With GLP-1 receptor agonists, we finally have safe and reliable weight-loss medications.

So of course most of the headlines you see focus on potential downsides.

Chief among them: weight loss with GLP-1s includes a disproportionate amount of lean body mass (LBM).

There’s no question that substantial weight loss, however you achieve it, will include a combination of fat and lean mass—muscle, fluid, bone, organ tissue, and everything else that isn’t fat.

As a general rule, you’d expect weight loss to include about 75 percent fat and 25 percent LBM.

With GLP-1s, reported LBM losses range from 25 to 40 percent. Crucially, very few studies have quantified the composition of that lost LBM. We don’t know how much is muscle, how much is fluid, or how much is from the LBM components of fat cells. (Fat isn’t entirely fat; the cells hold some water, and they’re held together by connective tissue.)

Without that data, the next-best way to assess the effects of reduced LBM is to look at how GLP-1s affect the people who lose weight with them.

A recent paper tries to do just that.2

How the study worked

The authors considered three possibilities:

• Is LBM loss with GLP-1 medications maladaptive—a negative side effect that could potentially mitigate the health improvements you’d expect with significant weight loss?

• Is it adaptive—pretty much proportional to what you’d expect if you lost weight through nutrition, physical activity, and other behavior modifications?

• Or is it perhaps beneficial, leading to improved muscle quality and function?

But first, what do we mean by “muscle quality”?

“Quality” most often refers to the functional properties of an individual’s muscle tissue, like the ratio of strength to mass. It might also include other measures like power, endurance, or even mobility.

The idea is to quantify what muscles can do relative to their size.

Someone with obesity will typically have more muscle mass than a leaner person but with less strength per unit of muscle. When that person loses a substantial amount of weight, it’s assumed that some of the LBM they’ve lost was expendable.

That’s partly because a smaller body needs less lean mass to operate, and partly because of the lower functional value of the lost muscle.

Quality can also refer to the muscle’s composition, specifically the extent of fat infiltration.

You expect a fit, healthy person’s muscle to include about 1.5 percent fat, but it might be 5 percent for someone with obesity.

With age, intramuscular fat infiltration increases by an average of 0.11 percentage points per year, reaching as much as 11 percent for an elderly person with sarcopenia or frailty.4

A reduction of intramuscular fat, which you’d expect with weight loss, would thus improve muscle composition.

What the study found

The authors went with Door #2:

Lean body mass loss with GLP-1 drugs is adaptive—neither dangerous nor beneficial.

They came to the same conclusion in their previous study since there’s currently no evidence that the loss of LBM affects muscle strength or function.3

They compared the adaptive loss of LBM to left ventricular hypertrophy (LVH):

When LVH is caused by chronic high blood pressure, it’s a serious health risk.5 But it can also be a result of high-level training, especially among cyclists, rowers, and swimmers, whose cardiac size quickly returns to normal when they stop training with extreme volume and intensity.

In those cases, LVH is adaptive, rather than unhealthy.

That said, the research is still extremely limited on the consequences of LBM loss with GLP-1s.

For example, studies that looked at grip strength (as a proxy for whole-body strength) following bariatric surgery have found little difference despite significant weight loss.9

But there’s a lot we still don’t know.

How does the loss of LBM affect someone’s long-term functional health? Would a significant loss of lower-body lean mass put someone at higher risk for disability or injuries years down the road?

That brings us to our first takeaway.

Takeaways

1. We don’t know how any of this applies to older adults.

Intentional weight loss doesn’t automatically lead to improved health and longevity.

That’s especially true for older adults, for whom a “normal” body mass index (between 18.5 and 24.9) is associated with a higher risk of death from any cause than a BMI above 25.6

We don’t know why a higher body weight, which is usually associated with negative health outcomes, might have the opposite effect on an older person.

Does the weight itself offer some kind of buffer against the wasting effects of illness or injury? Or is it because a heavier person has more LBM—not just more muscle, but also thicker connective tissues, larger organs, and sturdier bones?

If it’s the latter, what are the consequences of losing some of it? Especially for an older adult who’s already lost a substantial amount of muscle and strength? And most especially for an older woman who would already be at greater risk for sarcopenia?

Of course, if an older person takes a GLP-1 medication with the goal of losing weight, that’s between the client and their physician.

But if you take that step, be aware of the potential for LBM loss, and do what you can to minimize the risk.

That includes your two most potent tools, which we’ll discuss below. One is simple, and the other is surprisingly tricky.

2. Resistance training is awesome.

We wish every article written about how GLP-1s affect lean mass would inform their readers that there’s a reliable and relatively accessible solution to the problem.

Resistance training should be just as effective at preserving muscle mass for someone losing weight with a drug vs. a reduced-calorie diet. (At least, there’s no reason to think otherwise.)

In either circumstance, a well-designed strength program should minimize muscle loss while improving muscle quality and composition. For someone new to lifting, it doesn’t take a lot to help them retain muscle—just two 30-minute sessions a week should do the job.

3. Protein can be a problem for some clients taking GLP-1s.

GLP-1s promote weight loss by slowing down digestion.

