In her editorial comments and letter to the editor remarking on the Cordain et al paper on Plant-animal subsitence ratios of world-wide hunter-gatherers, Katherine Milton also made several other comments casting doubt on the idea that evolutionary human diets contained low proportions of plant foods and carbohydrate, and high proportions of animal protein and fat. Rather than quote, I will paraphrase:
1. Humans come from an ancestral lineage—i.e. primates–– in which plant foods have traditionally served as the primary source of energy.
2. The human gut displays a protracted transit time, averaging 62 hours with low-fiber diets and 40 hours with high-fiber diets. “In striking contrast to humans and all great apes, all extant Carnivora show a rapid turnover of ingesta. For example, a 370-kg polar bear takes ≈24 h to digest a seal carcass.”
3. “To date, few genetic adaptations to diet have been identified in humans, suggesting that, in their evolution, humans tended to resolve dietary problems primarily by using technology rather than biology.”
4. “The technologic abilities of humans derive from their unusually large, complex brain, a brain that, under normal conditions, is fueled by a steady supply of glucose. Consumption of digestible carbohydrate is the most efficient way for humans to obtain glucose for brain function. Potential alternatives—gluconeogenesis or the use of ketones to fuel the brain—represent alternative, more costly metabolic solutions.”
Our ancestral primate lineage consumed, so far as we can tell, a plant-dominated omnivorous diet. This does not mean that humans should eat like other extant, closely related primates (and Milton does not suggest that we should). We don’t have exactly the same digestive physiology as chimps or gorillas; we don’t have the equipment required for a raw plant-dominated omnivorous diet.
However, as Milton notes, we also lack the digestive physiology of the top carnivores. Even on a low fiber diet, due to our proportionately long small intestine, the passage of food through the human gut takes more than 2.5 times longer than transit of food through the guts of extant carnivores. In this respect, the human gut functions more like that of other primates than that of top carnivores. This strongly suggests that the human gut remains adapted to a plant-dominated diet.
As well, the fact that humans produce more salivary amylase and have more copies of the AMY gene that codes for amylase production than other primates also suggests that evolutionary diets contained substantial amounts of starch.
Although we have clear evidence that ancestral humans succeeded in expanding the animal-source component of their diets and good reasons to believe that this may have played a role in expansion of the human brain by providing neural fatty acids, this evidence does not tell us that ancestral humans had abandoned the primate tradition of plant-dominated nutrition.
The expansion of the human brain through evolution does not necessarily indicate a move to a diet composed largely of grassland animal meat and fat. The idea that “meat-based diets support brain development better than plant-dominated diets” doesn’t gain much support from comparative anatomy. Primates build larger brains (relative to body mass) than other species on largely vegetarian diets. The chimps living on 95 percent plant food diets have a body:brain ratio (by mass) of 100:1. Elephants eating plant-dominated diets have a ratio of 851:1. Tigers living on virtually 100 percent grassland animal food have a body:brain ratio of 1000:1.
Thus, the 95 percent vegetarian primate eating a has a brain 10 times larger than the carnivorous tiger on a relative basis. Moving the diet in the direction of the tiger’s (predominantly land animal flesh and fat) would seem to favor a smaller, not larger, brain. Notably, children recovering from malnourishment appear to build normal brains on diets containing as little as 8 grams of animal protein per day (one-third of total protein requirement) if provided adequate essential fatty acids.[1, 2, 3 ]
Some have suggested that consumption of brains and marrow supported development of hominid brains in evolution. Of interest, Ta'i chimpanzees regularly eat the brain, eyes, and marrow of long bones of colobus monkeys [4 pdf] and presumably have been doing this for millennia, but the chimp’s brain is only one-third the size of a human’s. Perhaps some other nutritional factor supported hominid brain expansion?
Dolphins have a brain slightly larger than human (1.8 kg vs 1.4) and a body:brain ratio similar to humans (dolphins, 42:1, humans, 56:1). For development and function, the human brain requires specific vitamins (B12, folate, B-complex) and minerals (iodide, iron, copper, zinc, and selenium) in addition to essential fatty acids. On a weight basis, shellfish, eggs, finfish, pulses, and cereals all provide greater concentrations of these minerals than meat.[5, pdf]
From this data, using single foods, a human can satisfy all requirements for brain-selective minerals by eating 2 pounds of shellfish, 5 pounds of eggs, 8 pounds of fish, or 11 pounds of meat. With this basis, one can imagine that only small amounts of seafoods to a plant-dominated diet would provide a hominid with adequate nutrition for building a large brain.
