Weight loss on GLP-1 agonists: why strength training matters

GLP-1 receptor agonists, such as semaglutide and tirzepatide, have completely changed the treatment of obesity. The weight loss results are impressive, but there is a detail that often gets left out of the conversation on social media: not all the weight lost is fat.

The lean mass problem

Studies with these medications show that a meaningful share of the weight lost, in some cases a quarter or more, is muscle mass rather than fat tissue. This happens because a drastic reduction in calorie intake, without enough stimulus to the muscle, leads the body to use muscle protein as an energy source.

Losing muscle mass has costs that go beyond appearance: lower functional strength, a lower resting metabolic rate, and a higher risk of regaining the weight once treatment ends or the dose is reduced.

What helps preserve muscle

Regular strength training: at least two to three sessions a week, working the major muscle groups, is the intervention with the strongest evidence for preserving lean mass during weight loss.
Adequate protein intake: generally between 1.2 and 1.6 grams of protein per kilogram of body weight per day, spread across meals, despite the appetite reduction typical of these medications.
Tracking body composition: when possible, monitor not just the number on the scale, but the ratio of fat mass to lean mass lost.

The goal is not just weighing less but losing fat while preserving muscle. Without enough strength training and protein, the body saves energy wherever it is easiest, and muscle is usually the first to go.

This article is for information only and does not replace a medical consultation. Changes to dose, diet or training should be discussed with the professional managing the treatment.

Sources

Wilding JPH, Batterham RL, Calanna S, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). New England Journal of Medicine. 2021
Jastreboff AM, Aronne LJ, Ahmad NN, et al. Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1). New England Journal of Medicine. 2022