The raw vs the cooked

I’ve been reviewing a 2009 article on the significance of cooking in human development.

Many folks argue that eating a diet of exclusively raw foods improves health. While it’s certainly true that humans traditionally consumed a lot of food types raw (e.g. fruit, some types of fresh-caught meat), they also cooked many (e.g. starchy roots, many other types of meat). (Interestingly, few raw food advocates suggest eating sashimi or something like the Inuit delicacy of raw organ meats.)

The authors of this study review the evidence that supports or refutes the significance of cooking in human evolution and health. Some key findings:

  • Humans on vegetarian diets gain more weight and exhibit higher reproductive performance when eating cooked food than raw food. Reproductive performance is generally a good proxy for overall health; when sex hormones are happy, everyone is happy.
  • Diets with a high proportion of raw food — even if that includes meat — nevertheless tend to result in lower body weights.
  • Women who eat a high amount of raw food have higher rates of amenorrhea or menstrual irregularities than those eating cooked food. Menstruation was absent in 23% of females of childbearing age who ate at least 70% of their food raw and in 50% of women reporting a 100% raw diet. Fascinatingly, this is not explained by vegetarianism alone, but rather by the proportion of raw food in the diet. As the authors note, “The poor ovarian performance of raw-foodists therefore cannot be attributed to their vegetarianism… [one researcher] concluded that women suffered because of their relatively low net energy gain as a consequence of eating their food raw.”
  • Many raw-foodists apparently do not limit their food intake, since they commonly describe themselves as experiencing persistent hunger despite eating frequently.
  • “We have found no records of individuals tending to gain weight while eating raw diets, even though the plant foods eaten by raw-foodists are mostly high-quality items such as germinated seeds, sprouts, fruits, nuts, and cereals, and tend to include oil. This is especially surprising since raw-foodists are typically members of urban communities, where habitual activity levels are lower than observed in traditional communities of hunter-gatherers or pastoralists. Furthermore, although raw-foodists are averse to cooking, they typically process their foods extensively by such methods as grinding, pounding, sprouting, and pressing, and even heating up to 48°C. A nutritional analysis suggested that on a diet of raw wild foods, which are generally lower in energy value and higher in fibre, energy intake in traditional communities would be so limited as to render survival and reproduction difficult.”
  • Cereals tend to be more digestible raw than tubers and legumes, but raw starches of all three types have important reductions in digestibility compared to cooked starches. In general, few cereals, tubers, or legumes are digestible raw — and many are highly toxic (such as raw kidney beans).
  • Archaeological evidence suggests that fat derived from bone marrow may have been preferred over muscle tissue as a source of energy and nutrients among early Homo. Moreover, it is known that diets deriving more than 50% of calories from lean protein can lead to negative energy balance, so-called “rabbit starvation,” due to the high metabolic costs of protein digestion, as well as a physiological maximum capacity of the liver for urea synthesis.
  • “The ileal [small intestinal] digestibility of raw eggs was found to be 51% in ileostomy patients and 65% in healthy volunteers. By contrast, the ileal digestibility of cooked eggs was 91–94%. These data indicate that cooking increased the digestibility of egg protein by 45–78%. This is a striking result considering that chicken egg proteins are commonly treated as having high biological value for humans whether they are consumed raw or cooked.”
  • “The poor performance of humans eating both raw vegetarian and raw omnivorous diets suggests that our species is biologically adapted to the consumption of cooked food; and importantly, some of the features preventing humans from utilising raw food efficiently include traits recognisable in fossils (i.e., small molars and relatively small total gut volume).”

In short, while we are still not 100% certain about small individual variations in digestibility, the balance of the evidence indicates that when it comes to health, fire is good.

Rachel N. Carmody; Richard W. Wrangham. The energetic significance of cooking. Journal of Human Evolution (October 2009), 57 (4), pg. 379-391