Why does your workout burn fewer calories than you think? Understand

For every 100 calories we can expect to burn as a result of exercise, most people will burn less than 72 calories, according to an enlightening new study on how physical activity affects our metabolism.

The study found that our bodies tend to automatically compensate for at least a quarter of the calories we expend during exercise, undermining our best efforts to lose weight when training.

The results also show that carrying extra pounds unfortunately increases calorie compensation, making weight loss through exercise even more difficult for those who are already overweight.

But the study also suggests that calorie compensation varies from person to person, and that learning how your metabolism responds to workouts could be key to optimizing exercise for weight management.

In theory — or in a gentler alternate universe — exercise would substantially help with weight loss. When we move, our muscles contract, requiring more fuel than at rest, while other organs and biological systems also expend extra energy.

Thanks to previous laboratory studies, we know approximately how much energy these processes require. Walking 1.5 km, for example, burns approximately 100 calories, depending on body size and walking speed.

Until recently, most people, including exercise scientists, assumed that this process would be additive — that is, walking 1.5 km burns 100 calories; walk 3 km, burn 200; and so on, logically and mathematically. If we don’t replace those calories with extra food, we should end up burning more calories than we consumed that day and start losing weight.

But this rational result rarely happens. In several studies, most people who start a new exercise program lose less weight than expected based on the number of calories they burn during workouts, even if they closely monitor their diets.

So some scientists began to speculate that energy expenditure might be less elastic than we thought. In other words, it can have limits.

This possibility was strengthened in 2012 with the publication of an influential study on African hunter-gatherers. He showed that although the tribesmen regularly walked or ran for hours, they burned about the same total number of calories daily as relatively sedentary Western men and women.

The study authors realized that somehow the bodies of the active tribes were making up for it, decreasing their total calorie burn, so they staved off hunger while chasing their food.

Other small studies since then have bolstered the finding that more activity doesn’t necessarily result in higher daily calorie expenditure. But few large-scale experiments have attempted to determine how much our bodies compensate for calories burned during movement, as measuring metabolic activity in people is complex and expensive.

As part of an ambitious new scientific initiative, however, dozens of researchers recently gathered metabolic data from several studies involving thousands of men and women. These studies involved drinking doubly labeled water, the gold standard in metabolic research. It contains isotopes that allow researchers to accurately track how many calories someone burns throughout the day.

For the new study, which was published in August in Current Biology, some of the scientists involved in the initiative set out to see what happens to our metabolisms when we move.

They extracted data from 1,754 adults that included their double-labeled water results, as well as measurements of their body compositions and basal energy expenditure, which is how many calories they burn simply by being alive, even if they are inactive. Subtracting baseline numbers from total energy expenditure gave the researchers an approximation of people’s energy expenditure on exercise and other movements, such as standing, walking, and general fidgeting.

Then, using statistical models, the researchers were able to calculate whether calories burned during activity increased people’s daily energy expenditure as expected — that is, whether people burned proportionately more total daily calories when they moved more. But, as the researchers found, they don’t tend to burn more calories. In fact, most people appeared to be burning only about 72% of the additional calories on average, as would be expected given their activity levels.

“People appear to be compensating with energy for the additional calories burned from activity by at least a quarter,” said Lewis Halsey, a professor of life and health sciences at the University of Roehampton in London and one of the lead authors of the new study. .

Unexpectedly, the researchers also found that energy compensation levels increased among people with relatively high levels of body fat. They tended to make up for 50% or more of the calories they burned by being active.

It is important to note that the study did not address people’s food consumption. It just focused on energy expenditure and how our bodies seem able to compensate for some of the calories burned during exercise by reducing biological activity in other parts of the body.

However, it remains unclear how we unconsciously orchestrated this feat and which internal systems might be most affected, Halsey said.

He and his colleagues speculate that the immune system’s operations, which require considerable energy, may be somewhat reduced. Or we may unknowingly become less restless or more sedentary in general on the days when we exercise. Perhaps, too, some of the inner workings of our cells may slow down, reducing our bodies’ overall energy expenditure.

But the new science of exercise and calorie compensation isn’t completely discouraging. Even people whose bodies make up 50% or more of the calories they expend during physical activity will burn more calories per day than if they sit still, Halsey pointed out.

A more intractable problem with using exercise for weight loss, he continued, is that exercise actually burns very few calories, period. To lose weight, we will also have to eat less.

“Half a cookie or half a can of soda” after half an hour of walking, and you’ll have eaten more calories than you burned, he said, no matter how much you compensate.

Luiz Roberto M. Goncalves