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.

More on the Shape of Weight Loss Curves

This installment about weight loss will focus on data for David (age 53, height six ft even). By way of background, I weighed about 160–172 lb in college (early 1970s), then tended to be in the 170s in my 20s, 180s in my 30s, 190s in my 40s, following the typical sort of pound-per-year pattern of adult male weight gain. I ballooned to close to 220 at the end of the holiday season at year-end 2007. My weight over the previous 2–3 years had become more erratic, fluctuating from about 205 to 220.

At the beginning of 2008, I adopted a non-calorie-restricted low-carbohydrate diet. If anything, I reversed the Atkins-recommended pattern of an “induction phase” of extremely low carbohydrates followed by a maintenance phase. Rather, I phased out carbohydrates from a previous life of 60% carbs (10% protein, 30% fat) to more like 15% carbs (25% protein, 60% fat) over a month or so by slowly finishing off leftover holiday sweets and breads and then not replacing them. Eating to satiety also occurred with an estimated total daily calorie reduction from about 2700 to about 2100 (all very rough numbers). The resulting weight loss is shown on the graphs of weight versus time (date).


I saw an initial weight loss rate of about 2 lb/wk for the first 2–3 weeks, similar to a typical induction phase. Thereafter the rate of loss dropped off to about 1 lb/wk, slowing perhaps to 0.75 lb/wk. It was starting to look like a possible exponential decay from an initial state to a final state resulting from a step change in diet, with a time constant of 8–14 months depending on what I chose as a likely asymptote. (Until there is measurable curvature in the data, it’s not possible to reliably fit a specific value to the asymptote.) I was trying to model the weight loss as a simple exponential decay, assuming that there was a single step change to account for. This would result in a linear fit on a semi-log plot of weight above assymptote versus time. This exponential approach to an asymptote (slowing of weight loss) might be expected because as body mass decreases, less total caloric intake is required just to maintain the body’s metabolic needs. Without further modifying the diet to decrease calories in, one would expect the weight loss to slow. You can see on the graphs that the data could be fit nicely to either an exponential model (straight line on the semi-log plot) or to a piece-wise linear model (set of straight lines on the linear plot) through July. The exponential fit shown assumes an asymptote of 165 lb. The linear plot has the exponential fit shown in orange. The light blue linear fit is drawn through the data from about February through May. The green line is a piece-wise linear fit with segments for the first two weeks, then mid-January through mid-March, mid-March through July, and a separate line for early-July through mid-August. The dashed dark green lines delimit a ±2 lb tolerance band about the mid-March through July fit line.

By mid to late July, it was still difficult to clearly identify which model fit the data best given the normal daily scatter in the data (even with some care to always measure under the same conditions, first thing in the morning). Certainly, a piecewise linear model where there was presumed to be some sort of “induction” phase for the first couple of months followed by a slower linear weight loss rate thereafter fit the data at least as well as the exponential fit.

Then, around the beginning of August, I was starting to get a cluster of data points that were looking unusually low, even though I hadn’t specifically made any major change in diet or lifestyle that I was particularly conscious of. Now, in mid-August, looking back, one can model the last six weeks of data as exhibiting a linear loss of about 1.5 lb/wk, about double what it had been for quite a while! So much for any simple linear or exponential fit to the data! Where will it go from here? And why? I’m not at all sure. Stay tuned for the next exciting episode!

I probably also eat somewhat less as my weight has decreased, although since I haven’t been keeping a careful food diary, that’s hard to prove. It’s clear that eating more dense calories helps you adjust to eating less food total, since you achieve satiety with smaller piles of food on your plate and fewer second helpings.

So, what’s going on in my life besides the low-carb diet? What could have caused the weight loss to accelerate? Clearly, life is more complicated than simple step changes and rapid return to a new equilibrium or steady-state.

A couple of things that may be different in the last 6 weeks with respect to diet are that I am probably eating a smaller late evening meal (meaning usually no second helpings), and about 3–4 days per week, my breakfast is now a protein shake, consisting of a couple of cups of whole milk with soy protein and/or whey protein plus low-caloric flavorings. My best estimate is that the number of calories in that breakfast is about the same as the typical cheese omelette or similar that I often eat otherwise, but there probably is a shift toward more total protein and less fat in the meal.

I’m now down about 30 lb in 33 weeks. I expect to be able to comfortably lose at least another 10 if not 20–25 lb before I reach a stable healthy weight. I suspect that I have more upper-body musculature than I had in my 20s, which may limit me from reaching my minimum athletic college weight, but I’m not sure. My waist circumference was about 34" in college, increasing to about 38" until recently when it got up to about 41". I’m back down to 36" now, and still have a little bit of a “spare tire.” I expect to get back down to 34". Almost all of my excess weight seems to have been stored as visceral fat in the abdomen and above the beltline for most of my pants—none of the weight gain or loss ever made all that much difference to the fit of most of my clothes, one of the reasons that my pattern of weight gain was rather insidious and easy to ignore. (This is probably true for most men.) Many of the symptoms of metabolic syndrome that I had have now gone away or abated (more on that after the next physical exam.)

