GLP 1 Weight Loss Medications and Your Muscle

GLP 1 medications such as semaglutide and tirzepatide have changed how much weight people can lose. They work largely by reducing appetite and slowing how fast the stomach empties, so people eat less without fighting hunger all day. The weight loss can be substantial. The question we care about is what that weight was made of.

Not all of it is fat

When you lose weight by any method, some of it comes from lean mass. In the DEXA substudy of STEP 1, a large semaglutide trial, roughly a third or more of the weight lost was lean mass. The DEXA substudy of SURMOUNT 1, a tirzepatide trial, reported closer to a quarter. Some lean mass loss happens with any large weight loss, and these figures sit within the range seen with other approaches, toward the higher end in the semaglutide study. Because the amount of weight lost can be large, the lean mass lost can be large too.

Lean mass on a scan is not all muscle. It includes organs, water and other tissue, and some of that decrease is expected as the body gets smaller. Still, skeletal muscle is a meaningful part of it, and muscle is hard to rebuild once it is gone, especially after 40.

Why it matters

Muscle is the main place insulin sends sugar from your blood after a meal. It supports your joints, keeps you strong enough to stay active, and protects against falls as you age. Losing a large amount of it while losing fat can leave a person lighter but weaker, with a higher body fat percentage than the scale suggests. If the weight returns later, it tends to come back as more fat than muscle.

How to protect muscle

The two main levers are the same with or without a medication.

• Strength training. Two to three sessions a week gives muscle a reason to stay. Without that signal, the body treats it as expendable during a calorie deficit.
• Protein. We set protein at about 1 gram per pound of lean body mass per day, spread across 3 to 4 meals of 30 to 50 grams. This is harder than it sounds on a GLP 1, because appetite is low. Protein often needs to come first at each meal.

Measure it

The scale cannot tell you whether you lost fat or muscle. A DEXA scan can. It measures fat mass, lean mass and bone density by region, so a baseline before or early in treatment and a repeat about every 6 months shows what the weight loss was made of. If lean mass in the arms and legs is falling faster than it should, that is a signal to adjust training and protein before more is lost.

A large weight loss is only a good result if the right tissue left. That is why we check it. You can pick your time right after you buy.

Book a DEXA scan at HYBRD Health ($199)

This article is educational and is not medical advice.