ARI SHAPIRO, HOST:
A major new study finds that people around the world burn roughly the same amount of calories each day, regardless of how active they are. As NPR's Maria Godoy reports, that strongly suggests that when it comes to obesity, it is what we eat, not a lack of exercise, that's the main driver.
MARIA GODOY, BYLINE: Back in the 1800s, obesity was almost nonexistent in the United States.
HERMAN PONTZER: If you look back, even just our grandparents' generation, the number of people who were overweight or had obesity and obesity-related health issues was much, much lower.
GODOY: That's Herman Pontzer, a professor of evolutionary biology and global health at Duke University. He says, over the last century, obesity has become common here and in most other industrialized nations. So what's changed?
PONTZER: So there's this ongoing, long-held belief in public health that as we've gotten more sedentary, as our populations have developed over the last few decades, that we've gotten less active. That's definitely true.
GODOY: Pontzer says that's led to the assumption that, as we sit at our desks all day typing on our computers, we're probably burning fewer calories than we used to, raising the risk of obesity. But is that really the case? To find out, Pontzer and an international team of collaborators looked at data gathered from more than 4,200 adults all over the world - some from countries with high obesity rates, others from populations where obesity is rare.
PONTZER: And we were able to do that in populations across the spectrum, from hunter-gatherers to rich countries, the developed countries like the U.S.
GODOY: Study participants were given a special water to drink that contained isotopes which came out in their urine. These isotopes allow scientists to determine exactly how much energy a person expends.
PONTZER: So we wanted to know how many calories people are burning over the course of a full day.
GODOY: Now, you would think that a person who was out foraging for berries all day, every day would burn a lot more calories than, say, your typical office worker. But as Pontzer and his colleagues report in the journal PNAS, that's not what they found.
PONTZER: Surprisingly, what we find is the total calories burned per day is really similar across these populations, even though the lifestyle and the activity levels are really different.
GODOY: Pontzer says this means if we aren't burning fewer calories than, say, the Hadza hunter-gatherers in Tanzania - a culture where obesity is rare - then it has to be differences in our diets that's driving weight gain.
DARIUSH MOZAFFARIAN: It's 100% the diet. And I think then the question is, what is it about the diet?
GODOY: That's Dr. Dariush Mozaffarian, director of the Food Is Medicine Institute at Tufts University. He was not involved in this study, but he says it adds to other recent research that suggests food is the biggest driver in obesity. He points out there's been a major shift in our food supply in recent decades. It's now dominated by ultraprocessed food.
MOZAFFARIAN: For decades, we've been telling Americans that you're lazy. It's your fault. You're not moving enough. You're eating too much. And I think what this study shows is that there's really complicated biology happening and that our food is driving this.
GODOY: Now, this doesn't mean there's no reason to exercise. After all, it's good for our mental and physical health in so many ways that have nothing to do with weight. But it does mean we can't outrun a bad diet. If we want to tackle the obesity epidemic, the changes have to begin on our plates. Maria Godoy, NPR News.
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