There is overwhelming evidence that we can not be a vegetarian species. In 1972 the publication of two independent investigations confirmed this.-1-2They concerned fats. About half our brain and nervous system is composed of complicated, long-chain, fatty acids. These are also used in the walls of our blood vessels. Without them we cannot develop normally. These fatty acids do not occur in plants, although fatty acids in a simpler form do. This is where plant-eating herbivores come in. Over the year, the herbivores convert the simple fatty acids found in grasses and seeds into intermediate, more complicated forms. By eating the herbivores we can convert their stores of these fatty acids into the ones we need.
About 2.5 million years ago animal foods began to occupy an increasingly prominent place in our ancestors' menus. Smaller molar size, less robust facial muscles and alterations in incisor shape from that time all suggest a greater emphasis on foods such as meat that require less grinding and more tearing.
An increasing proportion of meat in the diet would obviously have provided more animal protein, a factor perhaps related to the increase in stature which appears to have accompanied the transition from Australopithecines through Homo habilis to Homo erectus.-3
But greater availability of animal fat was probably a more important dietary alteration. Crude stone tools allowed early humans to break bones and allowed them access to brain and marrow fats from a broad range of animals obtained by scavenging or hunting. These and other carcass fats were probably as prized by early hominids as they are by modern human hunter-gatherers.-4
Not only did more animal fat in the diet mean considerably more energy, it was also a source of ready-made, long-chain, polyunsaturated fatty acids, including omega-6 arachidonic acid (AA), omega-3 docosatetraenoic acid (DTA) and omega-3 docosahexaenoic acid (DHA). These 3 fatty acids together make up over 90% of the fatty acids found in the brain matter of all mammalian species.-5
Our brain is considerably larger than that of any ape. Looking back at the fossil records from early hominids to modern man, we see a remarkable increase in brain size from 375-550 ml at the time of Australopithecus, to 500-800 ml in Homo habilis, 775-1,225 ml in Homo erectus, and 1,350 cc in modern humans (Homo sapiens). While there is still speculation about why this should have happened, this increase in brain size could not have been supported physiologically without an increased intake of preformed long-chain fatty acids which are an essential component in the formation of brain tissue.-6 It would never have occurred if our ancestors had not eaten meat - with its fat. Human breast milk contains the fatty acids needed for large brain development, cow's milk does not. It is no coincidence that, in relative terms, our brain is some 50 times the size of a cow's.
Where does the energy for our brain come from?
Between 20% and 25% of all the energy we use, is used by our brain. This is in contrast to the great apes whose brains use only about 8%. This makes our brains very expensive in energy terms. It means that our energy use compared to our body size should be considerably higher than that of other animals. Yet it isn't. This presents something of a puzzle: where do we humans get the extra energy to spend on our large brains? Researchers WR Leonard and ML Robertson concluded that the evolution of brain size imply changes in diet quality during hominid evolution. They say,
'The shift to a more calorically dense diet was probably needed in order to substantially increase the amount of metabolic energy being used by the hominid brain. Thus, while nutritional factors alone are not sufficient to explain the evolution of our large brains, it seems clear that certain dietary changes were necessary for substantial brain evolution to take place.' -7This confirms the Crawfords' work. While our enlarged brain was made necessary by our banding together into tighter communities with more individuals and, thus, a necessity to remember more individuals, what made it possible was a diet of sufficient quality to allow that brain expansion.
But there is another aspect. Two scientists, Aiello and Wheeler, measured the sizes of brains and other body organs against organ size relative to body size predictions.-8 What they found was that the larger-than-expected size of the human brain was compensated for by a smaller-than-expected gut size. Measuring the other energy-expensive organs in the body: heart, kidneys, liver, and gastrointestinal tract, as these use the most energy after the brain, and comparing those of a 65-kg non-human primate with the organ sizes of an average 65-kg human, they found dramatic differences between the expected and actual sizes of the human brain, and gut: 'the splanchnic [abdominal/gut] organs were approximately 900g less than expected'. Almost all of this shortfall was due to our gut being only about 60% of that expected for a similar-sized primate.
