Different limit to the body's ability of increasing fat-free mass

It is a common understanding that fat-free mass (FFM) increases with body w eight. However, limited information is available as to the relationship bet ween weight increase and changes in body composition. We performed the pres ent study to determine quantitatively the relationship between body composi tion, total body weight, age, and sex. Body composition data were obtained by isotopic dilution on 273 subjects ranging in body mass index (BMI) from about 13 to 70 kg/m(2). Adipose free tissue (AFT) was modeled as a nonlinea r, increase-limited function of body weight. Model parameters were evaluate d as functions of sex, age, and height. The relationship between AFT and bo dy weight was very well approximated by means of the nonlinear model (R-2 = .95), with maximal AFT being determined by both sex and height and with AF T growth rate determined only by sex. AFT clearly shows a nonlinear behavio r, tending to increase less and less with progressively increasing body wei ght. With the proposed model, an asymptotic maximal AFT may be postulated. The organism seems to have an intrinsic limitation to how much skeletal mus cle development may take place to accommodate the necessities of an ever-in creasing load. These limits are different between the sexes, with women ten ding to approach more rapidly than men a lower maximal AFT for the same hei ght. Copyright (C) 2001 by W.B. Saunders Company.