Dual-photon absorptiometry (DPA) allows separation of body mass into bone mineral, fat, and fat-free soft tissue. This report evaluates the potential of DPA to isolate appendages of human subjects and to quantify extremity skeletal muscle mass (limb fat-free soft tissue). The method was evaluated in 34 healthy adults who underwent DPA study, anthropometry of the limbs, and estimation of whole-body skeletal muscle by models based on total body potassium (TBK) and nitrogen (TBN) and on fat-free body mass (FFM). DPA appendicular skeletal muscle (22.0 +/- 3.1 kg, mean +/- SD) represented 38.7% of FFM, with similar proportions in males and females. There were strong correlations (all p less than 0.001) between limb muscle mass estimated by DPA and anthropometric limb muscle areas (r = 0.82-0.92), TBK (r = 0.94), and total-body muscle mass based on TBK-FFM (r = 0.82) and TBK-TBN (r = 0.82) models. Appendicular skeletal muscle mass estimated by DPA is thus a potentially practical and accurate method of quantifying human skeletal muscle mass in vivo.

OBJECTIVE:Although the body mass index (BMI, kg/m 2 ) is widely used as a surrogate measure of adiposity, it is a measure of excess weight, rather than excess body fat, relative to height. We examined the relation of BMI to levels of fat mass and fat-free mass among healthy 5-to 18-y-olds. METHODS AND PROCEDURES: Dual-energy X-ray absorptiometry was used to measure fat and fat-free mass among 1196 subjects. These measures were standardized for height by calculating the fat mass index (FMI, fat mass/ht 2 ) and the fat-free mass index (FFMI, fat-free mass/ht 2 ). RESULTS: The variability in FFMI was about 50% of that in FMI, and the accuracy of BMI as a measure of adiposity varied greatly according to the degree of fatness. Among children with a BMI-for-age Z85th P, BMI levels were strongly associated with FMI (r ¼ 0.85-0.96 across sex-age categories). In contrast, among children with a BMI-for-age o50th P, levels of BMI were more strongly associated with FFMI (r ¼ 0.56-0.83) than with FMI (r ¼ 0.22-0.65). The relation of BMI to fat mass was markedly nonlinear, and substantial differences in fat mass were seen only at BMI levels Z85th P. DISCUSSION: BMI levels among children should be interpreted with caution. Although a high BMI-for-age is a good indicator of excess fat mass, BMI differences among thinner children can be largely due to fat-free mass.

Multicompartment models are of growing importance in the study of body composition in humans. This study compares two improved four-compartment (water, protein, mineral, and fat) models that differ in expense, technological complexity, and radiation exposure. Primary data (from 31 subjects) for the first model were derived by dual-photon absorptiometry, 3H2O dilution, and hydrodensitometry and for the second model by delayed and prompt gamma neutron-activation analysis and 3H2O dilution. Estimates of fat, protein, and mineral from the first model were highly correlated with those from the second model (r = 0.98, 0.72, and 0.94, respectively; all p less than 0.001). The proportions of body weight represented by water, protein, mineral, and fat for the simpler first model (0.532, 0.155, 0.048, and 0.265) were similar to compartment fractions provided by the more complex and costly second model (0.532, 0.143, 0.046, and 0.279). Multicompartment body composition models can thus be developed from increasingly available techniques that compare favorably with similar results derived from limited-access instrumentation.

OBJECTIVE: To compare 16 currently used total body fat methods to a six-compartment criterion model based on in vivo neutron activation analysis. DESIGN: Observational, inter-method comparison study. SUBJECTS: Twenty-three healthy subjects (17 male and 6 female). MEASUREMENTS: Total body water (TBW) was measured by tritium dilution; body volume by underwater weighing (UWW); total body fat and bone mineral by dual-energy X-ray absorptiometry (DXA), total body potassium (TBK) by whole-body 40 K counting; total body carbon, nitrogen, calcium, phosphorus, sodium and chlorine by in vivo neutron activation analysis; skinfolds/circumferences by anthropometry (Anth); and resistance by single-frequency bioimpedance analysis (BIA). RESULTS: The average of total body fat mass measurements by the six-compartment neutron activation model was 19.7 AE 10.2 kg (mean AE s.d.) and comparable estimates by other methods ranged from 17.4±24.3 kg. Although all 16 methods were highly correlated with the six-compartment criterion model, three groups emerged based on their comparative characteristics (technical error, coef®cient of reliability, Bland-Altman analysis) relative to criterion fat estimates, in decreasing order of agreement: 1. multi-compartment model methods of Baumgartner (19.5 AE 9.9 kg), Heyms®eld (19.6 AE 9.9 kg), Selinger (19.7 AE 10.2 kg) and Siri-3C (19.6 AE 9.9 kg); 2. DXA (20.0 AE 10.8 kg), Pace-TBW (18.8 AE 10.1 kg), Siri-2C (20.0 AE 9.9 kg), and Brozek-UWW (19.4 AE 9.2 kg) methods; and 3. Segal-BIA (17.4 AE 7.2 kg), Forbes-TBN (21.8 AE 10.5 kg), Durnin-Anth (22.1 AE 9.5 kg), Forbes-TBK (22.9 AE 11.9 kg), and Steinkamp-Anth (24.3 AE 9.5 kg) methods. CONCLUSION: Relative to criterion fat estimates, body composition methods can be organized into three groups based on inter-method comparisons including technical error, coef®cient of reliability and Bland-Altman analysis. These initial groupings may prove useful in establishing the clinical and research role of the many available fat estimation methods.