EFFECTS OF DIET AND EXERCISE ON THE DENSITY OF THE FAT-FREE MASS IN OBESE WOMEN

Abstract

726 The validity of densitometry for assessing change in body composition is based on the assumption that the density of the fat-free mass (Dffm) remains constant at 1.100 g/ml. The purpose of this study was to determine whether (1) Dffm remains constant and (2) estimates of percent fat(%Fat) from body density using the Siri equation (%Fatd) are valid compared to estimates of%Fat from a four-component model (%Fat4c), following weight loss induced by diet, or diet and exercise. Measures of body density by underwater weighing, body water by bioelectrical impedance, and bone mineral by whole-body dual-energy x-ray absorptiometry were obtained in 27 obese women (BMI = 32.0 ± 6.1 kg/m2) assigned to a diet group(D; n = 9), diet and exercise group (DE; n = 8) or a control group (C; n = 10). Body mass change averaged -4.4 ± 3.2 kg, -7.2 ± 7.4 kg and+ 0.7 ± 2.1 kg and%Fat4c change averaged -2.7 ± 1.9%,-2.3 ± 1.8% and + 0.9 ± 2.3% for the DE, D and C groups, respectively. There was no significant change in Dffm or the difference between%Fat4c and%Fatd in any of the groups( p>0.05). In the total group, Dffm did not differ from 1.1 g/ml before (1.101 ± 0.007) or after (1.099 ± 0.006) the interventions ( p>.05), and Dffm changes were not significantly related to body mass change ( r = 0.14, p> 0.05) or%Fat4c change (r = 0.007, p > 0.05). We conclude that alterations in body mass and fatness by diet or diet and exercise do not affect Dffm, and estimates of changes in%Fat from body density are accurate, in obese women.