Lean mass loss

Lean mass is everything on your body that is not fat — muscle, organs, bone, and the water they hold. Lose weight by any method and some of it comes from lean tissue. The question that matters is the proportion, and whether you can shift it.

The drug labels address this directly. Both the Wegovy and Zepbound labels state that the medication lowers body weight with greater fat mass loss than lean mass loss. That is the reassuring half. The unreassuring half is that the absolute amount of lean tissue lost still scales with total weight lost, and these drugs produce large total losses — up to about a fifth of body weight in the tirzepatide trials.

How much that matters depends on who you are. For a 35-year-old with plenty of muscle to spare, losing some is a cosmetic and performance issue. For someone over 65, or anyone already close to sarcopenia, muscle loss threatens the ability to climb stairs and get out of a chair, and that is a different order of problem.

The evidence on doing something about it is now moving. In the EMBRAZE phase 2 trial published in 2026, 102 adults with overweight or obesity received tirzepatide plus either apitegromab — an investigational antibody that blocks myostatin — or placebo. At 24 weeks the apitegromab group lost 1.9 kg less lean mass despite similar total weight loss, a 54.9% retention of lean mass relative to placebo. That drug is investigational and not available, but the result shows the loss is not fixed.

What is available now is unglamorous and works: enough protein, and resistance training at least twice a week. Ask your clinician for a protein target in grams rather than a vague instruction, and consider a body-composition scan rather than tracking the scale alone. Our protein calculator and muscle-loss prevention protocol set out the specifics.