Navigation bar--use text links at bottom of page.

(Comparative Anatomy and Physiology Brought Up to Date--continued, Part 4B)

The Relationship of Dietary Quality
and Gut Efficiency to Brain Size

The Expensive Tissue Hypothesis of Aiello and Wheeler

Aiello and Wheeler [1995] is an excellent research paper that neatly ties together many of the threads addressed in the research papers discussed up to this point in this section. The insights available from this paper provide yet another view into the paradox of how humans can afford, in metabolic energy terms, the high "cost" of our large brains without a corresponding increase in BMR/RMR (basal metabolic rate, also known as resting metabolic rate).

Aiello and Wheeler begin by comparing the actual size of the "average" human brain (for a body weight of 65 kg). The actual weight is ~1.3 kg, while the predicted weight is 268 g, based on the scaling model of Martin [1983, 1990] (as cited in Aiello and Wheeler [1995]; the Martin model relates brain mass to body mass). They then note that despite the large brain size with its consequent disproportionate demands on metabolism, the total BMR for humans is nevertheless well within the range expected for primates and other mammals of comparable body size. This of course brings us back to the paradox described above: Where does the "extra" metabolic energy come from to power the enlarged human brain, if the human body's total BMR is no higher than that of other comparably sized mammals?

Large brain compensated for by decreased gut size. Aiello and Wheeler then analyze the other "expensive" organs in the body (expensive in terms of metabolic energy): heart, kidneys, liver, and gastrointestinal tract, noting that together with the brain, these organs account for the major share of total body BMR. Next they analyze the "expected" sizes of these major organs for a 65-kg non-human primate, and compare these with the actual organ sizes for an average 65-kg human. Their figure 3 (below) illustrates the dramatic differences between the expected and actual sizes of the human brain and gut: The larger-than-expected size of the human brain is compensated for by a smaller-than-expected gut size.

GRAPH: Observed vs. expected brain sizes for human.

Aiello and Wheeler [1995, pp. 203-205] note:

Although the human heart and kidneys are both close to the size expected for a 65-kg primate, the mass of the splanchnic [abdominal/gut] organs is approximately 900 g less than expected. Almost all of this shortfall is due to a reduction in the gastrointestinal tract, the total mass of which is only about 60% of that expected for a similar-sized primate. Therefore, the increase in mass of the human brain appears to be balanced by an almost identical reduction in the size of the gastrointestinal tract....

Consequently, the energetic saving attributable to the reduction of the gastrointestinal tract is approximately the same as the additional cost of the larger brain (table 4).

Less energy-intensive gut associated with higher dietary quality. The authors further point out that the reduction in gut size is necessary to keep the BMR at the expected level. Additionally, they argue that the liver, heart, and kidneys cannot be significantly reduced in size to offset the energy costs of encephalization because of their highly critical functions; only the gut size can be reduced. Since gut size is associated with dietary quality (DQ), and the gut must shrink to support encephalization, this suggests that a high-quality diet is required for encephalization. That is, a higher-quality diet (more easily digested, and liberating more energy/nutrients per unit of digestive energy expended) allows a smaller gut, which frees energy for encephalization.

Figure 5 from Aiello and Wheeler [1995], below, illustrates the diet/gut size/encephalization linkages.

FLOWCHART: High-quality diet and increased encephalization.

Aiello and Wheeler deduce that the obvious way to increase DQ among early hominids is to increase the amount of animal foods (meat, organs, insects) in the diet. They note [Aiello and Wheeler 1995, p. 208]:

A considerable problem for the early hominids would have been to provide themselves, as a large-bodied species, with sufficient quantities of high-quality food to permit the necessary reduction of the gut. The obvious solution would have been to include increasingly large amounts of animal-derived food in the diet (Speth 1989; Milton 1987, 1988).

Advent of cooking may have promoted further encephalization by reducing digestive energy required. Finally, in what will be controversial to raw-fooders, Aiello and Wheeler, after arguing that the first major increase in encephalization was due to increased consumption of animal foods, next propose that the second major increase in brain size (with the appearance of archaic Homo sapiens) was due to the appearance of cooking practices. (Archaic Homo sapiens coincides with a timeframe of roughly 100,000 to 400,000 years ago.)

