The human body is constantly working to maintain a balance between energy expenditure and intake, keeping daily energy use within certain limits and compensating for any deviations. To start burning fat reserves, it is necessary to combine physical activity with a calorie deficit.

There is a common belief that simply increasing physical activity is enough to lose weight. While exercise is undoubtedly beneficial for health, if your goal is to reduce your waistline by several centimeters, physical activity alone may not be sufficient. It is also important to monitor your diet. This conclusion was reached by evolutionary anthropologists from Duke University (USA), whose article was published in the journal Current Biology.

Additive and Constrained Models of Energy Expenditure

For a long time, scientists relied on the so-called additive model, according to which the total amount of energy burned is calculated using a simple formula:
Total energy expenditure = basal metabolism + physical activity.

This meant that every calorie burned during exercise was simply added to the calories needed to sustain life. For example, if you spend 2,000 calories a day on regular activities and then burn another 400 calories while jogging, according to this model, your daily expenditure would be 2,400 calories. It was believed that this additional expenditure would contribute to weight loss.

However, in recent years, an alternative, constrained model has emerged. According to this model, the body has a limit to how much energy it can expend without harming itself. If you burn more calories during exercise than this limit, the body begins to compensate for the excess by reducing energy spent on internal processes, such as cell repair, so that total expenditure remains within acceptable boundaries.

As a result, some of the calories that seemed to be burned during exercise and should have contributed to weight loss are actually compensated for by the body.

Research Findings

The authors of the study compared both models by analyzing data from 14 different studies involving 450 people who engaged in physical exercise, as well as results from animal experiments. By comparing the expected amount of energy burned with actual measurements, the scientists determined how much the body compensates for energy loss.

They found that with aerobic exercise, people’s total daily energy expenditure increased by only about 30% of what the additive model predicted. Moreover, compensation was even greater when physical activity was combined with dietary restrictions. In other words, when energy expenditure is high and calorie intake is low, the body is forced to burn more stored fat to maintain energy balance, which does indeed lead to weight loss.

Why Exercise Alone Isn’t Always Effective Without Diet

The researchers note that while physical activity does increase overall energy expenditure, it does not do so to the extent predicted by the simple additive model. In fact, compensation can be even higher when food intake is limited. This explains why exercise alone often results in less weight loss than expected, and why diet plays a crucial role in the weight loss process.

It is also worth noting that, as previous studies have shown, people in good physical shape use energy more efficiently than those who are not trained. Their bodies require less energy to perform the same everyday tasks.