Losing weight on a diet or Ozempic? Here's why you still need exercise.

Correction: An earlier version of the article referred to Ozempic as a weight-loss drug. Some patients are taking it to lose weight, but the drug has been approved by the Food and Drug Administration only for Type 2 diabetes and other medical conditions, not weight loss. The article has been corrected.

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It’s long been known that exercise doesn’t help most people lose weight. So why should anyone trying to shed pounds still keep exercising?

New research has the answer. A study published in the journal Nature Metabolism found that people who combine exercise with dieting can double the metabolic health benefits of their weight loss.

The study found that men and women with obesity and prediabetes who worked out while dieting improved their insulin sensitivity twice as much as people who only dieted, even though everyone lost comparable amounts of weight.

“These results demonstrate that regular exercise during a diet-induced weight loss program has profound additional metabolic benefits” compared with dieting by itself, the study’s authors wrote.

The researchers behind the study said they hope the findings will motivate dieters to exercise, including people who have been prescribed popular new drugs, such as Ozempic, for weight loss.

“Exercise should absolutely be on the agenda,” whether someone is using the drugs or not, said Samuel Klein, the chief of the division of geriatrics and nutritional science at the Washington University School of Medicine in St. Louis and senior author of the study.

The experiment was small, involving only a total of 16 men and women, but the findings strengthen a growing scientific consensus that being physically active while we cut calories changes our bodies in ways that dieting alone cannot match.

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Does exercise help you lose weight?

“It’s been reasonable to wonder” if we should bother exercising while losing weight, Klein said. The relationship between exercise and weight is famously uneasy, after all. “People are told to become physically active if they plan to lose weight,” Klein said. But for the most part, “physical activity doesn’t affect body weight much.”

In fact, exercise by itself, without simultaneous dieting, rarely leads to substantial weight loss, many studies show, and sometimes results in weight gain. Exercise generally expends fewer calories than we might expect. Walk for 30 minutes and you’ll burn 150 calories or so, which is easily replaced with a cookie or a sports drink. Exercise also often increases appetite.

Overall, Klein said, it is “easier to cut calories” for most people than to start exercising to drop pounds.

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Dieting vs. exercise

The research, however, offers new evidence on the benefits of exercising when you’re trying to shed pounds, even if it doesn’t speed or increase weight loss.

The Nature Metabolism study used data from several existing experiments about weight loss. In one, eight sedentary men and women with obesity and prediabetes began a supervised diet, low in fat and “plant-forward.” All meals were provided and individualized, so each person slowly shed 10 percent of his or her body weight.

A second group of eight men and women with obesity and prediabetes received the same meals, but also started exercising six times a week. Four of these sessions were supervised and included about an hour of moderate aerobic exercise, such as fast treadmill walking, twice a week; resistance training once a week; and interval training once a week. The other two days, participants worked out on their own at home.

The researchers adjusted exercisers’ meals to keep their weight loss similar to that in the other group, but the calories involved were few, Klein said.

The program lasted until each person was 10 percent lighter, which took most of them about five months. Before and after, the researchers drew blood, biopsied muscles, gathered fecal samples and checked people’s insulin sensitivity, fitness and other health measures.

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Helping weight-losers get twice as healthy

By the end, the men and women in the diet-only group were leaner, with better cholesterol profiles and other markers of enhanced metabolic health. Perhaps most important, their insulin sensitivity was markedly improved now.

“We’d expected they’d be healthier,” Klein said, “and they were.”

But the scientists were taken aback by the exercisers.

That group’s insulin sensitivity gains were twice as great as among the dieters. They also showed higher numbers of new blood vessels and mitochondria inside their muscles. Mitochondria are tiny organelles that power our cells, and the more we have, in general, the better.

The exercisers also added about 13 percent to their muscular strength and 10 percent to their endurance. The dieters, on the other hand, were now about 2 percent weaker and 6 percent less fit than at the study’s start.

“We’d wondered how much extra benefit exercise would add,” Klein said. “We were astonished at how potent it was.”

The study has caveats, though. It relied on “a very small sample size,” said John Thyfault, a professor and director of the KU Diabetes Institute at the University of Kansas. And all of these volunteers had obesity and prediabetes. Whether healthier people’s results would be similar isn’t clear.

The researchers also closely monitored everyone’s diet and exercise, making compliance with the programs “extremely high,” Klein said. But few of us would receive such supervision during weight loss, and some of us might feel intimidated by the extent and intensity of the workouts.

“We don’t know at this point if less exercise would be as effective,” Klein said.

He and his colleagues hope to explore some of these issues in future studies. But based on the evidence already available, if you’re dieting, but not exercising, he concluded, “you aren’t getting the full benefits for metabolic health, and our study shows those effects can be profound.”