Body composition assessment (revision 9)

Body composition assessment measures the fat and lean compartments of the body separately. Body weight cannot distinguish them, and a given weight change may comprise very different proportions of each.^[1]

The reference methods are dual-energy X-ray absorptiometry and multi-compartment models combining several measurements. Field methods — bioimpedance, skinfolds, circumference-based equations — are more accessible and considerably less precise.^[1]

The question this measurement is usually asked in this field is how much of the weight lost under incretin therapy is lean mass, and whether the proportion differs from that seen with other forms of caloric restriction. See Sarcopenia and Lean mass preservation.^[2]

Bioimpedance is the method most often used outside research and the one whose limitations are least appreciated. It infers composition from conductance, which depends on total body water; changes in hydration produce apparent changes in lean mass that did not occur.^[1]

For tracking change in one person, the same method on the same device under the same conditions is worth more than a nominally better method used inconsistently — the same principle that governs a laboratory baseline.^[3] Composition measured across a withdrawal period behaves differently again, since the compartments do not regain in the proportions they were lost.^[4]