Bill Harris, M.D. | Q&A
George, who remains a resolute avoider of commercial animal products in any case
The usual nutrient mentioned for big brains is alpha-linolenic acid (ALA) one of two essential fatty acids, the other being linoleic acid (LA). ALA is important because it elogates to EPA and DHA (docosohexaenoic acid) and DHA is a vital component in nervous tissue. However, since ALA is only synthesized in the chloroplasts of green plants I see no particular reason why meat eating would provide more DHA since the highest sources of DHA are flaxseed, soy, and green plants and we can make our own DHA from the plant source ALA that we eat on a greens based diet (not however on a grains based diet which is only high in LA ).
Your speaker may have been referring to the social advantages and problem solving requirements of hunting. If so I would agree that hunting conferred a selective advantage on those who hunted since one must outsmart the prey and then devise techniques to bring it back to camp. Meat eating also provides more Calories than plant gathering and that would also confer an enormous advantage in primitive times.
Here is an interesting website run by Tom Billings, a vegetarian who specializes in debunking bogus claims by other vegetarians. If I read his intent correctly he's saying that while we have the enzymes necessary to elongate ALA to DHA we can't do it fast enough to match the DHA content in meat. I regard this as possible but not probable except in infants who lack the necessary enzymes and must have preformed DHA either from breast or cow's milk. If however the allegation is correct then flax oil consumption is one option for committed vegans and "Neuromins" capsules that contain DHA made by algae are another.
I tried the Neuromins myself for a couple of months but it didn't make me any smarter.
-William Harris, M.D.
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.  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.  as cited in Farquharson et al. , Anderson and Connor , Woods et al. . Similar results are reported from studies using human infants as subjects: Carlson et al. , Farquharson et al. , Salem et al. . 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.
<http://www.beyondveg.com/graphics/shim.gif> To summarize <http://www.beyondveg.com/graphics/shim.gif> 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.
William Harris MD received a degree in physics from the University of California Berkeley, where he earned Phi Beta Kappa honors. He received his degree in medicine from the University of California at San Francisco, and received his postgraduate training at San Diego County Hospital. He holds a Medical License in the State of Hawaii. He has been an Emergency Department physican since 1963, and the Director of the Kaiser Permanente Vegan Lifestyle Clinic on Oahu until his retirement in 1998. Dr. Harris is the author of The Scientific Basis of Vegetarianism.
In addition, he was the 1950 Big Ten Trampoline Champion, is an accomplished hangglider and commercial pilot, and at age 70 became a skydiver with 108 jumps to date. Dr. Harris has been vegetarian since 1950, and vegan since 1963.