercise? Don’t athletes need carbs to provide short-term fuel for peak performance? Probably not! This one is fairly complicated to sort out in detail, though. If you are accustomed to a high-carbohydrate diet and switch, you will likely feel that you can’t get peak performance or even satisfactory performance from your muscles for some period of time while the body’s endocrine system and metabolism readjusts to use the fuels you do provide. For instance, there is evidence that a high fat diet increases mitochondrial biogenesis (through activation of PPARdelta) and induces muscle type switching from fast twitch to slow twitch (from glycolysis to fat burning), and importantly, may induce resistance to obesity. This adaptation period of time may last for 2–6 weeks depending on your particular circumstances, but you will get through it and eventually find that you can switch over to fat burning. The evidence supports low carb diets for endurance sports at least, as fat burning enzymes and mitochondria are increased resulting in carb sparing for when you really need it. Is “carb loading” before competition useful? Probably not, again! Making sure your muscles are fully loaded with glycogen may be valuable for maximal anaerobic efforts and sprints, and possibly for longer events, but do it by tapering your training, not by carb loading. What about right before or during the event (especially for events of more than 2–3 hours duration)? This one is less clear. It’s possible that you can achieve a personal best performance with the aid of some extra quick-release fuel from carbohydrates, but even that could be a myth. While we find that, now that we are adapted to low-carbohydrate-fueled exercise, we can go longer without any refueling, but we still eventually need to refuel. We still tend to use carbohydrates, because that’s what’s usually available. But we do try to avoid or at least minimize the simple sugars and make sure we get some protein and fat, too. We haven’t yet tried to maintain a high level of exercise over periods of more than four hours using mostly fat or fat and protein as fuel. Right now, we’re inclined to think it will work just fine, although it may be necessary to start refueling a little sooner to compensate for slower digestion. Post-event nutrition is important, too, but the key need then appears to be added protein to speed muscle repair and recovery, not carbs.

So then, if you’re not eating carbs, what do you eat? The short answer is fat! While you may not have been eating enough protein on your high-carb diet, it’s pretty hard to replace carbs with only lean protein, and can make you sick (the liver can’t dispose of that much ammonia). As a practical matter, a low-carb diet is almost always a high-fat diet. Anyone who tries to have it both ways by continuing to recommend restricting fat consumption while supporting reduced carbs is deluding themselves. It’s essentially a prescription for extreme total calorie restriction, and no one should maintain severe calorie restriction for more than very short periods (and probably not even that).

What kind of fat should you eat? That’s a much harder question, and we think it will be a few years before we have a good well-researched answer. From a simple macronutritional point of view, it appears that it probably doesn’t matter too much. It’s much more important to your health that you replace calories from simple sugars and starches with calories from fat than that you carefully choose which fats to eat. That’s the curious thing about how the popular press and the attention of the public at large focus on nutritional issues. Millions of people will start taking the latest supplement based on flimsy evidence that it slightly improved some condition or other in a handful of test subjects while completely ignoring the basic macronutrient imbalance in their diets. We will take the micronutrient research a lot more seriously when we finally get the macronutrient recommendations properly understood and straightened out.

That said, it is worth commenting on what some of the major issues are with regard to fat choices. Most obvious, of course, is the saturated fat controversy. As nearly as we can tell, there is no good scientific evidence that saturated fat is bad for you despite decades of pronouncements that it is. (This subject is too big and too controversial for one or two references; perhaps we'll write more about it in the future.) Higher consumption of saturated fat may be correlated with elevated blood lipids (particularly HDL, but that's "good" right?), but those elevated blood lipids are not well-correlated with health problems such as coronary artery disease. The assumption that saturated fat is bad is so ingrained in the culture and training of our “experts,” that one must read all related studies very critically, and understand that in the presence of excess carbohydrate, triglycerides will be elevated in most people, and will be formed primarily of saturated fat. You often find that authors are struggling to reconcile apparently contradictory results with the assumed “truth” that saturated fat is bad. Looking critically at the actual data often shows that, if there is any clear difference at all, the test subject consuming more saturated fat were better off on whatever measure was being monitored. So go ahead and enjoy your butter and meat fats, but limit the carbs when you do. Use lard if you like.

For a while we were taught that the more unsaturated a fat was, the better. Then we learned that omega-6 polyunsaturated fats are bad and so maybe monounsaturated fats were as good or better. Hydrogenated vegetable fats were initially considered a “healthy” substitute for the evil lard and butter, but were later found to be worse. “Trans-fats” have recently become taboo. More recently, there is concern that polyunsaturated fats are easily oxidized and implicated in inflammatory atherosclerosis processes ("oxidized LDL"). We are now focusing on the relative amounts of omega-3 and omega-6 fatty acids in our favorite fats and our overall diets. So what’s a high-fat eater to do? In the absence of good science, we still have to eat! Here’s our current working recommendations and hypotheses (aka best guesses):