Cooking neutralizes toxins and increases digestibility (of starch, protein, beta-carotene), and might make the digestion of cooked food (vs. raw) less "expensive" in metabolic energy terms--thereby freeing up energy for increased encephalization. It may be the case, therefore, that in evolutionary terms cooked food could have been responsible for some of the later increases in brain size and--since increased brain size is associated with increased technology--intelligence as well. The authors note [Aiello and Wheeler 1995, p. 210]:

Cooking is a technological way of externalizing part of the digestive process. It not only reduces toxins in food but also increases its digestibility (Stahl 1984, Sussman 1987). This would be expected to make digestion a metabolically less expensive activity for modern humans than for non-human primates or earlier hominids. Cooking could also explain why modern humans are a bit more encephalized for their relative gut sizes than the non-human primates (see fig. 4).

Note that while this particular aspect of Aiello and Wheeler's research is speculative, it is at least based on the existence of supportive evidence. Fruitarian readers might wish to compare the above analysis with the pejorative slogans about cooked food popular in the raw/fruitarian community that are often based on not much more than simply the strength of as much emotional vehemence as can be mustered.

Potential subterfuges by extremists. Note to readers: The 1995 paper by Aiello and Wheeler is excellent and, if you have an interest in this topic, makes very worthwhile reading; it is available in many university libraries. At the end of the paper there is a series of comments by other scholars, followed by a rejoinder from Aiello and Wheeler, which gives good insight into the process of scientific inquiry and debate. If you take the time to locate this paper, I would encourage reading not only the primary report, but the comments and rejoinder following.

Reason: In my experience and opinion, there is enough of a tendency among raw/veg*n extremists to avoid the primary evidence in cases like this, while finding minor points to nitpick, that it would not be surprising if certain (fruitarian) extremists were to try to use, in an illicit manner, the criticisms by the commenting scholars--e.g., use the material without proper attribution, without consideration of the rejoinder remarks by Aiello and Wheeler, and possibly (the worst extremists of all) the material could be plagiarized or used in violation of copyright. This may sound inflammatory to some readers--please be assured, however, that this comment is based on hard experience with extremists. (See the article Assessing Claims and Credibility in the Realm of Raw and Alternative Diets on this site for information on extremist rationalizations/behavior.)

Recent brain-size decreases in humans as further evidence of the brain/diet connection

Why has brain size decreased 11% in the last 35,000 years and 8% in the last 10,000? Another interesting observation about the brain/diet connection comes from recently updated and more rigorous analysis of changes in brain size in humans over the last 1.8 million years. Ruff, Trinkaus, and Holliday [1997] found that encephalization quotient (EQ) began reaching its peak with the first anatomically modern humans of approximately 90,000 years ago and has since remained fairly constant [see p. 174, Table 1]. Most surprisingly, however, absolute brain size--on the other hand--has decreased by 11% since 35,000 years ago, with most of this decrease (8%) coming in just the last 10,000 years. (The decrease in absolute brain size has been paralleled by roughly similar decreases in body size during the same period, resulting in EQ values that have remained roughly the same as before.)

The significance of constant EQ vs. shrinking brain size in context. This data suggests two points. The first point--relating to EQ--is subject to two possible interpretations, at least on the face of it. One interpretation (characterized by somewhat wishful thinking) might be that, if we disregard the absolute decrease in brain and body size, and focus only on EQ, we can observe that EQ has remained constant over the last 10,000-35,000 years. One could then further conjecture that this implies humans have in some sense been successful in maintaining dietary quality during this time period, even considering the significant dietary changes that came with the advent of the agricultural revolution (roughly the last 10,000 years). However, the problem with such an interpretation is exactly that it depends on disregarding the information that overall body size diminished along with brain size--a most important point which needs to be taken into account.

The alternate, and more plausible and genetically consistent interpretation begins by noting that EQ represents a genetically governed trait determined by our evolutionary heritage. Hence one would not expect EQ itself to have changed materially in just 10,000 years, as it would be unlikely such a brief period of evolutionary time could have been long enough for the actual genetics governing EQ (that is, relative brain size compared to body size) to have changed significantly regardless of dietary or other conditions.

Dietary/physiological mechanism may be responsible. This brings up the second point, which is that the specific question here concerns a slightly different issue: the absolute decrease in brain size rather than the issue of EQ. Since the greatest majority of this decrease took place in just the last 10,000 years, a genetic mutation is no more likely as an explanation for the decrease in absolute brain size than it is for relative brain size, or EQ. This leaves us once again with a physiological/biochemical mechanism as the responsible factor, which of course puts diet squarely into the picture. (Not to mention that it is difficult to imagine plausible evolutionary selective pressures for brain size--primarily cultural/social/behavioral--that could conceivably be responsible for the reversal in brain size, since human cultural evolution has accelerated considerably during this period.)

