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.
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