300 minutes of exercise a week is the right time to lose weight.

Can exercise help us lose weight? A study of overweight men and women found that working out can help you lose weight, in part by reshaping your appetite hormones.

But to benefit, as the study suggests, we probably have to exercise a lot — burning at least 3,000 calories a week. In the study, that meant working out six days a week for nearly an hour, or about 300 minutes a week.

The relationship between working out and our waistlines is always confusing. The process seems simple: we exercise, we burn calories, and if life and metabolism were fair, we would develop an energy deficit. At that point, we would start using stored fat to fuel the ongoing operations of our bodies, which would make us slim.

But our bodies are not always cooperative. Equipped by evolution to maintain energy stores in case of starvation, our bodies tend to undermine our attempts to lose weight. Starting to work out, the appetite increases, and so we consume more calories, to compensate for the lost ones.

The result, according to many previous studies of exercise and weight loss, is that most people who start a new exercise program without strictly monitoring what they eat don’t lose as much weight as they expected — and some do.

Kyle Flack, an assistant professor of nutrition at the University of Kentucky in the United States, began to wonder a few years ago whether this outcome was inevitable. Perhaps, he speculated, there was a ceiling on people’s caloric payoffs after exercise, meaning that if they increased their hours of exercise, they would make up for fewer calories expended and lose weight.

For a study published in 2018, he and his colleagues explored this idea, asking overweight, sedentary men and women to start exercising enough to burn 1,500 or 3,000 calories a week during workouts. After three months, the researchers checked everyone’s weight loss, if any, and used metabolic calculations to determine how many calories the volunteers had consumed in compensation for their efforts.

The total, after all, was an average of about 1,000 calories a week of compensatory eating, no matter how much people exercised. By that math, the men and women who burned 1,500 calories a week with exercise regained nearly all but 500 calories from their expenditure, while those who burned 3,000 calories with exercise ended up with a net weekly loss of about 2,000 calories. (Nobody’s overall metabolic rate has changed much.)

Not surprisingly, the group that exercised the most lost weight; the others do not.

But that study left many questions unanswered, Flack said. Participants performed similar, supervised workouts, walking moderately for 30 or 60 minutes, five times a week. Would the variation in duration or frequency of training be important for people’s caloric compensation? And what was stimulating their eating? Did different amounts of exercise affect your appetite hormones differently?

To find out, he and his colleagues decided to repeat much of the previous experiment, this time with new exercise schedules. So for the new study, which was published in late 2020 in the journal Medicine & Science in Sports & Exercise, they gathered another group of 44 sedentary, overweight men and women, checked their body compositions, and asked half of them to start. exercise twice a week for at least 90 minutes until they burn about 750 calories per session, or 1,500 per week.

They could exercise as they pleased — many chose to walk, but some chose other activities — and wore a heart rate monitor to track their efforts.

The rest of the volunteers began exercising six times a week for 40 to 60 minutes, burning about 500 calories per session, for a weekly total of about 3,000. The researchers also took blood samples to check levels of certain hormones that can affect people’s appetites.

After 12 weeks, everyone returned to the lab, where the researchers reviewed body compositions, repeated blood draws, and began calculating offsets.

And again, they found a compensatory threshold of about 1,000 calories. As a result, only the men and women in the group that exercised the most — six days a week, with a total of 3,000 calories — had significantly lost weight, about 4 pounds of body fat.

Interestingly, the researchers found an unexpected difference between the groups. Those who burned about 3,000 calories a week showed changes in their bodies’ levels of leptin, a hormone that can reduce appetite.

These changes suggested that exercise increased the exercisers’ sensitivity to the hormone, allowing them to better regulate their urge to eat. There were no comparable hormonal changes in men and women who exercised less.

In essence, Flack said, the new experiment “reinforces the earlier finding” that most people will eat more if they exercise, but only up to the tipping point of 1,000 calories a week. If we somehow manage to burn more than that amount with exercise, we will likely lose weight.

But, of course, burning thousands of calories a week with exercise is scary, Flack said. Furthermore, this study only lasted a few months and cannot tell us whether later changes in our appetites or metabolisms would increase or reduce any subsequent fat loss.

Still, for people hoping exercise can help slim their waistlines for their next vacation, it seems the more they can move around, the better.

Translated by Luiz Roberto M. Gonçalves