Studies of body-composition changes during weight loss have had conflicting results with regard to changes in bone mineral content (BMC) and bone mineral density (BMD). We examined BMC and BMD for changes during weight loss. Fourteen women enrolled in a 15-wk weight loss program. Dual-energy X-ray absorptiometry (DXA) measures of the total body were made at baseline (T1), the midpoint of weight loss (T2), and at the end of weight loss (T3). Body weight changed significantly throughout the 15 wk, declining from a high of 89.7+/-3.6 to 74.1+/-3.2 kg. Fat-free mass declined initially (47.8+/-1.7 kg at T1, 45.7+/-1.4 kg at T2, and 46.0+/-1.5 kg at T3) and then stabilized. Fat mass changed significantly during the study (39.2 kg at T1, 32.4 kg at T2, and 29.3 kg at T3). No significant differences were observed in BMC or bone areal measurement during the study. However, BMD declined significantly from baseline (1.217 g/cm2 at T1, 1.197 g/cm2 at T2, and 1.200 g/cm2 at T3). The changes in BMC and BA were in opposite directions, resulting in a significant decline in BMD without a loss of BMC. These data suggest that changes in BMD observed with weight loss may be the result of a lack of instrument sensitivity when body weight and composition change and are simply an artifact and not a physiologic change in BMD. Further research is needed to determine the full effect of weight loss on BMC, bone area, and BMD.
