The idea that if you burn more calories than-you consume you’ll lose weight is the mainstay of most diet programs. And public health officials have long cited the developed world’s increasingly sedentary lifestyle as a key factor behind the global obesity epidemic of the past few decades.
But should we be giving diet (calories in) and exercise (calories out) equal weight in the fight against obesity? No, suggests a growing body of research. Regular exercise may be crucial for our physical and mental health, but it does little to fight obesity, these studies indicate. In other words, we’re not getting fat because we’re less physically active, but because we’re eating too much energy-dense foods, particularly foods high in sugar.
In a commentary published last Friday in the New York Times, Hunter College anthropologist Herman Pontzer offers yet another piece of evidence that debunks the idea that our sedentary lifestyles are a driving factor behind the obesity epidemic. He describes an intriguing study he and his colleagues recently conducted in Tanzania (and published earlier this summer in the journal PLoS ONE).
For the study, the researchers measured 11 days of energy expenditure of 30 members (13 men, 17 women) of north-central Tanzania’s Hadza community, one of the few remaining full-time hunter-gatherer societies in the world.
“The Hadza live in simple grass huts in the middle of a dry East African savanna,” explains Pontzer in the Times commentary. “They have no guns, vehicles, crops or livestock. Each day the women comb miles of hilly terrain, foraging for tubers, berries and other wild plant foods, often while carrying infants, firewood and water. Men set out alone most days to collect honey or hunt for game using handmade bows and poison-tipped arrows, often covering 15 to 20 miles.”
Would the Hadza — who live similarly to our ancient ancestors (and who never get obese) — expend more energy (calories) during their everyday activities than the rest of us do in our daily routine?
Previous attempts to quantify daily energy expenditure among hunter-gatherers have relied entirely on estimation. By contrast, our study used a technique that calculates the body’s rate of carbon dioxide production — and hence the calories burned per day — by tracking the depletion of two isotopes (deuterium and oxygen-18) in an individual’s urine over a two-week period. …
We found that despite all [the Hazda people’s] physical activity, the number of calories that the Hadza burned per day was indistinguishable from that of typical adults in Europe and the United States. We ran a number of statistical tests, asccounting for body mass, lean body mass, age, sex and fat mass, and still found no difference in daily energy expenditure between the Hadza and their Western counterparts.
How can that be? Here’s Pontzer’s explanation:
We think that the Hadzas’ bodies have adjusted to the higher activity levels required for hunting and gathering by spending less energy elsewhere. Even for very active people, physical activity accounts for only a small portion of daily energy expenditure; most energy is spent behind the scenes on the myriad unseen tasks that keep our cells humming and our support systems working. If the Hadza’s bodies somehow manage to spend less energy in those areas, they could easily accommodate the elevated energy demands of hunting and gathering. And indeed, studies reporting differences in metabolic-hormone profiles between traditional and Western populations support this idea (though more work is needed).
“Our findings add to a growing body of evidence suggesting that energy expenditure is consistent across a broad range of lifestyles and cultures,” Pontzer concludes.