That Americans are sedentary at work and at home has long been a part of the explanation for the obesity epidemic.
For Britt Creech, this has meant getting creative with how she fits movement in her day. In college, she played rugby for NC State. And in her early career as an engineer, she was also on her feet working with technicians on the manufacturing floor. But, that changed as she started advancing in her career.
“You’re just doing the day to day things from your desk, like invoicing, so you’re sitting a lot more. And so naturally, my step counts were much lower than they had been,” she said.
This career transition is analogous to something epidemiologists call the obesity transition – as countries get richer, obesity becomes more prevalent. Today, over 40% of US adults are obese. Scientists are still puzzling over why.
In a study published in July in the journal PNAS, evolutionary anthropologists at Duke University looked at the question of how economic development contributes to obesity. They compared the number of calories consumed and burnt by people living in economies across the development spectrum.
Forager-horticulturalists in Bolivia and hunter-gatherers in Tanzania were compared to people from Nepal, South Africa, the US, and beyond. In all, 36 populations spanning 6 continents were compared. Despite the differences in development across groups, researchers found no real difference in energy expenditure, or calories burned.
Basically, those who hunt and gather for their dinner are burning as many calories as those of us who can afford to order it as takeout.
Yet, those in the developed world do have higher rates of obesity. If it’s not the lack of “calories out” that is causing obesity in the developed world, then it must be the other side of the energetic equation, “calories in.” Eating excessive calories is 10 times more influential than sedentariness in driving the obesity epidemic, the researchers say.
When it comes to losing weight, it’s not a 50-50 mix of diet and exercise
Physical activity has always been a part of public health guidance on obesity prevention from organizations like the WHO or CDC. But Herman Pontzer, professor of evolutionary anthropology and global health at Duke University, wanted to get more specific. His lab, the Pontzer Lab, was the one responsible for the PNAS study.
“The guidance always gives diet and physical activity equal weight in trying to explain why obesity is such a problem,” Pontzer told WUNC. “So I think people lean into the exercise part of that diet and exercise guidance. But this study asked the question, ‘Well, is it really a 50-50 mix’?”
To answer the question, Pontzer’s team drew from a global archive of data on metabolism called the International Atomic Energy Agency Doubly Labelled Water Database. It contains measurements of thousands of research subjects’ caloric expenditure. It’s collected through a method called doubly labelled water, which tracks the path of special, recognizable water molecules (isotopes) as they are consumed and expended by the body, cluing scientists into the metabolic or processing rate of the body.
Amanda McGrosky, lead author of the PNAS study and assistant professor of biology at Elon University, explained what story emerged from the data.
“It seemed that across all of these populations, people weren't really differing in the amount of energy that they were expending on activity,” she said, adding, “We did, however, see differences in total body fat percentage.”
What they found was that obesity was more prevalent in developed economies. If there was no change in energy expenditure across populations, “it has to be a product of taking in more calories as you become more economically developed,” said McGrosky.
'It takes an hour of walking to burn off calories from a sugary doughnut'
Though the researchers did not have access to dietary data for each of the populations included in their study, they point to existing research about how consuming ultra processed foods (UPFs)--- which is characteristic of diets in developed countries— can lead to obesity.
“They push us to eat too much,” said Pontzer. “They're engineered to be delicious, and they're engineered to be energy dense, and all these things that make it hard for our bodies to regulate how many calories we're eating.”
NPR recently reported that on average in the US, kids get 70% of their calories from UPFs.
Adam Drewnowski is a professor of epidemiology at the University of Washington, where he is also the director of the Center for Obesity Research. In an email to WUNC, he wrote that “There is a link between obesity and poverty which must not be ignored.” This is because “low income people in rich societies have low-cost diets full of empty calories.”
Still, Drewnowski maintained that no one country is obese, rather, specific vulnerable populations within the country are. Nonetheless, he said that the main thesis of the PNAS study is largely correct.
“Physical activity was never the principal means of weight loss,” he said. “It takes an hour of walking to burn off calories from a sugary doughnut. It was always diet.”
This does not mean we should stop exercising.
“Physical activity is still really important for health. This study doesn't change anything about that,” said Pontzer. Exercise is important for developing strength in our bones and muscles so we can be healthy throughout life stages. It been shown to mitigate chronic disease and improve mental health as well.
Pontzer said we need to think about diet and exercise as “two different tools that do two different jobs.”
The future of basic science
The data from the PNAS study was not collected all in one go, by one team. It was the result of a massive international scientific collaboration spanning decades.
Pontzer estimates that there are ten labs in the world that use the doubly labelled water method for metabolism research, including his lab. It is understood to be the gold standard of metabolism research because it can be done outside of the lab. This gives a more accurate picture of how people spend their energy day-to-day.
Amanda McGrosky collected samples from a hunting and gathering group in Tanzania. This data is reflected in the PNAS study, alongside the work of other scientists who have been working with similarly rural communities for years.
“This is not the sort of thing that any one lab could ever do,” said McGrosky, adding that the databases that result from this international network “are the product of many, many people working over many decades.”
As funding for scientific research faces cuts from the Trump administration, research and collaborations like this one are on the line. Inside HigherEd has reported that the National Science Foundation, one of the funders of the PNAS study, is facing cuts of $16 million even after a Senate committee pushed back on the president’s initial $5 billion cut.
“I think that we also need to maintain a focus on basic research, research that doesn't have an obvious clinical outcome right now, but that might be important in the future,” Pontzer said.