We have a carnivore gut
Not only is our gut smaller than predicted compared with other primates, it is also configured very differently. Our small intestine is the major organ used to digest food and extract its nutrients for absorption into our bodies. Not surprisingly, it is more than 50% of the total volume of our gut. Our colon (large intestine) plays little part in the process of digestion: it is used mainly to extract and, so conserve, water. For this reason, it represents only around 20% of our gut's volume. In contrast, the ratios in other primates are exactly the opposite: The small intestines of orangs and chimps, which play a minor role in digestion, are around 25% of gut volume, and their colons, where bacteria are used to ferment plant fibre and where most digestion takes place, are around 53% by volume.-9
This is not the only measurement that matters. So far I have compared our gut to that of our primate cousins which eat mostly plant food. If we also compare them to the great carnivores, we find that our gut is actually very much like theirs. The comparisons are done with respect to body weight as weight is closely related to the metabolic energy requirements of an animal. This ratio, known as Kleiber's Law, expresses the relationship between body mass (weight) and the body's metabolic energy requirements. The size of any organ that is directly concerned with metabolic turnover should comply with Kleiber's law. If we measure the size of these and they are in accordance with Kleiber's law, each part's gastrointestinal (GI) quotient should be 1.00. A GI greater than 1.00 means the organ is larger than expected, and GI less than 1.00 indicates a size smaller than expected.
In the gut, it is the surface area of various parts of the digestive tract which determines their relative absorptive ability. A test of major areas of the human digestive tract was published in 1985 with the following results:-10
- Stomach quotient - 0.31
- Small intestine quotient - 0.76
- Caecum quotient - 0.16
- Colon quotient - 0.58
Our gut is not the only part of our bodies to be analysed in this way. It is in our brain size and high intelligence that we humans are unique. Relative to our body size, our brains are truly enormous. If we measure our brain quotient in the same way we did for the gut, we can get some idea of just how big it really is.
In order to measure encephalization as it is called, statistical models were developed which compared brain size and body size in a wide range of species. This allowed an accurate estimation of the brain size for a given species based on its body mass. This is important because it allows the quantitative study and comparison of brain sizes between different species by automatically adjusting for body size. For example, elephants, which are plant eaters, and whales whether herbivores or carnivores, have larger brains than we do - but they also have much larger bodies. In this exercise it was noticed that the brain sizes of these animals also followed Kleiber's law.
When this test was conducted on humans, it put humans right at the very top of the primate scale. Our Encephalization Quotient was an outstanding 28.8.
With a brain so out of proportion to the rest of our bodies, it's not surprising that it uses such a large proportion of our total energy. As brain size and energy use is so high, and our gut size so small, the amount of energy available to the brain is dependent not only on how the body's total energy budget is allocated between the brain and other energy-intensive organs and systems, but on the ability of our gut to extract sufficient energy from our food. That also confirms that the kind of diet we should eat must have the high nutrient density found in foods such as meat and fat.
Our brains are now getting smaller
With such a small gut with which to absorb all the nutrients and energy our bodies need, a modern low-calorie, low-fat, fibre-rich, plant-based diet is woefully inadequate as an energy source for our energy-hungry system to function at peak efficiency. That lack has begun to show.
Since the advent of agriculture, there has been a worrying trend as our brains have actually decreased in size. A recently updated and rigorous analysis of changes in human brain size found that our ancestors' brain size reached its peak with the first anatomically modern humans of approximately 90,000 years ago. That then remained fairly constant for a further 60,000 years.-11 Over the next 20,000 years there was a slight decline in brain size of about 3%. Since the advent of agriculture about 10,000 years ago, however, that decline has quickened significantly, so that now our brains are some 8% smaller.