Far-reaching dietary changes over the last 10,000 years. This leaves us with the indication that there has likely been some kind of recent historical shortfall in some aspect of overall human nutrition--one that presents a limiting factor preventing the body/brain from reaching their complete genetic potential in terms of absolute physical development. The most obvious and far-reaching dietary change during the last 10,000 years has, of course, been the precipitous drop in animal food consumption (from perhaps 50% of diet to 10% in some cases) with the advent of agriculture, accompanied by a large rise in grain consumption--a pattern that persists today. This provides suggestive evidence that the considerable changes in human diet from the previous hunter-gatherer way of life have likely had--and continue to have--substantial consequences.

Brain growth dependent on preformed long-chain fatty acids such as DHA. The most plausible current hypothesis for the biological mechanism(s) responsible for the absolute decrease in brain size is that the shortfall in consumption of animal foods since the late Paleolithic has brought with it a consequent shortfall in consumption of preformed long-chain fatty acids [Eaton and Eaton 1998]. Specifically, for optimal growth, the brain is dependent on the fatty acids DHA (docosahexaenoic acid), DTA (docosatetraenoic acid), and AA (arachidonic acid) during development to support its growth during the formative years, particularly infancy. These are far more plentiful in animal foods than plant.

Eaton et al. [1998] analyze the likely levels of intake of EFAs involved in brain metabolism (DHA, DTA, AA) in prehistoric times, under a wide range of assumptions regarding possible diets and EFA contents. Their model suggests that the levels of EFAs provided in the prehistoric diets was sufficient to support the brain expansion and evolution from prehistoric times to the present, and their analysis also suggests that the current low levels of EFA intake (provided by agricultural diets) may explain the recent smaller human brain size.

Rate of synthesis of DHA from plant-food precursors does not equal amounts available in animal foods. Although the human body will synthesize long-chain fatty acids from precursors in the diet when not directly available, the rates of synthesis generally do not support the levels obtained when they are gotten directly in the diet. This is particularly critical in infancy, as human milk contains preformed DHA and other long-chain essential fatty acids, while plant-food based formulas do not (unless they have been supplemented).

Animal studies indicate that synthesis of DHA from plant-source precursor fatty acids does not equal the levels of DHA observed when those are included in the diet: Anderson et al. [1990] as cited in Farquharson et al. [1992], Anderson and Connor [1994], Woods et al. [1996]. Similar results are reported from studies using human infants as subjects: Carlson et al. [1986], Farquharson et al. [1992], Salem et al. [1996]. For a discussion of the above studies, plus additional studies showing low levels of EFAs in body tissues of vegans, see Key Nutrients vis-a-vis Omnivorous Adaptation and Vegetarianism: Essential Fatty Acids.

To summarize

The data that human brain size has fallen 11% in the last 35,000 years--with the bulk of that decrease (8%) coming in the last 10,000 years--furnishes, by extension, suggestive, potential corroborative support for the hypotheses explored earlier in this section that increasing brain development earlier in human evolution is correlated positively with the level of animal food in the diet. It also indicates that animal food may be a key component of dietary quality (DQ) that cannot be fully substituted for by increasing other components in the diet in its absence (such as grains).

This indication is important to consider, because evidence available on the changes in food practices of more recent prehistoric humans (and of course, humans today) can be assessed in more depth and with a higher degree of resolution than dietary inferences about earlier humans. In conjunction with data about DHA synthesis in the body vs. obtaining it directly from diet, this provides a potentially important point of comparison for assessing hypotheses about the brain/diet connection.


(Fruitarian Evolution: Science Fact or Science Fiction?)

Return to beginning of article



GO TO PART 1 - Brief Overview: What is the Relevance of Comparative Anatomical and Physiological "Proofs"?

GO TO PART 2 - Looking at Ape Diets: Myths, Realities, and Rationalizations

GO TO PART 3 - The Fossil-Record Evidence about Human Diet

GO TO PART 4 - Intelligence, Evolution of the Human Brain, and Diet

GO TO PART 5 - Limitations on Comparative Dietary Proofs

GO TO PART 6 - What Comparative Anatomy Does and Doesn't Tell Us about Human Diet

GO TO PART 7 - Insights about Human Nutrition & Digestion from Comparative Physiology

GO TO PART 8 - Further Issues in the Debate over Omnivorous vs. Vegetarian Diets

GO TO PART 9 - Conclusions: The End, or The Beginning of a New Approach to Your Diet?

Back to Research-Based Appraisals of Alternative Diet Lore

   Beyond Veg home   |   Feedback   |   Links