Obesity is more likely to be caused by a high-calorie diet than by a lack of physical activity

It is well known that obesity is a global epidemic, especially in industrialized countries, and that it is a major cause of disease and poor general health. However, there is a tug of war in the debate over whether the main cause of obesity is diet or lack of physical activity.

Calories consumed must be expended as energy, either through physical activity or through basal activity—internal processes in the body that expend energy, such as breathing or digestion. In the context of obesity, it is unclear whether too many calories are consumed on average to be expended through normal activity, or whether physical activity is insufficient to expend a reasonable amount of calories.

A new study published in the journal Proceedings of the National Academy of Sciences focuses on the diet versus exercise dilemma. Amanda McGrosky and her team analyzed data from 4,213 adults aged 18 to 60 from 34 populations on six continents. The sample is impressively diverse, covering people from hunter-gatherer groups, herders, farmers, and industrialized societies. It includes people with a wide range of diets and activity levels.

The researchers looked at total energy expenditure (TEE), activity energy expenditure (AEE), basal energy expenditure (BEE), and two measures of obesity: body fat percentage and body mass index (BMI). AEE represents the energy expended through exercise and was calculated by subtracting BEE from TEE. The results were grouped by economic development level using the United Nations Human Development Index (HDI), accounting for general differences in lifestyle and diet across economic development groups.

Initially, the results showed that TEE, AEE and BEE were higher in more developed populations, as were body weight, BMI and body fat percentage, meaning that the prevalence of obesity was higher in more economically developed countries, but they also seemed to be using more energy overall. However, this is not the full picture.

The researchers noted that overall body size, including height, is generally higher in industrialized areas. There are also variations in weight and energy expenditure depending on age and gender. The scientists further refined their data, adjusting them to account for these factors.

The results then showed that TEE and BEE decreased slightly, by about 6–11%, with increasing economic development. After adjusting for age, sex, and body size, AEE remained generally higher in more economically developed populations, indicating that lack of exercise is unlikely to be the cause of higher BMI or body fat percentage.

The researchers found that total energy expenditure was only weakly associated with obesity, explaining about 10% of the increase in obesity in more economically developed countries. Instead, they pointed to higher consumption of ultra-processed foods (UPFs), such as processed meats, ready meals and cakes, as the culprit, noting that “the percentage of ultra-processed foods in the diet was positively correlated with percentage body fat.”

While exercise is not a major factor in obesity, the study's authors call for regular exercise as it is still considered key to disease prevention and mental health. The study also highlights the need to focus on reducing calories from highly processed foods to combat the obesity crisis, as well as continuing to study why certain foods lead to obesity, as ultra-processed foods appear to have unique effects on the body.

They write: “The hyperpalatability, energy density, nutrient composition, and appearance of ultra-processed foods can disrupt satiety signals and promote overeating. Processing has also been shown to increase the proportion of calories consumed that are absorbed by the body rather than excreted.”