It may come as a surprise, but not all weight loss is healthy.

And one key question for people over 65 is whether the weight-loss drugs that have become so popular also pose a risk of melting away much-needed muscle.

It’s almost inevitable when a person loses weight — be it from medication, strict dieting, or bariatric surgery — that not all of it will be from shedding fat. About a quarter is typically from the loss of lean mass, like muscle and bone, which is essential for metabolism and preventing injury as we age.

That’s true for weight loss from so-called GLP-1 medications like Ozempic and Wegovy. For younger adults, the loss of muscle may not cause problems right away, but in people over 65, it’s critical for maintaining strength and mobility. Muscle weakness is a risk factor in falls among older adults, one of the leading causes of injury death for that age group, according to the US Centers for Disease Control and Prevention. As Medicare grapples with whether or not to cover the drugs for people 65 and older, studies of this population may be key.

“While there may be many important benefits of weight loss metabolically, we need, particularly in older adults, to think about muscle and how important it is for the older adult population,” says John Batsis, an associate professor of medicine in the Division of Geriatric Medicine and Department of Nutrition at University of North Carolina Chapel Hill. “Losing muscle mass and strength with aging is a natural phenomenon – everyone though has a threshold where it causes a problem.”

So far, there’s not enough data to know just how big of a problem this is for older adults taking GLP-1 medications for weight loss, experts say. About 42% of US adults aged 60 and older have obesity, according to the CDC. In a clinical trial of semaglutide, the active ingredient in Ozempic and Wegovy, researchers looked at loss of lean muscle mass in a subgroup of 140 participants. On average, participants lost about 15 pounds of lean muscle and 23 pounds of fat during the 68-week trial. However, the mean age of participants in that group was 52.

Batsis says that very few GLP-1 studies, in general, have looked at differences between older and younger adults — including how loss of fat and muscle may vary. And more studies focused on older adults are needed, he says.

Although semaglutide reduced lean mass, patients lost even more fat, which helped improve their overall body composition, said Martin Havtorn Petersen, a spokesperson for Ozempic and Wegovy maker Novo Nordisk. He also said “there has not been a safety signal identified in relation to loss of muscle mass.”

Drugmakers are already looking at how next-generation weight loss drugs might be able to overcome this challenge. Eli Lilly & Co., for example, acquired obesity drug startup Versanis Bio for up to $2 billion in July, further extending the drugmaker’s reach into the weight loss market. Versanis’ experimental drug, bimagrumab, aims to help people lose weight while preserving their muscle mass.

It will be studied in combination with Lilly’s drug tirzepatide, which is expected to gain approval in obesity by year’s end. The combination “could be the next major step in innovative treatments for those living with cardiometabolic diseases, like obesity,” Ruth Gimeno, Lilly’s vice president for diabetes, obesity and cardiometabolic research, said in a statement last month.

For older adults who may be taking weight loss medications, increasing protein intake, resistance exercises and other measures may help mitigate the loss of lean muscle, Batsis says.