Weight loss drugs can lead to muscle loss, too. Is that a bad thing?

Weight loss drugs have soared in popularity in the past year, helping some lose dramatic amounts of weight — but not all that weight is fat. 

Some of that is actually lean mass, which is everything in the body that isn’t fat, including your bones, organs and, importantly, muscle. 

“When we look at weight reduction from any intervention, about one-third of the weight we lose tends to be lean mass, and that can be problematic,” said Dr. Jaime Almandoz, an associate professor of internal medicine in the Division of Endocrinology at UT Southwestern Medical Center in Dallas. That lean mass loss is generally from muscle. “Lean mass is healthier and associated with better metabolism, so when we lose lean mass, we may lose some of that function.”

The more muscle mass a person has, the better the resting metabolic rate, or the number of calories a person burns at rest. When a person loses muscle mass, the resting metabolic rate decreases, too.

“If you can maintain your muscle mass, you could have more sustained weight loss because your metabolic rate would not be reduced,” said Dr. Louis Aronne, director of the Comprehensive Weight Control Center at Weill Cornell Medicine in New York. 

Two drugs in particular have taken off for weight loss: semaglutide and tirzepatide. Semaglutide is the drug found in Novo Nordisk’s Ozempic and Wegovy; tirzepatide is the drug in Eli Lilly’s Mounjaro. The medications work in slightly different ways, but both suppress appetite, causing people to consume fewer calories.

Losing weight has complex effects on the body, especially during the initial phases. The more weight a person loses during a short period of time, the more muscle mass they’ll lose in addition to fat, Almandoz said.

When the body isn’t getting as much energy from food, it pulls from reserves of energy stored as fat and muscle, starting primarily with fat. While it’s normal for some of this extra energy to be pulled from muscle, the body may need to pull more energy from muscle if a person is losing a significant amount of weight quickly, such as 5 to 10 pounds a week instead of around 2 pounds.

“That’s why we try to emphasize slow, consistent weight loss rather than quick weight loss, where you’re more likely to pull from your energy stores” in muscle, said Dr. Jonathan Purnell, an endocrinologist and professor of medicine at the Oregon Health and Science University Knight Cardiovascular Institute in Portland.

“It’s also a function of how low you go,” he said. “Once you enter into the healthy BMI range, you’re more likely to be pulling from the lean stores.”

Why muscle mass is important 

Despite concerns about muscle mass loss, at the end of the dramatic weight loss period, people may actually have a healthier body composition, that is, the ratio of lean mass — especially muscle —compared to fat. 

“While lean mass loss is concerning, at the end, you may actually end up with a little more lean mass compared to other components such as body fat,” said Almandoz, who has worked as a consultant for Novo Nordisk and Eli Lilly. 

That’s important because “the damage from obesity doesn’t come from increased muscle, it comes from increased fat mass, particularly in the abdomen,” according to Aronne, who is a consultant for Eli Lilly. Figuring out how to lose fat but not muscle would be the “holy grail” of weight loss, he said.

A 2021 clinical trial that tested the weight loss effects of Wegovy found that while about 40% of the weight people lost tended to be lean mass, including muscle, at the end of 68 weeks, people who took the drug had a lower risk of heart disease and better physical function compared to a placebo group.

“Although lean body mass, which includes muscle mass, decreased in absolute terms, the proportion of lean body mass relative to total body mass increased,” Allison Schneider, a spokesperson for Novo Nordisk, the maker of Wegovy and Ozempic, said in an email.

Studies showed similar results for the drug tirzepatide, which is sold under the brand name Mounjaro. In a phase 3 clinical trial, “a reduction in lean mass was reported at a similar rate as what is seen in lifestyle-based treatments for obesity,” Jessica Thompson, a spokesperson for tirzepatide-maker Eli Lilly, said in an emailed statement.

A new analysis of data from the same trial found that after 72 weeks of taking the drug, about 25% of the weight participants lost was lean mass. However, the participants had an overall healthier body composition. Aronne, who worked on the trial, presented the findings this week at the European Congress on Obesity in Dublin. The analysis has not yet been published in a peer-reviewed journal. 

How to maintain muscle mass when losing weight

There are ways to mitigate lean mass loss during weight loss, and people older than 60 who are already more prone to muscle loss need to be particularly mindful of building muscle, said Dr. Juliana Simonetti, co-director of the Comprehensive Weight Management Program at the University of Utah in Salt Lake City. 

One way is to make sure you’re eating enough protein, which can be difficult to do while using weight loss drugs. Protein is the building block of muscle; without enough of it, muscles aren’t able to build and repair themselves.  

People should aim to get 60 to 80 grams of protein every day, which can be difficult to do through diet alone, Purnell said. 

Simonetti said she recommends looking for protein supplements, such as protein powders, that are low in sugar and have at least 20 grams of protein per serving.

Exercise, specifically resistance training, is also a critical part of maintaining muscle mass during weight loss, she said. 

Complicating matters is that it’s difficult to determine whether you are losing weight from fat or muscle without special equipment. That’s why people on these drugs should stay in close contact with their health care team, Simonetti said. If you’re losing weight at a rapid pace, you are likely losing muscle mass, as well. 

“People should have a medical team involved when they use these drugs so they can monitor not only muscle loss, but also kidney function, salt and electrolytes,” all of which can be thrown off by dramatic weight loss, she said. Nausea — a common side effect of weight loss drugs — can also indicate acute kidney failure if a person isn’t drinking enough water, she added.

There are currently no guidelines for physicians counseling patients on weight loss drugs. For now, many doctors, including Purnell, base their recommendations on guidelines for bariatric surgery patients, since the weight loss effects of surgery are comparable to those seen with the new weight loss drugs. 

Almandoz said that since the drugs can cause dramatic weight loss, physicians need research-backed recommendations about whether or not they should tell people on the drugs to take extra vitamins or other nutrients. It’s also unclear how these recommendations may be different for older people. 

“There are a lot of unknowns with these medications but we do know that they are effective and they do bring metabolic benefits and improvements in quality of life,” he said. Follow NBC HEALTH on Twitter & Facebook.