For every 100 calories we might expect to burn while exercising, most of us will actually cleanse less than 72 calories burned, according to a revealing new study on how physical activity affects our metabolism.
The study finds that our bodies tend to automatically compensate for at least a quarter of the calories we burn during exercise, compromising our best efforts to lose weight through exercise. The results also show that carrying extra pounds unfortunately worsens calorie compensation, making weight loss through exercise even more difficult to achieve for those who are already overweight.
But the study also suggests that calorie compensation varies from person to person, and learning how your metabolism responds to workouts can be key to optimizing exercise for weight control.
In theory – or in a more lovable alternate universe – exercise would go a long way in helping to lose weight. When we move, our muscles contract, requiring more fuel than at rest, while other organs and biological systems also expend extra energy. From previous laboratory studies, we know approximately how much energy these processes require. Walking a mile, for example, burns around 100 calories, depending on body size and someone’s walking speed.
Until recently, most people, including exercise scientists, assumed this process would be additive, that is, walking a mile would burn 100 calories. Walk in pairs, 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 eat that day and start losing weight.
But this rational result rarely occurs. Study after study, most people who start a new exercise program lose less weight than expected based on the number of calories they burn during their workouts, even if they strictly watch their diet.
So, some scientists have started to speculate that energy expenditure might be less elastic than we thought. In other words, it can have limits. This possibility gained ground in 2012, with the publication of an influential study on African hunter-gatherers. He showed that although the tribe members walked or jogged regularly for hours, they burned about the same number of total daily calories as relatively sedentary Western men and women. Somehow, the study authors realized, the bodies of the active tribe members compensated, reducing overall calorie burning so that they avoided starving while they were stalked their food.
Other small studies since have reinforced the conclusion that more activity doesn’t necessarily lead to more daily calorie expenditure. But few large-scale experiments have attempted to determine how well our bodies compensate for calories burned while moving, because measuring metabolic activity in people is complex and expensive.
However, as part of an ambitious new scientific initiative, dozens of researchers recently pooled their metabolic data from multiple studies involving thousands of men and women. These studies involved drinking double-labeled water, the gold standard in metabolic research. It contains isotopes that allow researchers to accurately track the number of calories someone burns during the day.
For the new study, published in August in Current Biology, some of the scientists involved in the initiative set out to see what happens to our metabolism when we move. They extracted data for 1,754 adults that included their double-labeled water results, as well as measures of their body composition and baseline energy expenditure – that is, the number of calories they burn. simply by being alive, even if they are otherwise inactive. Subtracting the basic numbers from total energy expenditure gave the researchers an approximation of people’s energy expenditure resulting from exercise and other movements, such as standing, walking, and general restlessness.
Then, using statistical models, the researchers were able to determine whether the calories burned during activity increased people’s daily energy expenditure as expected – that is, whether people burned proportionately more total daily calories. when they move more. But, the researchers found that they didn’t tend to burn more calories. In fact, most people seemed to only burn around 72% more calories on average, as you would expect given their activity level.
“People seem to make up at least a quarter for the extra calories burned by activity,” said Lewis Halsey, professor of life and health sciences at the University of Roehampton in London and one of the main authors of the new study.
Unexpectedly, the researchers also found that energy compensation levels increased in people with relatively high body fat levels. They tended to offset 50% or more of the calories they burned by being active.
It’s important to point out that the study did not examine people’s food consumption. He only focused on energy expenditure and how our bodies seem able to compensate for some of the calories burned during exercise by reducing biological activity elsewhere in the body. However, it remains unclear how we subconsciously orchestrate this feat and which internal systems could be affected the most, Halsey said. He and his colleagues believe that the operations of the immune system, which require considerable energy, can be reduced somewhat. Or we might unwittingly move less or become more sedentary overall on the days we exercise. It may also be that some of the inner workings of our cells can slow down, thereby reducing our body’s overall energy expenditure.
But the new science of exercise and calorie compensation isn’t completely daunting. Even people whose bodies compensate for 50% or more of the calories they burn during physical activity will burn more calories per day than if they stay still, Halsey pointed out. A more difficult problem to solve with using exercise for weight loss, he continued, is that exercise realistically burns low calories, period. To lose weight, you will also need to eat less.
“Half a cookie or half a can of cola” after half an hour of walking, and you’ll have absorbed more calories than you burned, he said, whether you compensated a lot or a little.