Part of what had resulted in the relatively rapid weight gain in the last 2–3 years were some major changes in my life that made me more sedentary after a period of higher activity, along with an appreciative eating audience who enjoyed and encouraged my baking habits. (We have yet to find or develop a new bread recipe that is an acceptable low carb substitute, though we’re making progress.) One always tends to reduce eating more slowly than one reduces activity, so in principle, the weight could have accumulated simply because of excess calories. (Or was it excess carb consumption?)

In the last few years, I have eaten more of my calories late in the day, if that makes any difference. (I think it did, if for no other reason, than because I was more likely to need extra snacks to hold me until a late evening meal. Previously, I ate an earlier dinner, so I didn’t need an afternoon snack, and rarely ate anything for several hours before sleeping.) But I was hardly leading a couch-potato lifestyle while gaining weight. I was typically running at least 25–30 mi/wk on hilly terrain, if not always very fast. The running certainly seemed to be getting harder as I got either older or heavier or both. For my weight (averaging about 210 during this time), this amount of exercise should translate into 3300 kcal expended each week, or an average of 472 kcal/day (assuming no additional caloric expenditure for the hills—my best estimate is that our particular hills added another ~60 kcal/day). I estimate I was eating an average of about 2700 kcal/day during this time, which is about as much as the Runner’s World diet page says is needed to maintain my weight with only light exercise. Without getting into a critique of the Runner’s World diet recommendations (another posting), I should not have been gaining weight on this amount of calories with the amount of exercise I was getting!

Not surprisingly, my running ability has improved significantly with 30 fewer pounds to cart around. My breathing is easier and slower. My heart rate is slower and recovers faster. I can run uphill again, where I previously pretty much slowed to a walk most of the time. Longer runs (12–18 miles) don’t wipe me out anymore. I’m probably logging closer to 40 mi/wk than 30 mi/wk right now, though I’m still not going to win any races.

Regardless, what I am doing works! I’m not struggling at all to lose weight. I never feel like I am starving myself, and except for the usual carb temptations in the world at large, I am finding the diet easy and comfortable to live with and sustain.

More on the Thermodynamics of Body Weight

The previous post was intentionally general with no specific finger-pointing to misuse or good use of thermodynamic arguments. The misuse is so widespread, that I still prefer not to provide specific references. However, it is worth pointing to a few recent papers that have taken the issue more seriously.

First, I should note that to a high percentage of even trained scientists and doctors, "thermodynamics" means "equilibrium thermodynamics." This is all that is taught in a typical introductory course. Unfortunately, the human body can rarely be modeled accurately as being in any sort of equilibrium. At best, it may be reasonable (at least for some time scales) to model it as being in something approaching a "steady-state." As such, a complete analysis must include a study of "dynamic," "kinetic," or "nonequilibrium" effects, i.e., you must look at all inputs and outputs, and potentially the rate of change of inputs, outputs, and the state of what's inside.

One recent paper which tackles the issue head-on is by Feinman and Fine: "Nonequilibrium thermodynamics and energy efficiency in weight loss diets," Theoretical Biology and Medical Modelling 2007, 4:27. The authors specifically focus on the greater weight loss observed in low-carbohydrate diets with an emphasis on the specific "kinetics" of fat storage and dissipation arguing that simple equilibrium models fail, because real bodies, and especially real bodies that are in transition (gaining or losing weight) can be far from equilibrium requiring the consideration of dynamic effects. While they find that no experiment exists that measures all relevant variables, they are able to find evidence that dietary carbohydrate controls fatty acid storage and release via its effect on hormone levels (particularly insulin) and that this "nonequilibrium" effect can explain the greater weight loss of low-carb diets.

Another paper by Schulz and Schoeller, "A compilation of total daily energy expenditures and body weights in healthy adults," Am J Clin Nutr 1994, 60:676, reviews 22 studies which use an important (if somewhat expensive) technique for measurement of what they call "total expenditure energy" (how much energy is used over, say, a day or a week for all physical activity. The technique uses "doubly labeled water" (both the hydrogen and oxygen atoms are non-standard isotopes so that the differential kinetics of hydrogen and oxygen can be measured). The authors present data for various populations from elite athletes to normal and overweight individuals. Most notably, the total daily energy expenditure is found to vary by a factor of more than 3. To a limited extent, they were able to separate out the contributions of basal metabolic rate and physical activity, fat-free body mass and excess fat. While not specifically focused on thermodynamic equilibrium, it does add further evidence against any simple "calorie is a calorie" model of diet and weight.

