Following the trend of humans being an outlier from great ape digestive anatomy, it is perhaps not surprising to find that our nutrient requirements are also unusual. The nutritional makeup(19) of human breast milk is shown in the table below along with that of great apes.
Primates Total Protein Fat Sugar Ash Solids(%) (%) (%) (%) (%)
Human(*1) 12.5 1.0 4.4 6.9 0.2
Great Apes 11.5 2.8 3.0 5.5 0.2 (*2)
The composition of primate milks. (*1) Homo sapiens, Packard, 1982; (*2) Pongo pygmaeus, Pan troglodytes, Ben Shaul, 1962; Gorilla gorilla, Tailor & Tomkinson, 1975;
The great apes produce milk that contains nearly 3 times as much protein as human milk, and slightly less sugar and fat. This is not surprising because human babies are born in a comparatively immature state of development because their relatively large head must pass out through the limited aperture of the cervix, while the body is still immature. The lack of development of the human infants body, and its relative immobility, is parsimonious with a low protein and energy milk that is consumed frequently.
The characteristic lack of development of the human body versus head development is called desomatisation, and is a characteristic of primates in general, as Terrance Deacon explains in his book 'The Symbolic Species'(20). According to Harper's Biochemistry, 24thEd., the average male human body is 17% protein (p. 6), of which most is muscle. Human muscle is from 18 to 20% protein, whereas brain tissue is only 8% protein(21) but has double the fat of muscle. The proteins in brain tissue are also far longer lived than in muscles. Brain tissue is therefore cheap in terms of protein requirements compared to muscles.
Approximate figures derived from work by Nancy Lou Conklin-Brittain et al.(22), show that wild fruits eaten by chimpanzees are on average 0.9% protein, 4% carbohydrate (1% sugar and 3% fibre) and about 0.4% fat. As adult requirements for nutrients are lower than those required for growth and development, it is easy to see why chimpanzees can manage to live off fruits as a high proportion of their diet. By analogy a similar situation should follow for humans, with a lower protein requirement, and perhaps a greater requirement for sugars to fuel the brain, as glucose is the primary metabolite of brain tissue.
Human biochemistry is poor at dealing with high protein intakes, as one might expect of a species adapted to a high fruit diet. When dietary protein intake exceeds about 100 to 150 grams of protein per day infant birth weight has been found to be reduced(23). At higher levels of protein intake a deadly condition called 'rabbit starvation' is induced(24), although figures vary from 35% of daily calorie(24) intake to 50% of daily calorie intake(23). No similar conditions have been reported in faunivores or unequivable omnivores. In contrast, in dogs that are fed grain based pet foods they are reputed to develop skin and hair problems.
One idea of many anthropologists is that use of animal matter has allowed humans to provision themselves with sufficient additional calories per unit food weight over their original plant based diet, to allow them to evolve larger brains. But as Deacon points out(20), humans grow smaller bodies, not larger heads. Furthermore, this energy boost by consumption of high calorie foods is alleged to be part of a maternal dietary strategy(2). However, according to Speth(23), during this period women commonly experience aversion to meat and meat odours, and cravings are for the most part, for carbohydrate foods. Speth also speculates that high levels of meat during pregnancy may be deleterious to the foetus.
Humans are also unable to synthesise vitamin C, a characteristic unique to herbivores, including the great apes. This, and the fundamental differences in anatomy and physiology described above, should exclude humans from being grouped along with other omnivores, and fit the theory of a strong frugivory in humans. Vitamin C has only a 30 minute half-life in blood plasma, so humans must regularly ingest fresh food in order to maintain a significant pool(31). It is no surprise that fruit was found to be an effective remedy against scurvy.
One of the most common diseases associated with the human consumption of animal products is cardiovascular disease. Atherosclerosis is the most prevelant of the deadly degenerative diseases. According to Harpers Biochemcistry, 4th Ed., "The rabbit, pig, monkey and humans are species in which atherosclerosis can be induced by feeding cholesterol. The rat, dog, and cat are resistant.", and further that "Diets rich in palmitate inhibit the conversion of cholesterol to bile acids."Meat is a rich source of palmitate. On page 281, it is further stated that infrequent large meals versus more continuous feeding, adversely affects cholesterol statis,
Another dangerous affect of eating diets high in meat (or other sulphurous compounds), over plant based diets is the production of sulphuric acid in the colon. Bacteria in the gut will convert undigested sulphur amino acids into hydrogen sulphide (the rotten egg smell),this combines with water and makes sulphuric acid. This is thought to promote a number of diseases as reported in New Scientist(26). It seems that the human colon is after all better adapted to plant foods than meat.