New study links obesity to diet, not lack of exercise

For decades, conventional wisdom has blamed obesity on inactivity. But a new study published in the Proceedings of the National Academy of Sciences of the United States of America (PNAS) challenges that narrative, arguing that what’s on your plate plays a much bigger role than your step count.

The findings suggest that while movement is still important for health, calorie intake and food quality have a significantly greater impact on obesity rates.

Obesity affects 2 in 5 US adults

According to the U.S. Centers for Disease Control and Prevention (CDC), two out of every five adults in the United States are obese. Obesity is defined as having a body mass index (BMI) over 30 and is considered a serious but common disease. It also disproportionately impacts certain communities, including Hispanic and Black Americans.

BMI has been criticized by some health experts due to its limited considerations. A healthy woman who is of average height but is a competitive bodybuilder will have a weight that’s considered obese.

What the study found

Researchers examined more than 4,000 men and women across 34 diverse populations — from hunter-gatherer tribes to office workers in urban areas.

Using a method known as doubly labeled water, scientists were able to track total energy expenditure (TEE), basal metabolic rate (BMR) and activity energy expenditure (AEE). This technique involves giving participants water marked with stable isotopes of hydrogen and oxygen, then measuring how quickly these isotopes leave the body through urine, sweat and respiration.

After adjusting for age, sex and body size, researchers discovered something surprising: people in highly active societies burn about the same total number of calories per day as those living more sedentary lives. This supports the “constrained energy model,” which suggests the body adapts by saving energy in other areas when physical activity increases.

Diet plays the biggest role

The study’s lead author, Herman Pontzer, told The Washington Post that calorie intake is the biggest contributor to obesity in industrialized societies. The researchers cite estimates that suggest overeating is 10 times more influential than decreased energy expenditure in terms of weight gain.

Ultra-processed foods — like sugary snacks, frozen meals and fast food — are high in calories but low in nutritional value. They are significantly altered from their original state and often include added sugars, oils, preservatives and artificial ingredients.

These foods are heavily linked not only to weight gain but also to long-term metabolic and cardiovascular issues.

Takeaways for everyday Americans

This study does not suggest that exercise is unimportant. Physical activity remains essential for heart health, mood and muscle function.

But when it comes to obesity, researchers say what you eat matters far more than how much you move. Reducing intake of ultra-processed foods and being mindful of calorie quality may be the most effective path toward reversing the obesity epidemic.