They say “abs are made in the kitchen” and “you can’t outrun a bad diet”—and a recent study in the Proceedings of the National Academy of Sciences suggests there’s science to back up those cliches.
Diet—not physical inactivity—seems to be the primary driver of the obesity epidemic, according to the new research.
“This is not saying that physical activity isn’t important. It absolutely is,” said Amanda McGrosky, PhD, an assistant professor of biology at Elon University who co-authored the new study. But if weight loss is your goal, her research suggests the gym may not hold the solution.
Both diet and exercise are important to overall health and wellness. But when it comes to weight management, does one matter more than the other?
McGrosky and her colleagues set out to answer that question. They compared data on daily calorie burn, body fat percentage, and body mass index (BMI) from more than 4,000 people living across six continents. Some were highly active farmers or hunter-gatherers, while others lived in industrialized nations and were far more sedentary.
People in industrialized nations tended to have higher body masses, BMIs, and body fat percentages, they concluded. But after adjusting for body size, the researchers found that they were burning only slightly fewer calories per day than people in less-developed areas—even though their lifestyles were much less active.
“We didn’t see [big] differences in total energy expenditure across populations, but we do see this increase in body fat,” McGrosky told Health. “If we’re not changing the number of calories out, we have to be changing the number of calories in.”
In other words: Obesity may be on the rise in some countries not because people are failing to burn enough calories, but because they are consuming too many.
The study relied on a mix of observation and modeling, meaning it can’t definitively prove cause and effect. Still, outside experts say there’s plenty of other research to back its conclusions.
“Almost everyone that comes to me and wants to talk about their weight begins by talking about their efforts at physical activity,” said Max Petersen, MD, PhD, an assistant professor in the Washington University in St. Louis Medicine Division of Endocrinology, Metabolism & Lipid Research, who was not involved in the new study. “And yet it is abundantly clear from decades of research that physical activity really makes a minor contribution to our weight.”
Not all studies have found that diet drives obesity more than inactivity. One study from 2014 came to the exact opposite conclusion, and other researchers have found that a combined approach—focused on both diet and exercise—is the best system for sustained weight loss.
But the bulk of evidence supports a diet-first approach to weight management, agreed Rekha Kumar, MD, MS, an endocrinologist and obesity medicine specialist at Weill Cornell Medicine who was also not involved in the new study.
“My first impression [of the new study] was, ‘I think we already know this,’” Kumar said. The study’s breadth and international design, looking at populations across the economic spectrum, only strengthen its conclusions, she added.
While the study argues that diet is the more important driver of obesity, researchers are still working to understand why, McGrosky said. One likely explanation ties back to the body’s natural ability to regulate energy usage.
“Even when people are very active, their bodies become more efficient,” Kumar told Health. “Your body eventually figures out how to adapt to increased physical activity—so over time, people who continually increase their physical activity don’t proportionately burn more and more calories.”
On the flip side, McGrosky said, the body may find ways to burn extra calories if you’re not moving much throughout the day.
Plus, it simply takes more energy to power a larger body compared to a smaller one, McGrosky said, which helps explain why a population made up of larger individuals burns roughly as many calories as one with smaller but very active people.
The take-home message isn’t that exercise doesn’t matter. On the contrary, exercise is clearly linked to a wide range of physical and mental health benefits. Petersen called it “the most powerful life-extending intervention we have in medicine—more powerful than any drug, any pill, any injection that we can offer.”
When it comes to weight management, however, McGrosky’s study and others suggest it’s best to focus on the number of calories you’re consuming, rather than the number you’re burning. “If the diet’s not conducive to weight loss, there’s no physical activity regimen that can override that,” Kumar said.
To improve your diet, experts typically recommend eating a mix of fresh fruits and vegetables, whole grains, nuts, legumes, and other lean proteins, while cutting down on ultra-processed foods. Research suggests people are more likely to overeat and gain weight when they consume lots of ultra-processed foods.
Individuals shouldn’t be responsible for making these changes alone, though. Governments and public health officials also have an important role to play. “It’s a lot easier or cheaper to buy a bag of chips, sometimes, than a bag of carrots,” McGrosky said—so at the systemic level, it’s important to make fresh, minimally processed foods readily accessible, available, and affordable.
