Back in the 1800s, obesity was almost nonexistent in the United States. Over the last century, it’s become common here and in other industrialized nations, though it remains rare among people who live more traditional lifestyles, such as the Hadza hunter-gatherers of Tanzania.
So what’s changed? One common explanation is that as societies have developed, they’ve also become more sedentary, and people have gotten less active. The assumption is that as a result, we burn fewer calories each day, contributing to an energy imbalance that leads to weight gain over time, says Herman Pontzer, a professor of evolutionary biology and global health at Duke University who studies how human metabolism has evolved.
But in a major new study published in the journal PNAS, Pontzer and an international team of collaborators found that’s not the case. They compared the daily total calorie burn for people from 34 different countries and cultures around the world. The people involved ran the spectrum from hunter-gatherers and farming populations with low obesity rates, to people in more sedentary jobs in places like Europe and the U.S., where obesity is widespread.
“Surprisingly, what we find is that actually, the total calories burned per day is really similar across these populations, even though the lifestyle and the activity levels are really different,” says Pontzer.
And that finding offers strong evidence that diet — not a lack of physical activity — is the major driver of weight gain and obesity in our modern world.
“This does sort of really fly in the face of what a lot of us anecdotally assumed was driving a lot of the weight gain and obesity today,” says Deirdre Tobias, an obesity and nutritional epidemiologist at the Brigham and Women’s Hospital in Boston and the Harvard T.H. Chan School of Public Health. Tobias was not involved in the new research.