Supporting this feature, we have the insulin system. Although many paleodiet thinkers focus on the lipogenic effects of our insulin response to carbohydrate, lipogenesis via insulin only occurs if we have no immediate use for the glucose and we have full glycogen stores. If we don’t have full glycogen stores, insulin will convert glucose to glycogen first. This interaction between insulin, dietary glucose, and glycogen indicates that human physiology evolved in an environment in which conversion of dietary glucose into glycogen stores conferred a survival advantage.
Starch, Glycogen, and Hunting Success
Glycogen storage appears to have a significant effect on physical performance. Phinney investigated the effect of a ketogenic diet on physical performance, and found that, given adequate adaptation time and mineral nutriture, a ketogenic diet can support endurance performance equivalent to a high-starch diet. However, he also noted that athletes on ketogenic diets show decrements in anaerobic performance—e.g. weight training, weight lifting, or sprint performance, particularly at the end of an endurance event. This occurs because sprinting or lifting requires the production of ATP in the absence of oxygen, and human mitochondria cannot derive ATP from fat in the absence of oxygen. Thus, we require intramuscular storage of glycogen to retain and develop maximum strength or sprint performance, as needed in the “fight or flight” response.
Aerobic or steady state endurance activity has very little impact on glycogen stores unless carried out for one hour or more. In contrast, high intensity activity--like sprinting or resistance training--rapidly depletes muscle glycogen. 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 (Fournier et al, 2004).
Humans can replenish glycogen stores without dietary carbohydrate, and even while fasting (Fournier et al, 2004), which emphasizes the importance of glycogen stores for survival (specifically, in fight or flight situations). In the absence of food, lactate and endogenous amino acids provide the substrate for glycogen replenishment--you sacrifice lean mass for glycogen. In the absence of dietary carbohydrate, dietary amino acids would supply the substrate for glycogen replenishment.
Since we can store approximately 400 g of glycogen, and it takes one gram of protein to produce one gram of glucose/glycogen, in the absence of dietary carbohydrate, replenishment of just one-third of muscle glycogen (100 g) depleted by one maximal effort each day would require intake of 100 g of protein above dietary requirements, or the breakdown of up to 100 g of lean mass––nearly one-quarter of a pound of muscle.
People with physical activity levels similar to hunters doing persistence hunting require 1.0 to 1.6 g of protein per kilogram of body mass to maintain nitrogen balance if eating a mixed diet containing adequate glucose (Tarnopolsky, 2004). A 150 pound (68 kg) individual would thus require 68 to 108 g of protein to maintain lean mass assuming a mixed diet containing carbohydrate. If his diet had no carbohydrate, he would require 133 g additional protein each day on which he made efforts requiring him to replenish just one-third of his total (muscle and liver) glycogen. So, his total requirement would be at least 201 up to 241 g of protein daily to prevent loss of lean mass. This would require the consumption of about 28 to 34 ounces of lean meat – 1.75 to 2 pounds. If he depleted one-half of his glycogen daily, he would require 268 to 308 g of dietary protein, or 38 to 44 ounces of lean meat (2.4 to 2.75 pounds) every day.
Such an individual would expend about 3000 calories daily. An intake of 201 grams of protein provides 1064 calories, 27% of the total caloric intake. An intake of 241 g of protein would provide 32% of calories. This gets near the maximum intake of protein possible without exceeding the liver’s ability to detoxify the ammonia that results from deaminating the amino acids in the process of converting them to glucose. An intake of 308 g of dietary protein would provide 1232 calories, 41% of total caloric intake, an amount that would likely cause protein poisoning.
These apparent facts raise the intriguing possibility that the use of tubers fueled human success in hunting. Simply put, people who had fuller intramuscular glycogen stores would have superior sprinting, wrestling, and lifting ability compared to people without them. Such ability certainly would not improve human ability to harvest tubers, since they don’t run away or resist capture. But I imagine that when humans hunted on foot, with spears and such, using some type of persistence hunting, those who had the best “kick” into a sprint would frequently have had greater hunting and reproductive success than those who did not.
Consider this feat of strength recounted in The Paleolithic Prescription:
“In 1805, the Lewis and Clark expedition witnessed an Indian bison kill ….A small herd was stampeded over a cliff into a deep, broad ravine. As the bison fell one on top of the other, dazed and injured, hunters killed those on top with spears; the others were crushed and suffocated underneath. The ravine was twelve feet wide and eight feet deep; most of the bulls weighed over a ton, yet a team of five Indian hunters pulled nearly all the bison out of the ravine onto level ground for butchering.”
This is all anaerobic activity. Our current knowledge indicates that men who had more stored glycogen would have had greater success in feats like this, compared to men who did not.
In addition, if attacked by a predator, natural selection would have favored the survival of those who had a strong “kick” fueled by glycogen over those who had less stored glycogen and limited sprint ability.
I can think of several other advantages people using starch from tubers instead of protein from meat to supply glucose for replenishing glycogen would have realized:
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.
2. Less burden on the liver for ammonia detoxification.
3. Easier to avoid protein poisoning while at the same time maintaining greater glycogen stores.
4. Easier to maintain and increase lean mass in response to the stresses of high intensity activity, with a lower dietary protein requirement.
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.
Thus it seems likely that people who ate both meat and tubers regularly and had increased amylase and had the ability to store glucose as intramuscular glycogen, rather than as fat, would have scored more often in hunting, more often avoided predators, had access to more prey, and had ability to support more offspring, compared to those who did not eat tubers, or did not have enough amylase, or did not store glucose from tubers as glycogen.
Glucose, glycogen, insulin resistance, and intermittent feeding
Modern people easily consume 300 to 400 grams of glucose daily, which means that they will always have full glycogen stores unless they do something to deplete them on a daily basis. When muscle have full glycogen stores, they exhibit insulin resistance, but if you deplete the muscles of glycogen, they become insulin sensitive.
I have already noted that hunters engaged in the type of high intensity activity required to deplete glycogen stores and maintain insulin sensitivity. In addition, hunter-gatherers typically ate only once or twice daily. When you fast, the liver store of glycogen gets used up in 8 to 10 hours, and muscle glycogen reduces by about 50 percent in 24 hours. Thus, the primal combination of high intensity exercise and intermittent feeding would likely have maintained insulin sensitivity even with regular intake of glucose-rich starches.
Also consider that Holt et al (see Mendosa’s report) performed a study to determine the satiety value of various foods. White potatoes turned out to have the highest satiety index of any food tested, twice as satisfying as cheese or eggs, nearly twice as satisfying as beef, and about 50% more satisfying than ling fish, measured two hours after feeding. If that to which we have adapted gives us the greatest satisfaction, then this supports the idea that foods like the potato played a very important role in satisfying our ancestors. Since meat also had a high satiety value (nearly 75% greater than that of white bread), roasted meat and potatoes would have provided our ancestors with very satisfying fare.
Thus, I have come to accept that tubers probably played important roles in human evolution, and propose that the use of tubers actually increased success in hunting by improving physical performance. This seems to me the best explanation of all the unique features of human physiology. I think it may explain why many people find meals of meat and potatoes deeply satisfying “comfort” food.
The next question is, will a diet high in tubers adversely affect dental and therefore general health? I will address that question in Primal Potatoes, part 3.