And not being completely unable to resist highlighting one paper that is unwilling to draw the right conclusions from its own data, consider Brehm et al, "The role of Energy Expenditure in the Differential Weight Loss in Obese Women on Low-Fat and Low-Carbohydrate Diets," J Clin Endocr & Metab, 2005, 90:1475. Their paper reports the results of weight loss comparisons for obese women following the two diets for six months. The Low-carb group lost 50% more weight despite not being specifically calorie-restricted. (Food diaries indicated that the caloric intake was nevertheless similar between the two groups.) Not being able to measure any significant difference in resting energy expenditure or physical activity, the authors fell back on the old standby: the low-fat dieters must have cheated and underreported actual food consumption! The possibility that the results could be real apparently wasn't acceptable.

"Thermodynamics" of Weight Gain and Loss

A common belief among the proponents of calorie restriction and exercise as a means of weight loss is that "thermodynamics" (or more particularly, conservation of energy) says it must be so. The basic premise is that:

Energy-in - Energy-out = Energy-stored-as-fat

Therefore, if you want to lose weight (make Energy-stored-as-fat negative), then Energy-out must exceed Energy-in. This is then interpreted to mean that you must either eat less or exercise more to change this balance. People then go into elaborate calculations as to how many Calories you burn with various sorts of exercise and how many Calories are in different diets.

A few authors have recognized that this same equation can be interpreted a little differently. They argue that no one can actually regulate their energy intake precisely enough to have a constant weight over any length of time. Therefore Energy-stored-as-fat must be regulated by the endocrine system and be more or less fixed. Then, the thing that has to change when you eat more or less is the Energy-out; i.e., the body adjusts its energy expenditure (without a specific conscious exercise program) to burn available excess energy, and conversely, if you get an unusually large amount of exercise (run a marathon, for example), then you will naturally eat more to replace the energy used.

I believe the real picture is a good deal more complicated than any of these simple descriptions. As a physicist and thermodynamicist, I look at a "control volume" (a human body in this case), and consider all of the flows of mass and energy in and out. Any net differences must be stored internally. Inputs include the food and drink consumed and the air breathed in (most of the time, direct energy inputs [mechanical or thermal] can probably be ignored as small). Outputs include waste excreted, air breathed out (with a different composition!) and any mechanical work performed. It is important also to recognize the energy associated with differences in the internal potential energy associated with the any chemical changes that may occur between matter taken in and matter excreted. These represent a form of possible difference between energy-in and energy-out.

So, consider someone who is eating, say, an "extra" 1000 Calories a day. He could store it as a quarter pound of fat and gain a couple of pounds per week. He could get restless and move around a lot more, expending at least some of the excess as mechanical energy. His peripheral capillaries could dilate so that he loses more energy thermally through his skin. He could simply excrete some of it as undigested. I have always suspected, for example, that there is an upper limit to the rate at which food can be processed in the body, and if you exceed that limit by binge-eating or feasting on a particular day, then most of the excess is simply excreted rather than stored.

People often report that even large increases in the amount of exercise they get does not result in weight loss. Some portion of this may be a consequence of muscle development ("good" weight gain). But to a large extent, it is very hard not to just eat more in response to the body's repair and recovery processes after exercise.

Similarly, if you try to lose weight by starving yourself, your body tends to try to conserve energy to compensate (and you may be chronically suffering as your body tells you to eat more!). People who literally go on starvation diets (either intentionally or as a result of disaster) often report a permanent reduction in their "metabolism" to the point where they eat much less after the period of starvation (or quickly gain back any weight lost).

All of this means, that if you really want to change body weight (either up or down), you really need to be tinkering with the endocrine system to stimulate release or storage of fat. That's rather complicated, and I don't understand it very well myself. It's a bigger topic than I want to tackle today, anyway. Moderate amounts of exercise are probably beneficial. But probably the simplest thing that most people can do is to control carbohydrate consumption, particularly simple sugars and starches (including those found in "whole" grains and most fruits). These tend to set off a chain of hormonal events including a large spike in insulin production, triglyceride production and ultimately fat storage. If you keep your insulin and triglyceride levels low, you tend to take fat out of storage instead.

Anyway, the purpose of this post is not to promote any particular approach to weight management, but merely to argue that if you're going to invoke "thermodynamics" in discussing it, make sure you are properly accounting for all the energy inputs, outputs, and transformations that are occurring, and be very careful that you are drawing the right conclusions.