A new international study challenges one of the most common assumptions about obesity: that it’s caused mainly by inactivity. Instead, the findings point to caloric intake, not reduced physical activity, as the primary driver of rising obesity rates in wealthy countries.
The study, published in Proceedings of the National Academy of Sciences, analyzed data from more than 4,200 adults across 34 populations in six continents. Participants ranged from hunter-gatherers and subsistence farmers to people living in highly industrialized countries.
Despite differences in lifestyle and economic development, researchers found that people in wealthier nations expended just as much energy, if not more, on a daily basis as people in less industrialized settings. What differed was their caloric intake and levels of body fat.
“Despite decades of trying to understand the root causes of the obesity crisis in economically developed countries, public health guidance remains stuck with uncertainty as to the relative importance of diet and physical activity,” said Herman Pontzer, professor of evolutionary anthropology at Duke University and principal investigator of the study. “This large, international, collaborative effort allows us to test these competing ideas. It’s clear that changes in diet, not reduced activity, are the main cause of obesity in the U.S. and other developed countries.”
The study used direct measurements of total energy expenditure and body fat percentage from adults aged 18 to 60. Researchers also used the United Nations Human Development Index to assess each population’s level of economic development.
While some small differences in energy expenditure were seen across economic levels, they did not explain the rise in obesity.
“While we saw a marginal decrease in size-adjusted total energy expenditure with economic development, differences in total energy expenditure explained only a fraction of the increase in body fat that accompanied development,” said Amanda McGrosky, lead author and assistant professor of biology at Elon University. “This suggests that other factors, such as dietary changes, are driving the increases in body fat that we see with increasing economic development.”
The findings don’t suggest physical activity is unimportant, only that exercise alone may not be enough to counteract the effects of overeating. The researchers emphasized that diet and movement should be seen as partners in good health.
“Diet and physical activity should be viewed as essential and complementary, rather than interchangeable,” the study notes.
Researchers plan to next investigate which specific aspects of diet in developed countries are most responsible for excess weight gain.
This research was supported by the U.S. National Science Foundation, the Chinese Academy of Sciences, the International Atomic Energy Agency, Taiyo Nippon Sanso and the Sercon Group.