Advantage of knee height over height as an index of stature in expression of body composition in adults.

It is not clear how body composition should best be compared between subjects, but it is accepted that simply comparing total mass or percentage fat-free mass (FFM) or fat mass (FM) is not adequate. Indexing body composition to stature seems more appropriate, but the decline in height with age or osteoporosis could confound comparisons of body composition across age or disease groups. Using bioimpedance data from 600 adults in the Framingham Offspring Study, we examined the ability of height and knee height indexes to correctly describe the loss of FFM seen with increasing age between 28 and 75 y. Indexing body composition to height obscured the loss of FFM with age (r = -0.065, P < 0.26 for men; r = -0.050, P < 0.394 for women), whereas indexing body composition to knee height preserved the correct information (r = -0.154, P < 0.007 for men; r = -0.161, P < 0.006 for women). These data suggest that knee height is a reliable surrogate for stature and should be used to adjust body composition measurements when bioimpedance is used to estimate body composition.