Lean mass is the non-fat part of the body. It includes muscle, organs, bone, water, and other tissues, but in everyday clinical use it usually refers to the amount of muscle a person has. It is a key part of body composition, which describes how much of the body is fat versus everything else.
Lean mass matters because losing weight is not always the same as improving health. During dieting or GLP-1 treatment, people may lose fat, but they can also lose muscle if nutrition and activity are not managed well. Preserving lean mass helps maintain strength, mobility, and metabolic function. Clinicians may track it with tools such as DEXA scans, bioelectrical impedance, or other body-composition checks to make sure weight loss is healthy and sustainable.