1. Go ahead and eat as much saturated fat as you like. It won't become oxidized and will increase HDL levels.
2. Avoid trans-fats and hydrogenated vegetable fats.
3. We tend to favor olive oil, then canola oil, then nut and seed oils in our cooking right now, but we’re open to new data that might shift the emphasis around. We've been tempted to try the rendered goose fat available in our local market, but haven't yet. We’re likely to choose an oil based on its performance or taste characteristics as much as anything and to favor cheaper oils when performance or taste don’t dictate a clear choice.
4. Omega-3-rich fats and oils are probably desirable when and where you can get them. Wild oily fish such as salmon and mackerel are probably the best sources. (Farmed fish may not be, depending on the feed that is used.) If you want to go with supplements, use fish oil or cod liver oil by preference over flax seed. Flax seed and flax seed meal or oil are probably the best readily available vegetable sources of you prefer them or want to use them in addition, but it’s not clear how much of the omega-3 fats in flax are even converted in our bodies to the desirable forms.
5. “Free-range” and “grass-fed” meat probably has “better quality” fat than the factory or feed-lot (corn-fed) equivalent. You may also find that it tastes better. It’s also typically a lot more expensive if you’re not raising your own. If your budget allows, and your local stores offer it, by all means go for it, but don’t worry too much if you find yourself eating mostly the cheap stuff.

Getting the macronutrients generally right is still the most important thing to do. Since fats become a dominant source of nutrition in a low-carb diet, it is apparent that the choice of which fats to eat must become a close second. Absent clear scientific evidence to support a strong emphasis on one or two sources of fat, the best we can conclude for the moment is that you should diversify and consume a least some fat from a lot of different sources.

So what about protein? It’s pretty clear that you do need some protein, and in particular, you need protein that contains certain “essential” amino acids that the body cannot manufacture for itself, including vitamins such as B12 that are difficult to find anyplace else. Animal protein (especially organ meats) is a very rich source of most vitamins (except folic acid and vitamin K). Chronic protein and vitamin deficiency can cause severe malnutrition, poor muscle development, and premature aging. Vegetarians need to be particularly careful since good vegetable protein sources tend to be limited and tend to have much lower protein and vitamin content than meat and fish. From a purely nutritional point of view, humans aren’t really designed to be vegetarians, though if you choose to be so on moral or religious grounds, it is still possible to avoid malnutrition as long as you make sure you get enough of the right proteins and supplement to avoid vitamin deficiencies.

You will also find warnings against eating too much protein, particularly if you have impaired kidney function. For most people, this is not a serious concern in that it’s fairly difficult to actually eat too much protein. (See, for example, Martin et al. (2005).) Perhaps, if you tried to cut both carbs and fat out of your diet, then you could do it, but most reasonable ad libitum diets emphasizing fat and protein are not likely to give you too much protein.

Obviously, there’s no good reason to follow the low-fat crowd to the leanest cuts of meat. You don’t have to remove the skin from chicken and turkey of you don’t want to, and go ahead and leave your beef, pork, and lamb untrimmed if you like the fat. You can also eat cheese and other dairy products for protein as well as eggs, nuts, seeds, and tofu. These tend to come with a generous serving of fat, but that’s OK.

If you’re an athlete, then you may need to consciously increase your protein intake to help build and repair muscle. If you have trouble getting enough protein from your standard dietary sources, you can supplement by using either whey protein or soy protein powders. Whey protein is probably the better choice from a protein quality point of view, and there is some concern that soy protein, if consumed in large quantities, may provide excessive hormones. We use both with an emphasis on the whey protein. Whey protein tends to be sticky and gooey (at least when concentrated), and soy protein isolate behaves more like a flour; both are largely tasteless by themselves. They can be added to anything from eggs to baked goods and drinks. They are often packaged for retail sale in various adulterated (and overpriced) forms with added flavorings and sweeteners. Buy them unadulterated if you can (one on-line source: Honeyville Grain).

One last useful ingredient that we recently discovered is polydextrose. This is a long-chain sugar polymer that is largely indigestible and functions nutritionally like fiber. It is also tasteless (i.e., not sweet by itself). However, it can provide a lot of the mouth feel and body that artificial sweeteners generally fail to do. So far, we’ve tried it in homemade artificially sweetened ice cream and carrot cake with very satisfactory results. (You may need to disperse it in warm liquid first; it tends to clump.) We never met a low-fat ice cream that we thought was worth eating, but as long as you use flavors that adequately mask the objectionable bitterness and aftertaste of your artificial sweetener, you can make a very respectable low-carb ice cream! You can buy polydextrose as a commercial food ingredient, for example, from Honeyville Grain.

We find we don’t really miss most carbs. We don’t crave things like cookies or noodles at all, but occasionally find it necessary to eat a sweet potato or make polenta to quiet the cravings for a few days. It also depends on how much we are exercising. Of late, with our 50+ per week running mileage, it seems less important to cut carbs, but we generally keep them low anyway, compared to the usual endurance athlete's diet). On the other hand, the more we exercise, the less the weight comes off! We're not sure if it's due to eating more carbs or increased cortisol levels from overdoing it of late (three 50K trail races since Aug 23 and a 50 miler planned!).