Fat-Free Mass

Fat-free mass (FFM) is the component of body mass that remains after fat mass is excluded. In the commonly used two-component model of body composition, total body mass is divided into fat mass and fat-free mass. Fat mass in this model refers primarily to stored body lipids such as triglycerides .^1,2

Fat-free mass is not a single tissue. It includes body water, proteins, minerals, glycogen, and other nonfat components distributed throughout organs and tissues. Despite the name, FFM can also include some structural lipids that are not classified as fat mass in this model .^1,2

Fat-free mass should not be equated with skeletal muscle. Skeletal muscle is a specific tissue, whereas FFM is a body-composition compartment that includes muscle as well as bone, organs, water, and other components. FFM also differs from lean soft tissue, a quantity commonly estimated by dual-energy X-ray absorptiometry. Lean soft tissue excludes bone mineral content, whereas FFM includes it .¹

The term lean mass is sometimes used as a synonym for fat-free mass, although terminology has varied. Current body-composition standards favor the term fat-free mass when referring specifically to this compartment and discourage less precise terminology when it may create ambiguity .^1,2