Monday, October 20, 2014

Famine & the evolution of diet

One cornerstone assumption of a lot of diet theories based on emulating hunter-gatherer diets is that famine was a constant companion. Predictable seasonal famines occur in all environments, the dry season in the tropics and winter in the high latitudes. The just-so story goes like this: in a predictable feast-famine environment "thrifty genes" that take advantage of feast times to store fat in preparation for the lean times would be selected for. And when you take a population that has these thrifty genes and put them in a constant "feast" environment, they get fat and develop diabetes. Like the Pima Indians in North America*, and the Tokelau people in the south pacific. To avoid triggering these "thrifty genes", paleo diet proponents suggest avoiding the products of agriculture, both modern and historical.

Colette Berbesque and co-authors [1] decided to test the frequent famine assumption by looking at anthropological evidence of famine in a fairly sophisticated way. In particular, they controlled for the effects of habitat richness by only comparing hunter gatherers from warm climates (Effective Temperature > 13C) with agriculturalists, and by using a metric of habitat productivity (a linear combination of Net Primary Productivity and Effective Temperature) as a covariate. From their abstract, they found:
... if we control for habitat quality, hunter–gatherers actually had significantly less—
not more—famine than other subsistence modes.
Digging into the details though, we find that this is true for some measures of famine but not others. For instance, on the variables Occurrence, Severity, Persistence, Recurrence, and Contingency of famine warm climate hunter gatherers do better (that is, less famine). In terms of short term or seasonal famine however, there is no difference between warm climate hunter gatherers and agricultural peoples. And that's important, because it is seasonal famine that would lead to the evolution of thrifty genes. And there was plenty of time for such genes to arise in paleolithic people before the advent of agriculture, so the fact that agricultural peoples are no better at avoiding seasonal famine simply means the selection for these genes wouldn't go away after agriculture.

Although Berbesque et al. paint their research as a critique of the "paleo diet" approach, it seems to me that it reinforces a key tenet: paleolithic peoples had it better than their agricultural neighbors. Hunter gatherers do significantly better on  "ordinary nutritional conditions and endemic starvation", have no difference in seasonal famine, and less long term and unpredictable famine. Huh.

[1] J. Colette Berbesque, Frank W. Marlowe, Peter Shaw and Peter Thompson. 2014. Hunter−gatherers have less famine than agriculturalists. Biology Letters 10, 20130853.

*The Pima aren't necessarily a good example, as they were agricultural before becoming "modern western". It is interesting to me that the National Institute of Diabetes and Digestive and Kidney disease article I linked to focuses on the increase in fat in the Pima diet, rather than the exchange of whole grains for refined carbohydrate.

1 comment:

  1. I recieved an email comment from someone too busy to post here:
    "The first line in the link to the pima suiggests that they and their forebears arrived in na over 30,000 years ago, WTF? how credible is the rest of the research based on that first line then to suggest they grew and harvested wheat??

    I didnt bother reading any further"

    Thanks for picking that first line up, too early by at least several thousand years. To be fair, this is something written by a PR person on behalf of a group of scientists who aren't anthropologists.

    I agree the Pima certaintly didn't grow wheat when they first settled down ~2 millenia ago, but they could have been growing it when European settlers trashed their irrigation systems in the 19th century. Wheat had arrived in NA by then.

    However, there are plenty of other things in that website that would raise your ire as an adherent to the carb hypothesis of diabetes and obesity!