This suggests some kind of recent historical deficiency in some aspect of overall human nutrition. The most obvious and far-reaching dietary change during the last 10,000 years is, of course, the enormous drop in consumption of high-energy, fat-rich foods of animal origin which formed probably over 90% of the diet, to as little as 10% today, coupled with a large rise in less energy-dense grain consumption.-12 This pattern still persists; it is even advocated today: it is the basis of our so-called 'healthy' diet.
If any more convincing that we have to be a meat-eating species is needed, there is one other essential nutrient that is not found in any plant food. That nutrient is Vitamin B-12.
Vitamin B-12 is unique among vitamins in that while it is found universally in foods of animal origin, where it is derived ultimately from bacteria, there is no active vitamin B-12 in anything which grows out of the ground. Where trace amounts of vitamin B-12 are found on plants it is there only fortuitously in bacterial contamination of the soil. And even that is lost if plants are washed thoroughly before eating them.
Bacteria in the human colon make prodigious amounts of vitamin B-12. Unfortunately, this is useless as it is not absorbed through the colon wall. Dr. Sheila Callender tells of treating vegans with severe vitamin B-12 deficiency by making water extracts of their stools which she fed to them, thus affecting a cure.-13 An Iranian vegan sect unwittingly also makes use of this fact. Investigators could not understand how members of this sect remained healthy, until their investigations showed that they grew their vegetables in human manure - and then ate the vegetables without being too fussy about washing them first.-14
To enable vegans to survive, vitamin B-12 is added artificially to breakfast cereals in Britain and may be bought in pill form. This is hardly a natural way to get food and in many cases it is self-defeating. Unlike most other vitamins, Vitamin B-12 occurs as a number of analogues, very few of which are active for humans. In collecting human stools for analysis Dr. Victor Herbert found that of each 100 micrograms of vitamin B-12 extracted, only 5 micrograms were analogues active for humans.-15 Thus even in this most prodigious source of the vitamin, 95% was composed of analogues which were useless.
Several fermented products such as tempeh, a soya bean product and spirulinas, used by strict vegans as a source of vitamin B-12, either do not contain significant amounts of the vitamin or contain analogues of the vitamin which are not active for humans.-16 Over half of the adults from a macrobiotic community tested in New England had low concentrations of vitamin B-12. Children were short in stature and low in weight. The community relied on sea vegetables for the vitamin.
This reliance on vegetables sources gives a false sense of security and could actually bring on the symptoms of B-12 deficiency more quickly.
The amount of vitamin B-12 we need is tiny: about 1 microgram per day. Eating more than this results in a reserve being built up in the body. When a person becomes a vegan, those stores are depleted - but only gradually. Thus it can be several years before the onset of symptoms. In England a carefully conducted study carried out on vegans showed that they all got vitamin B-12 deficiency eventually.-17
Brain shrinkage among vegetarians
But, getting back to brain size, the decline which started with the advent of agriculture and our greater reliance on foods of plant origin has now seen a dramatically greater decline in those who have adopted a 'healthy', vegetarian diet.
Scientists at the Department of Physiology, Anatomy and Genetics, University of Oxford, recently discovered that changing to a vegetarian diet could be bad for our brains - with those on a meat-free diet six times more likely to suffer brain shrinkage.-18
Using tests and brain scans on community-dwelling volunteers aged 61 to 87 years without cognitive impairment at enrolment, they measured the size of the participants' brains. When the volunteers were retested five years later the scientists found those with the lowest levels of vitamin B12 intake were the most likely to have brain shrinkage. Not surprisingly, vegans who eschew all foods of animal origin, suffered the most brain shrinkage. This confirms earlier research showing a link between brain atrophy and low levels of B12.
Vegans are the most likely to be deficient because the best sources of the vitamin are meat, particularly liver, milk and fish.
Confirmation of this was provided the following year by another study by the Oxford Project to Investigate Memory and Ageing, the Department of Physiology, Anatomy and Genetics, University of Oxford, UK.-19 Noting that vitamin B-12 deficiency is often associated with cognitive deficits, they reviewed evidence that cognition in the elderly may also be adversely affected at concentrations of vitamin B-12 above the traditional cutoffs for deficiency. Their suggestion is that the elderly in particular should be encouraged to maintain a good, rather than just an adequate, vitamin B-12 status by dietary means.
