lean mass

This term is primarily used in clinical, athletic, and nutritional contexts to distinguish between weight gained from muscle and weight gained from fat. It carries a positive connotation in fitness circles, as increasing lean mass is typically associated with improved metabolic health and physical strength.

Because it refers to a collective measurement of various body tissues (bone, water, organs, and muscle), it is treated as an uncountable mass noun. It is not used in the plural to describe multiple people's measurements, but rather as a singular metric of body composition.