Thursday, June 28, 2007

Will a raw vegetarian diet make you dumber?

Well, no, but according to a recent news article in Science, the addition of meat and cooked foods to the Homo erectus diet may have led to the dramatic expansion of our ancestors' brains and cognitive abilities.

Between 1.9 million and 200,000 years ago, the brains of our ancestors tripled in size (from 500 cc in Australopithecus to about 1500 cc in Neanderthals), a feat that required a massive increase in energy supply. Brains are rather greedy structures, utilizing 60% of a newborn baby's energy expenditure, and 25% of a resting adult's. In contrast, the average ape brain uses only 8% of the animal's total energy expenditure, despite similar basal metabolic rates. So what led to this glorious caloric upsurge?

One well-supported theory proposes that calorie-dense meat provided the necessary fuel. The high caloric return (not necessarily the high protein content) of meat made it a far more efficient fuel, capable of supporting a 35-55% increase in caloric needs. Moreover, a diet with a greater proportion of meat permits a smaller gut, allowing the allocation of energy saved from digestion and tissue maintenance to feeding the voracious brain. One line of evidence that supports this hypothesis stems from correlational primate studies: capuchin monkeys, which eat an omnivorous diet and have small guts, are considered the most intelligent New World monkeys.; in contrast, Howler monkeys, while bereft of significant brainpower, have large guts to accompany their vegetarian diets.

According to Harvard primatologist Richard Wrangham, "a diet of wildebeest tartare and antelope sashimi alone isn't enough." By breaking down collagen and starches, cooking is a form of pre-digestion, thus lightening the load for the GI tract and allowing greater energy expenditure elsewhere. In one study, pythons fed cooked, ground meat spent 23.4% less energy digesting relative to those which ate raw meat; in another, mice raised on cooked meat gained 29% more weight than mice fed raw meat.

Theoretically, cooking and meat could have provided a great enough surge in calories to fuel the major expansion of our ancestors' brains and cognitive abilities, but the idea is still controversial. Back then, cooking required fire, and evidence for the earliest controlled fires is a bit ambiguous. The earliest such evidence is from about 800,000 years ago, and the earliest evidence for cooking (e.g. hearths) is from no earlier than 250,000 years ago, with questionable evidence dating to 300,000 to 500,000 years ago.

Nevertheless, it's an intriguing explanation for this feature of our evolutionary history. Of course, as we are no longer subjected to the same evolutionary pressures, it's not exactly a recipe for intelligence in modern society. It's possible, according to Wrangham, that "Western food is now so highly processed and easy to digest that...food labels may underestimate net calorie counts and may be another cause of obesity." That said, I love a good barbecue, and in the land of "raw foodies" and "fake stake [sic]," it's refreshing to see meat and cooking receive some due recognition for their delicious role in our natural history.

*For those without a Science subscription, Jake at Pure Pedantry has some key excerpts from the article

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