It is obvious that we need to be eating more, not less, meat and animal-sourced foods.
If vegetarians - and vegans in particular - berate you for 'murdering' and eating animals, please be kind to them. They are almost certainly suffering from self-inflicted brain atrophy, and have little recognition of both the damage they are doing to themselves and the harm that are doing to others who follow their advice.
. Crawford M, Crawford S. The Food We Eat Today. Spearman, London, 1972.
. Leopold AC, Ardrey R. Toxic Substances in Plants and Food Habits of Early Man. Science 1972; 176(34): 512-4.
. McHenry HM. How big were early hominids? Evol Anthropol 1992; 1: 15-20.
. Stefansson V. The fat of the land. MacMillan, New York, 1960. 15-39.
. Sinclair AJ. Long-chain polyunsaturated fatty acids in mammalian brain. Proc Nutr Soc 1975; 34: 287-91.
. Crawford MA, Cunnane SC, Harbige LS. A new theory of evolution: quantum theory. In: Sinclair A, Gibson R, eds. Essential fatty acids and eicosanoids. American Oil Chemists Society, Champlaign, Ill, 1992. 87-95.
. Leonard WR, Robertson ML. Evolutionary perspectives on human nutrition: the influence of brain and body size on diet and metabolism. Am J Human Biol 1994; 6: 77-88.
. Aiello LC, Wheeler P. The expensive tissue hypothesis: the brain and the digestive system in human and primate evolution. Current Anthropology, 1995; 36: 199-221.
. Milton K. Primate diets and gut morphology: implications for hominid evolution. In: Food and Evolution: Toward a Theory of Food Habits, eds. Harris M, Ross EB; Temple University Press, Philadelphia, 1987, 93-115.
. Martin RD, et al. Gastrointestinal allometry in primates and other mammals. In: Size and Scaling in Primate Biology. Jungers WL ed., Plenum Press, New York, 1985, 61-89.
. Ruff CB, Trinkaus E, Holliday TW. Body mass and encephalization in Pleistocene Homo. Nature 1997; 387: 173-176.
. Eaton, S Boyd, Eaton, Stanley B III. Evolution, diet and health. Presented in association with the scientific session, Origins and Evolution of Human Diet. 14th International Congress of Anthropological and Ethnological Sciences, Williamsburg, Virginia, 1998.
. Callender ST, Spray GH. Latent pernicious anaemia. Br J Haematol 1962; 8: 230.
. Halstead JA, et al. Serum and tissue concentration of vitamin B 12 in certain pathologic states. N Eng J Med 1959; 260: 575.
. Herbert V. Vitamin B-12: plant sources, requirements and assay. Am J Clin Nutr 1988; 48: 852.
. Miller DR, et al. Vitamin B-12 status in a macrobiotic community. Am J Clin Nutr 1991; 53: 524-9.
. Chanarin I, O'Shea AM, Malkowska V, Rinsler MG. Megaloblastic anaemia in a vegetarian Indian community. Lancet 1985; ii: 1168.
 Vogiatzoglou A, et al. Vitamin B12 status and rate of brain volume loss in community-dwelling elderly. Neurology 2008; 71(11): 826-32
 Smith AD, Refsum H. Vitamin B-12 and cognition in the elderly. Am J Clin Nutr 2009; 89: 707S-11S.
Dr. Barry Groves is a nutritional author, lecturer and journalist; doctorate in nutritional science; 2002 Sophie Coe Prize winner; currently: a director of the Foundation for Thymic Cancer Research; a founder member of the Fluoride Action Network; a founder member of THINCS - The International Network of Cholesterol Sceptics; and an honorary member of the board of the Weston A Price Foundation. E-mail: [email protected]