The evolutionary expansion of the brain actually increased the demand for glucose provided by plant foods. Nonhuman primate brains only use 8-9% of resting energy expenditure, but the modern human brain uses about 20-25% of resting energy expenditure and two-thirds of all glucose used by the body, while the mass of the human gut is only 60% of expected for a similar sized primate. Although the brain can adapt to a large extent to use of ketones instead of glucose, this is an inefficient way to fuel the brain and appears to stimulate the stress i.e. fight or flight response. For example, it appears that the ketogenic diet’s antiseizure effect depends on stimulation of the sympathetic nervous system since disabling norepinephrine destroys the anti-seizure effect of the diet. [6 ]
The nervous system and red blood cells use an estimated 150 g of glucose daily. Non-neural tissues will also use glucose for fuel when provided. The conversion of amino acids to glucose unnecessarily burdens the liver with ammonia to detoxify, and the kidneys with sulfuric acid to eliminate. Therefore, for smoothest operation, humans need food that supplies large amounts of glucose in a compact and readily usable form (i.e. not amino acids). This is supported by the beneficial effect that high carbohydrate diets appear to have on mood , and reflected in the fact that the body preferentially burns glucose as fuel and stores glucose as glycogen in the liver and muscles rather than as fat.[8 , 9 ] Animal tissues are universally low in glucose, therefore not serving as the best source of substrate for brain metabolism.
The body also needs glucose as a substrate for the manufacture of a number of other important functional and structural compounds including:
- Glucuronic acid, which binds to substances to facilitate their transport around the body. In this way glucuronic acid is largely responsible for the elimination of toxins such as drugs, excess hormones, and foreign chemicals.
- Hyaluronic acid, a constituent of extracellular fluids, needed for synovial fluid, cartilage, and skin.
- Chondroitin sulfates (a type of glycosaminoglycan), constituents of cartilage
- DNA and RNA
- Heparin (another glycosaminoglycan), an endogenous anticoagulant
None of this means we should “go vegan.” Animal products of the best quality—particularly seafoods-- provide important nutrients not adequately provided by plants, not the least of which is vitamin B-12. However, as a student of evolutionary nutrition, I find it noteworthy that we have enterohepatic recirculation of vitamin B-12. As noted by Herbert [8 ], this can prevent B-12 deficiency from occurring in a previously omnivorous adult vegan for 20-30 years:
“The enterohepatic circulation of vitamin B-12 is very important in vitamin B-12 economy and homeostasis (27). Nonvegetarians normally eat 2-6 mcg of vitamin B-12/d and excrete from their liver into the intestine via their bile 5-10 mcg of vitamin B-12/d. If they have no gastric, pancreatic, or small bowel dysfunction interfering with reabsorption, their bodies reabsorb ~3-5 mcg of bile vitamin B-12/d. Because of this, an efficient enterohepatic circulation keeps the adult vegan, who eats very little vitamin B-12, from developing vitamin B-12 deficiency disease for 20-30 y (27) because even as body stores fall and daily bile vitamin B-12 output falls with body stores to as low as 1 mcg, the percentage of bile vitamin B-12 reabsorbed rises to close to 100%, so that the whole microgram is reabsorbed.”
Looking at this from an evolutionary perpective, why would the body have such efficient recycling of vitamin B-12 but not of other B-complex vitamins? Logically, nutrient recycling probably represents a response to scarcity and infrequent consumption, so I would guess that this system would most likely have developed in response to a diet that did not have a continuous rich daily supply of vitamin B-12, with high B-12 intake occurring sporadically.
Of further interest, which animal products supply the most vitamin B-12 per serving? From the Linus Pauling Institute page on vitamin B-12  :
Shellfish, not land animals, appear the richest sources. An adult human requires about 2.4 mcg of vitamin B-12 daily. Given the fact that we recycle vitamin B-12 with a net loss of only 2-5 mcg per day from body stores, three ounces of beef or salmon or 4 large eggs daily, 3 ounces of crab once every other day, 3 ounces of mussels once every 3-4 days, or 3 ounces of clams once every ~20 days, would satisfy the requirement.
So, reverse engineering from current human vitamin B-12 metabolism seems to suggest that for a very long period of time, long enough to establish our baseline B-12 metabolism, our ancestors infrequently consumed foods rich in vitamin B-12.
On the other hand, applying the same thought process to modern human carbohydrate (glycogen) and fat storage suggests frequent consumption of carbohydrates and infrequent consumption of fats in ancestral diets. But I’ll save that topic for a future entry.