Body Composition (Lean Body Mass vs. Fat Mass)

Body weight consists of many components whose relative proportions vary among individuals. Total body weight — which includes bone, muscle, fat, blood, and viscera — is conveniently divided into the lean body mass and the fat mass.

Lean body mass is the weight of all body tissue except fat. Body fat is stored in various organs (heart, liver, lungs, and brain) and in adipose tissue, including the fat surrounding internal organs and the subcutaneous layer of fat just beneath the skin.

Some body fat is essential — as an energy store, for protection of internal organs, as a component of nerves and cell membranes, and as insulation against heat loss. See essential-fat for the exact essential-fat percentages.

• Total body weight = lean body mass + fat mass.
• Lean body mass = weight of all body tissue except fat.
• Total weight includes bone, muscle, fat, blood, and viscera.
• Fat is stored in organs (heart, liver, lungs, brain) and in adipose tissue (around organs and subcutaneously).
• Essential fat functions: energy store, organ protection, nerve/cell-membrane component, heat insulation.

• essential-fat — the lower limits of fat needed for health.
• obesity-thresholds — body-fat percentages defining obesity.
• overweight-vs-overfat — why % fat matters more than scale weight.
• skinfold-measurement — how body fat is estimated.
• skinfold-accuracy — sources of error and assumptions underlying that estimate.
• regional-fat-distribution — where the fat sits matters too, not just how much.
• body-composition-overview — this source's skinfold-based framework is one of several methods now available (BIA, DEXA, hydrostatic/Bod Pod); see there for a full accuracy/cost comparison and current selection guidance.