Barbara J. King
In the wake of the 439 comments on my last post about obesity and weight-bias in our society, I've been thinking about issues of comparative health around the world and, as I have before, about the Paleo diet.
A vigorous thread on last week's post (see the top of the comments section) assailed the multi-million-dollar food industry and its manipulation of consumers in this country. That theme is echoed in a book I've just read, Michael Moss's Salt Sugar Fat. Moss's central point is that the giant food companies pursue a "deliberate and calculating" approach to ensure that the "allure" — indeed, the "bliss point" — of our favorite foods comes about through salt, sugar and fat in ways that keep us coming back for more, more, and more.
In risk factors for obesity and poor health, though, poor diet isn't alone — lack of exercise is considered a major culprit as well. After all, we in the West have diverged from that original hunter-gatherer lifestyle that Paleo advocates love to evoke, and we know that hunter-gatherers not only eat minimal levels of salt, sugar, and fat but also expend far more energy than we ourselves do when they forage actively across large swaths of the landscape.
Thanks to a pointer by Adam van Arsdale in a great review article in the new issue of American Anthropologist, I made time this week to read a paper from 2012 that offers a surprising answer to this question.
In "Hunter-Gatherer Energetics and Human Obesity," anthropologist Herman Pontzer and his co-authors take direct aim at one aspect of the popular association between obesity and modern lifestyle. Pontzer et al., compared energy expenditure levels of Hadza hunter-gatherers living in Tanzania with people in the United States and Europe living more sedentary lifestyles.
Because the Hadza are vigorously active as they forage — the men hunt with bows and also gather honey, the women gather plants — researchers expected to find that the Hadza expended way more energy than we do.
Except, that's not at all what they found.
As expected, the Hadza have lower body fat levels than Westerners and they did have greater levels of physical activity. Their average daily energy expenditure, though, didn't differ from Westerners', even when body size was controlled.
In other words, the Hadza are more active than most of us but don't burn more calories. That "counter-intuitive" result, Pontzer et al., write, challenges "current models of obesity suggesting that Western lifestyles lead to decreased energy expenditure." The researchers hypothesize that there's a sort of ingrained human set-point for energy expenditure, impervious to cultural variation.
In a piece for The New York Times, Pontzer speculates that the Hadza's bodies compensate for high energy output in foraging by conserving energy on other physiological tasks.
Of course, this study focuses on a single group of hunter-gatherers: 13 Hadza men and 17 women whose energy levels were measured over 30 days. The tired phrase "more research is needed" fits well here.
Pretty obviously, the take-home message of the Hadza study is not that physical activity isn't beneficial. Massive amounts of data show the positive effects of exercise on cognitive and physical well-being.
So what are the take-home messages? I'll mention two.
In the Times, Pontzer flatly concludes that the obesity epidemic is not about being sedentary but about diet. In my words, it's about those alluring, blissful foods the food industry encourages us to eat. I know that's a sweeping generalization, one that deserves testing by way of more research into diet, exercise, and health.
The Hadza data also points, yet again, to the poor reasoning that underlies attempts by some "paleo" advocates to emulate a hunter-gatherer lifestyle. The surprising study results remind us that, when tested, our expectations about hunter-gatherer physiology and health may yield data quite divergent from our assumptions.
Data from anthropology do indicate that there has always been enormous variety in hunter-gatherer diet, foraging and health; any notion of the hunter-gatherer model is an illusion. I took flak from some quarters for saying so here at 13.7 in 2011. But a great post this month by Ferris Jabr at Scientific American brings the case home in an informed and engaging way.
"The Paleo diet," Jabr writes, "is founded more on privilege than on logic."