REGULAR EXERCISE DISSOCIATES FAT MASS AND BONE-DENSITY IN PREMENOPAUSAL WOMEN

Body weight is one of the principal determinants of bone density and f racture frequency, but there is significant disagreement in the Litera ture regarding the relative contributions of the lean and fat componen ts of body weight to this relationship. As previous studies have not c onsidered the possible role of exercise in soft tissue-bone density in terrelationships, we measured areal bone mineral density (BMD), fat ma ss, and lean mass in eumenorrheic premenopausal women and determined w hether the interrelationships of these variables are influenced by the subject's exercise status. Subjects with mean activity levels of more than 140 kilojoules/kg . day (equivalent to undertaking vigorous phys ical activity for > 1.5 h/week) were classified as exercisers. In the nonexercising subjects (n = 36; age, 36 +/- 8 yr), BMD was markedly we ight dependent (0.45 < r < 0.62), and this was contributed to by both fat and lean tissue. Because this finding may have arisen from the mut ual dependence of soft tissue mass and areal BMD on body size, fat and lean masses were reexpressed as a percentage of body weight. The perc ent fat tended to be positively related to areal BMD (0.23 < r < 0.35) , whereas the percent lean was inversely related to this index. A seco nd way of obviating the mutual dependence of soft tissue mass and area l BMD on body size is to derive BMD/height as an index of volumetric b one density. This parameter was only related to lean mass in the femur , whereas the correlations with fat mass were little changed. The perc ent fat was positively (0.29 < r < 0.43) and the percent lean was nega tively (-0.43 < r < -0.29) related to BMD/height throughout the skelet on, including the femur. In the exercising subjects (n = 63; age, 33 /- 8 yr), fat mass and lean mass were unrelated to BMD/height (r < 0.2 3). However, the percent lean was positively correlated with BMD and B MD/height in the femoral neck (r = 0.28 and r = 0.31, respectively). I t is concluded that bone density is only associated with fat mass in s edentary women. In exercisers, femoral neck density is related to lean mass, possibly through the effects of weight-bearing exercise on both of these variables.