According to a study done at the University of Oxford, those on a meat-free diet six times more likely to suffer brain shrinkage. The reduction in size is thought to be caused by vitamin B12 deficiency.
Other studies have confirmed similar findings:
http://www.ncbi.nlm....pubmed/20187536
http://www.ncbi.nlm....pubmed/19817650
http://www.ncbi.nlm....pubmed/16197315
http://www.ncbi.nlm....pubmed/15918275
http://www.ncbi.nlm....pubmed/12460231
http://www.ncbi.nlm....pubmed/11194532
http://www.ajcn.org/...short/78/1/3#R2
http://www.ajcn.org/...pe2=tf_ipsecsha
http://www.ajcn.org/...stract/50/4/818
Some studies show up to 58% of vegetarians are deficient in B12. This is surprising given that many modern vegetarian foods are supplemented with B12 and it infers that obviously vegetarians / vegans aren't supplementing enough. But the brain shrinkage trend doesn't just exist in the meatless group. In adults over 50, some studies have found vitamin B12 deficiency in nearly 40% of the sampled individuals. The implication is that our modern diet may not be providing adequate B12.
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 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. Most surprisingly, however, absolute brain size 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.)
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.
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.
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.
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]. 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.
Of course a lack of animal foods also precipitates a lack of B12 consumption. So we could have two possible mechanisms for the reduction in brain mass seen in modern humans.