On the plus side, that means you feel full longer, with less hunger. The downside is that common side effects include gastrointestinal issues.

So, practically speaking, the protein choices you’d typically consume in a calorie deficit probably won’t be appealing to someone who’s experiencing nausea from their weight-loss medication.

If you’re taking a GLP-1 and can’t stomach lean meat or poultry, or gag at the thought of a fish fillet, consider softer foods, like eggs or dairy. (Note: If you are experiencing significant, enduring GI issues, talk to your doctor about prescribing a lower dose until the problem subsides.)

Protein supplements, whey or plant-based, can also make protein more palatable—and convenient.

Brian St. Pierre, Precision Nutrition’s director of nutrition, recommends a minimum of 1.2 grams of protein per kilogram of body weight (0.55 grams per pound) for folks on GLP-1 meds—though 1.2 to 1.6 grams of protein per kilogram of body weight (0.75 grams per pound) tends to be the sweet spot.

Closing thoughts
GLP-1 drugs are truly transformative in the weight-loss market. It’s important to remember that for much of human history, particularly until the mid-20th century, our primary food concern was famine. Hunger has been one of the greatest threats to humanity throughout history. Thanks to advancements in science, particularly the Green Revolution of the 1960s and 1970s, we made significant strides in improving food production. Through innovations like selective breeding and, later, genetic modification, we can now produce enough food to feed nearly 10 billion people with fewer resources than were required to feed half that number just 50 years ago.

In many ways, we’ve shifted from the problem of scarcity to one of abundance. While obesity and metabolic diseases are certainly significant challenges, they represent the kinds of issues we deal with when basic survival is no longer a daily concern. The same scientific innovations that helped solve the problem of famine have also led to the development of GLP-1 drugs. These medications are not a cure-all, but they are an important step in addressing the consequences of overabundance. As I write this in early 2025, we now have a drug that can significantly improve weight management and metabolic health—though it’s not without limitations.

Is it perfect? No. But in the span of human history, just 50 years of progress is extraordinary. It’s a testament to our capacity to solve complex problems when we apply science and work together.

To progress,

Michael Beiter

Personal Trainer

Nutrition, Sleep, Stress Management, and Recovery Coach

References

1. Horn, Deborah B., Jaime P. Almandoz, and Michelle Look. 2022. What Is Clinically Relevant Weight Loss for Your Patients and How Can It Be Achieved? A Narrative Review. Postgraduate Medicine 134 (4): 359–75.

2. Linge, Jennifer, Andreas L. Birkenfeld, and Ian J. Neeland. 2024. Muscle Mass and Glucagon-like Peptide-1 Receptor Agonists: Adaptive or Maladaptive Response to Weight Loss? Circulation 150 (16): 1288–98.

3. Neeland, Ian J., Jennifer Linge, and Andreas L. Birkenfeld. 2024. Changes in Lean Body Mass with Glucagon-like Peptide-1-Based Therapies and Mitigation Strategies. Diabetes, Obesity & Metabolism 26 Suppl 4 (S4): 16–27.

4. Rahemi, Hadi, Nilima Nigam, and James M. Wakeling. 2015. The Effect of Intramuscular Fat on Skeletal Muscle Mechanics: Implications for the Elderly and Obese. Journal of the Royal Society, Interface 12 (109): 20150365.

5. Lovic, Dragan, Puneet Narayan, Andreas Pittaras, Charles Faselis, Michael Doumas, and Peter Kokkinos. 2017. Left Ventricular Hypertrophy in Athletes and Hypertensive Patients. Journal of Clinical Hypertension (Greenwich, Conn.) 19 (4): 413–17.

6. Veronese, N., E. Cereda, M. Solmi, S. A. Fowler, E. Manzato, S. Maggi, P. Manu, et al. 2015. Inverse Relationship between Body Mass Index and Mortality in Older Nursing Home Residents: A Meta-Analysis of 19,538 Elderly Subjects. Obesity Reviews: An Official Journal of the International Association for the Study of Obesity 16 (11): 1001–15.

7. Prado, Carla M., Stuart M. Phillips, M. Cristina Gonzalez, and Steven B. Heymsfield. 2024. Muscle Matters: The Effects of Medically Induced Weight Loss on Skeletal Muscle. The Lancet. Diabetes & Endocrinology 12 (11): 785–87.

8. Zhu, Yihua, Yue Hu, Yalan Pan, Muzhe Li, Yuanyuan Niu, Tianchi Zhang, Haitao Sun, et al. 2024. Fatty Infiltration in the Musculoskeletal System: Pathological Mechanisms and Clinical Implications. Frontiers in Endocrinology 15 (June): 1406046.

9. Jung, Han Na, Seon-Ok Kim, Chang Hee Jung, Woo Je Lee, Myung Jin Kim, and Yun Kyung Cho. 2023. Preserved Muscle Strength despite Muscle Mass Loss after Bariatric Metabolic Surgery: A Systematic Review and Meta-Analysis. Obesity Surgery 33 (11): 3422–30.