For decades, experts have debated whether rising obesity rates were caused more by dwindling physical-activity levels or by increasing calorie intake. A sweeping new analysis led by Duke University’s Pontzer Lab brings fresh clarity as well as a bit of nuance to that discussion. Drawing on data from more than 4,000 adults from 34 populations that span hunter-gatherer camps to affluent suburbs, the investigators used doubly labeled water [a scientific method that uses naturally occurring, stable isotopes of hydrogen and oxygen in water to accurately measure total daily energy expenditure (TDEE)] and paired those data with objective assessments of body composition and economic development indicators.

What the Study Found

The results upended a common assumption that people in more industrialized nations are moving less than they used to. In fact, this study found that absolute TEE, basal expenditure and activity-related expenditure were actually higher—not lower—in economically developed settings.

In their conclusion, the authors wrote that their study showed “that daily energy expenditures are greater in developed populations, and activity energy expenditures are not reduced.” They also concluded that dietary intake plays “a far greater role than reduced expenditure” in development-related obesity.

Yet after adjusting for fat-free mass, TEE dipped only about 6 to 11% across the economic spectrum, which is considered far too small a decline to explain the jump in obesity. Body-fat percentage and body mass index (BMI) climbed sharply with a country’s economic development, but variation in expenditure accounted for barely one-tenth of that rise. Instead, estimated energy intake was substantially higher in wealthier groups, and in populations with diet records, the proportion of ultra-processed foods (UPFs)correlated directly with body-fat levels.

Why Diet Takes Center Stage

“Despite decades of trying to understand the root causes of the obesity crisis … it’s clear that changes in diet, not reduced activity, are the main cause of obesity in the U.S. and other developed countries,” says principal investigator Herman Pontzer, PhD. Lead author Amanda McGrosky, PhD, agrees, saying that, “Differences in total energy expenditure explained only a fraction of the increase in body fat … suggesting that other factors, such as dietary changes, are driving the increases” with development.

Their data adds to a growing consensus that UPFs and easy access to calorically dense fare disrupt satiety cues and boost net energy absorption. Among 25 populations with diet detail, a higher share of UPF predicted greater body fat, independent of age, sex, activity and national prosperity.

Physical Activity Still Matters

None of this diminishes the myriad benefits of movement. The authors caution that physical activity “is an essential component of a healthy lifestyle,” noting its well-documented roles in cardiovascular, metabolic and mental health. What the study challenges is the long-standing focus on exercise alone as the antidote to obesity. Rather, diet and physical activity should be viewed as essential and complementary, rather than interchangeable.

What the Research Means for Health and Exercise Professionals

For health coaches and exercise professionals, these findings reframe the energy-balance equation. Physical activity remains essential, but the data suggest that dietary intake and food quality carry more weight in driving population-level weight gain. That means your work with clients should pair smart, sustainable movement programming with coaching that helps them build skills around food choices, meal structure and environments that make those choices easier.

You can start this process by recalibrating the conversation clients often have about “burning off” food with exercise. Physical activity is a lever, but it has limits: a vigorous one-hour session might expend 500 to 700 calories, while a single fast-food meal can quickly exceed that. Use those comparisons to set expectations without shaming food choices. Emphasize that exercise isn’t punishment for eating; it’s a cornerstone of long-term health that works best alongside thoughtful nutrition.

Urge your clients to prioritize diet quality, while being sure to stay within your scope of practice. The association between UPFs and higher body-fat levels strengthens the case for focusing on minimally processed options, adequate protein and fiber, and beverages with little or no added sugar. Encourage clients to keep simple food logs, read labels and notice satiety cues. When needed, refer clients to a registered dietitian nutritionist (RDN) if they need individualized meal planning, medical nutrition therapy or support beyond your scope. Focusing on small, skills-based targets, such as adding a serving of fruit or vegetables to lunch, pre-planning protein-rich snacks or swapping one sweetened drink for water, often produce meaningful change without overwhelming the client.

At the same time, protect and promote physical activity for outcomes that extend far beyond the scale. Regular moderate-to-vigorous movement improves cardiorespiratory fitness, lowers cardiometabolic risk, preserves lean mass during weight loss, supports bone health and enhances mood and sleep. Help your clients incorporate activity into their whole week, aiming for at least 150 minutes of moderate-intensity aerobic activity (or 75 minutes vigorous), plus two or more days of total-body strength training. Layer in simple ways to maximize nonexercise activity, such as having walking meetings, taking brief mobility breaks and creating purposeful step goals, all of which can help to support adherence and recovery.

Finally, integrate behavior-change strategies so nutrition and training reinforce each other. Use motivational interviewing to help your clients uncover their values and barriers and then translate those into SMART goals the client owns. Track process metrics (e.g., meals cooked at home, servings of vegetables, minutes walked, sets completed) alongside outcomes (e.g., blood pressure, resting heart rate, waist circumference, performance markers). Avoid “earn-or-burn” framing and instead connect choices to energy, mood, performance and health. Build a referral network that may include an RDN, mental health professional or sleep specialist, and coordinate care when appropriate.

In practice, this might look like pairing a progressive strength-and-walking plan with two or three nutrition-focused habits for the next month, a brief weekly check-in to review what went well and what got in the way, and a plan to adjust one variable at a time. Over time, clients learn that consistent movement plus higher-quality intake—not heroic workouts to offset eating—produce the durable health, performance and body-composition changes they’re after.

The Takeaway

This study offers strong evidence that eating patterns, more than lower physical-activity levels, are a primary cause of increasing levels of obesity around the world. As a health and exercise professional, the message is not to minimize the importance of movement but to place equal—or greater—emphasis on supporting clients’ nutritional environments. By pairing evidence-based exercise programming with deliberate, skills-based nutrition guidance and strong referral networks, you can better tackle the complex, diet-dominated drivers of obesity and help clients reap the full spectrum of health benefits that both pillars provide.