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For decades, conventional wisdom held that, to lose weight, you must eat less and move more. Of course, that is easier said than done, which is why drugs like Wegovy are so revolutionary. By suppressing appetite, they help tackle the first part of that equation, typically leading to dramatic weight loss. But what about the second? Do we still need to hit the gym if weight-loss drugs are causing the number on the scales to drop? And what impact do these drugs have on our ability to exercise?

What is becoming clear is that exercise may be even more crucial for people who are on these medications than it is for those who aren鈥檛. The ability of semaglutide to induce rapid weight loss (see 鈥淗ow they work鈥�, below) also leads to notable declines in muscle mass. For instance, a 2021 study of 95 people who were overweight or had obesity and were taking semaglutide found that lean body mass , on average, after 68 weeks.

Lean body mass encompasses body tissues like muscle and bone. So, these results suggest that both deteriorate when taking weight-loss drugs, says at the University of Copenhagen. Because these drugs lead people to consume fewer calories, the body must break down fat, muscle and even bone for nutrients.

鈥淥ur understanding is that up to about 40 per cent of the overall weight loss that is seen from semaglutide is thought to be potentially from the loss of muscle mass,鈥� says at the University of Colorado. This effect isn鈥檛 unique to weight-loss drugs, however. It also occurs during other rapid weight-loss interventions, such as highly restrictive diets and bariatric surgery [weight-loss surgery to shrink the stomach], says Torekov.

Muscle loss

What is less clear is whether these declines in muscle mass affect muscle function and strength. One of the only studies to explore this was carried out by Kulik and her colleagues last year, when they assessed the psoas muscle, located in the lower back, in 51 people taking semaglutide. After 24 weeks of treatment, their muscle volume decreased by more than 9 per cent, on average, yet there was . Kulik says this might be due to weight loss making certain physical activities easier.

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However, more research is needed to understand the influence of these drugs on muscle function, especially in older people, who are already prone to muscle and bone loss. As such, they may be at an even greater risk of falls when taking weight-loss medications, says at the University of Utah.

The impact of these drugs on elite athletes is also under scrutiny. Last year, the World Anti-Doping Agency 鈥� the organisation that fights against drug misuse in sports 鈥� to track its use, as well as possible performance-enhancing effects or outcomes that endanger the health of athletes.

Despite the many unknowns, physical activity seems crucial for preventing weight regain as well, a common issue for anyone who stops using these medications. For instance, in a by Torekov and her colleagues, 98 people with obesity took the GLP-1 drug liraglutide, and half of them also participated in a supervised exercise programme. A year after treatment stopped, those in the exercise group had regained 2.5 kilograms of weight while those only taking liraglutide regained 6 kilograms.

The difference is most likely due to those in the supervised exercise group continuing some active habits after the programme ended, says Torekov. Physical activity probably helped them maintain muscle as well. 鈥淲hen you have a loss of muscle mass, then you automatically use less energy,鈥� says Torekov. 鈥淭hat also means it is even more difficult to preserve weight loss because your energy needs are lowered.鈥�

For these reasons, it is important that people on weight-loss drugs prioritise exercise where possible. Doing so will help preserve muscle and bone mass, says Torekov, who recommends that such individuals do at least 2 hours of vigorous exercise a week.

Motivation to exercise

The trouble is that these medications may reduce the motivation to work out 鈥� according to studies in mice, at least. at Yale University and his colleagues found that mice given semaglutide ran about half the distance on a running wheel as those given a placebo.

In another experiment, the wheel periodically locked up, requiring the mice to press a lever to release it. Each time the wheel jammed, it became more difficult to unlock, requiring additional lever presses. The maximum number of times mice given semaglutide pressed the lever was, on average, 25 per cent lower than those in the control group, suggesting they were less motivated to exercise, says DiLeone, who presented these findings at a Society for Neuroscience meeting in October 2024.

These results may stem from side effects like nausea, which could make exercise less appealing. But they may also be due to semaglutide affecting the brain鈥檚 reward pathways. Medications like Ozempic that curb food cravings may dampen other urges too, affecting our desire to exercise (see 鈥淲hat do GLP-1 drugs really tell us about the brain鈥檚 reward system?鈥�).

But humans are much more complex than mice, says DiLeone. For instance, research shows that after rapid weight loss due to bariatric surgery, probably because they experience improved mobility and better physical function.

We might be only beginning to understand how weight-loss medications influence fitness. What鈥檚 clear, though, is that hitting the treadmill and swinging kettlebells remains vital.