Why exercise often leads to less weight loss than expected, and why diet plays such a key role

Some people complain that they are not losing much weight despite exercising.  

Why is this? Science has another view on this now.

According to conventional wisdom, a great way to lose weight is to do some exercise. While being active is beneficial in many ways for our health, it may not be very helpful if you want to shed a few inches off your waistline. And now, a new study published in Current Biology offers an explanation for why.

For decades, scientists have used a simple mathematical formula to calculate how much energy we use, which is essentially: Total Burn = Living Cost + Exercise. This is known as the Additive Model, and it means that every calorie you burn during a workout is simply added to the calories you use just to stay alive.

So, for example, if you burn 2,000 calories a day during your normal activities and then go for a run and burn 400 calories, you'll burn 2,400 calories in a day according to the formula. The thought has been that this extra burn could lead to weight loss.

However, in recent years, another model has emerged called the Constrained Model. It says that our bodies have a limit on how much energy they spend. So, if we burn more calories through exercise, the body reduces internal tasks, such as cell repair, to keep total energy expenditure within a narrow, predictable range. It means that the extra calories you thought you were burning at the gym are partially offset.

Researchers decided to compare these two ideas to see which one is supported by the data.

They analyzed 14 different studies involving 450 people who participated in exercise programs, as well as several animal studies. By comparing the energy these subjects were expected to burn with the energy they actually burned, the scientists calculated how much the body was compensating. They also compared data from different populations.

Their results suggested that the Additive Model often overestimates how much total daily energy expenditure rises with exercise. Instead, they found that as people and animals become more active, they may compensate by reducing energy spent on other processes or activities.

On average, about 72% of the calories burned during exercise are added to the total daily burn. The remaining 28% may be offset through compensation. However, this is partial. Exercise still increases total energy use, but less than a simple additive calculation would predict. The researchers also noted that the 28% figure is an average and varies widely between individuals.

"Humans and other animals respond to increased physical activity by reducing energy expenditure on other tasks, supporting a constrained model of energy expenditure," commented the researchers in their paper.

These findings may explain why exercise often leads to less weight loss than expected, and why diet plays such a key role.

Herman Pontzer et al, The evidence for constrained total energy expenditure in humans and other animals, Current Biology (2026). DOI: 10.1016/j.cub.2026.01.025