Am I going to lose muscle on a GLP-1?

You will lose some lean mass along with the fat, but it's not as bad as it sounds if you train and eat enough protein. The March 2026 Cell Reports Medicine paper (Langer et al.) found that fat accounts for about 70% of the weight lost on GLP-1s, and muscle strength stays preserved in people who keep up resistance training. In the STEP 1 DEXA sub-study, total lean mass dropped 9.7%, but as a percentage of body weight, lean mass actually went up 3 points. Protect muscle with resistance training 2 to 3 times a week and at least 1.6 g of protein per kg of body weight per day.

• STEP 1 DEXA substudy (n=140): total lean body mass down 9.7% from baseline at week 68; proportion of lean to total body mass up 3.0 percentage points
• STEP 1 DEXA: total fat mass down 19.3%; visceral fat down 27.4%
• Langer et al. 2026 (Cell Reports Medicine): obese mice on tirzepatide lost 35% body weight; fat down 73%, lean mass down only 13%; running performance preserved
• 12-week human pilot in Langer 2026: thigh muscle size decreased slightly, absolute and relative leg strength unchanged
• Practical guidance: resistance training 2 to 3x/week; protein 1.6 g/kg/day minimum, 2.0+ g/kg/day during active weight loss

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