What About Muscle Loss?

The concern over muscle loss from GLP-1 drugs is a hot topic. Influencers often mention research showing that 25-40% of the weight lost with these medications can be muscle instead of fat.

Let's review these concerns and discuss how to measure body composition correctly.

What Is Lean Mass and Muscle Mass?

First, we need to understand an important difference that often gets overlooked. Many people use Lean Mass and Muscle Mass interchangeably, but they're actually different measurements.

Think of Lean Mass as everything in your body that isn't fat. This includes your muscles, bones, organs, water, and even the small amounts of protein and water stored in fat tissue itself.

Muscle mass, on the other hand, refers specifically to your skeletal muscle tissue. When you're trying to build strength or maintain muscle during weight loss, muscle mass is what you're really concerned about.

Why This Debate Started?

It began a few years ago when Dr. Peter Attia highlighted findings from a STEP-1 and SUSTAIN-8 studies, which looked at the weight loss with Semaglutide (Wegovy). Scans from the STEP-1 study showed that 39% of the weight lost on semaglutide was lean mass. The SUSTAIN-8 trial found nearly the same at 40%.

Since GLP-1 trials track only total pounds lost and not the type of weight, Dr. Attia calls this a major oversight.

His concern has been echoed by other physicians, sparking widespread discussion about the quality of weight loss from weight loss drugs.

The response to this concern was put forward by body composition experts Dr. Grant Tinsley, who highlighted an important detail: the lean mass measured on scans includes much more than functional muscle tissue. It captures water, organs, and connective tissue too. The actual loss of muscle protein - the stuff that matters for your strength and metabolism - is much smaller than these total lean mass numbers suggest.

To put this in perspective, the SURMOUNT-1 trial showed participants losing an average of 22.5% of their body weight, with about 75% coming from fat and 25% from lean mass.

A Very Common Mistake

During weight loss, water fluctuations create one of the most misunderstood phenomena in body composition analysis. When you lose weight through any method, whether diet, exercise or Mounjaro you initially lose a substantial amount of water weight and stored glycogen. This shows up as lean mass loss on scans, but it's not muscle tissue disappearing. It's your body using up stored carbohydrates and releasing the water that was bound to them.

Early weight loss phases often show what appears to be concerning lean mass loss ratios, sometimes 40 percent or more of total weight loss. But much of this represents the body's carbohydrate stores normalizing and excess fluid being eliminated. As weight loss continues and stabilizes, these ratios typically improve dramatically, especially when proper nutrition and resistance training are maintained.

Here’s a great podcast on the topic if you’d like to learn more: Tirzepatide and Muscle Loss with Dr. Grant Tinsley

Interesting Fact About Fat Tissue

Here's something that might surprise you: fat tissue isn't 100% fat. It's actually about 85% fat and 15% water and protein. When fat shrinks during weight loss, you naturally lose some of this water and protein content too.

This gets counted as "lean mass loss" on scans, even though it's really just fat tissue becoming more concentrated.

Scientists have developed mathematical corrections for this effect, and when they apply these adjustments to weight loss studies, the reported lean mass loss often drops by 30% or more. In some cases, what initially appeared to be significant muscle loss became statistically insignificant once properly adjusted.

What is DEXA Scan?

DEXA scans have earned their reputation as the gold standard for measuring body composition and bone density. This technology use two different X-ray beams that your tissues absorb at different rates. Bones block more radiation than soft tissue, while fat and muscle absorb at different levels. A computer analyzes these patterns and creates a detailed map of your body composition, down to specific regions like your arms, legs, and trunk.

The scanning process is simple: you lie on a table while a scanner passes over your body, creating a detailed image that shows exactly where your fat, muscle, and bone are distributed.

DEXA vs Alternatives

InBody machines at gyms work using a method called bioelectrical impedance. These devices send a tiny, harmless electrical current through your body. Since muscle is full of water and electrolytes, it conducts electricity well, while fat resists the current. The device measures this resistance to estimate your body composition.

This method is incredibly convenient and relatively affordable, but it's also the most sensitive to factors you might not even think about. Room temperature, how much water you've consumed, whether you've exercised recently, if you've eaten, even if you've had a bowel movement can all influence the results.

Home smart scales like Renpho work on the same principle but with less sophisticated technology. They're calibrated for population averages, so individual accuracy varies.

How Often Should I Get a DEXA Scan While on GLP-1?

Here's my recommendation: think of DEXA scans like taking "before and after" photos of your body composition. You don't need them every week, but strategic timing can give you valuable insights.

I recommend getting an initial DEXA scan to establish your baseline before starting GLP-1 meds. This gives you a clear starting point to compare against later. Then, wait until you've lost a significant amount of weight before your next scan, typically after losing 20-25 pounds or about 10% of your starting weight.

The cost of a DEXA scan in the US typically ranges from $65 to $130 per scan, depending on your location and provider.

Things to Remember About Muscle Loss

Let me wrap this up with four straightforward principles that cut through all the noise around Mounjarol, Ozempic and all GLP-1 medications and muscle loss:

Scientific References for this article:

^{Systematic Review on Semaglutide Lean Mass with link 2024: Expert Opinion on Pharmacotherapy. 2024
https://pubmed.ncbi.nlm.nih.gov/38629387/}

^{Grant Tinsley Body Composition Expertise with link 2022: Deconstruct Nutrition. The down low on body composition measurements with Dr. Grant Tinsley.
https://www.deconstructnutrition.com/the-down-low-on-body-composition-measurements-with-dr-grant-tinsley/}

^{Muscle matters: the effects of medically induced weight loss on skeletal muscle, 2024 https://www.thelancet.com/journals/landia/article/PIIS2213-8587(24)00272-9/abstract}

^{Peter Attia Detailed Analysis with link 2024: Attia P. Lean mass loss on GLP-1 receptor agonists.
https://peterattiamd.com/the-downside-of-glp-1-receptor-agonists/}

^{BIA vs DEXA Comparison with link 2022: Journal of Exercise and Nutrition. Reliability and validity of bioelectrical impedance analysis compared to dual energy x-ray absorptiometry.
https://journalofexerciseandnutrition.com/index.php/JEN/article/view/133}

^{InBody Validation Studies with link 2018: Journal of Clinical Densitometry.
https://pubmed.ncbi.nlm.nih.gov/30472111/}

^{Glycogen-Water Relationship with link 2017: Multiple sources confirm the 3-4 grams of water per gram of glycogen storage ratio
https://wellwisp.com/why-do-you-lose-water-weight-first/}

^{SURMOUNT-1 Body Composition Substudy with link 2025: Diabetes, Obesity and Metabolism. 2025
https://pmc.ncbi.nlm.nih.gov/articles/PMC11965027/}