Obesity Epidemiology

60 STUDY DESIGNS AND MEASUREMENTSThe accuracy of DXA has been further confirmed by small experiments in whichexogenous fat was added to either central or peripheral body regions during the imagingprocess. 35 DXA estimates of total abdominal fat (TAF) and abdominal visceral fat(AVF) have been validated against a single-slice CT scan. 36 The DXA estimates oftrunk and abdominal FM were strongly correlated with TAF (r = .94 to .97) and AVF(r = .86 to .90) as assessed by CT.In the Health ABC Study of elderly men and women, Snijder et al. 31 compared themeasurements of visceral fat from DXA and CT. Total body fat and trunk fat were measuredby DXA with a Hologic QDR 1500. Visceral fat and TAF were measured witha 10 mm CT scan at the L4-L5 levels. The study showed a strong correlation betweenTAF measured by DXA (subregion) and CT (ranging from 0.87 in white men to 0.98 inblack women). The DXA subregion underestimated TAF by 10% compared with the CTscan. This study supports the value of DXA as a good alternative to CT for predictingTAF in an elderly population. For the prediction of visceral fat, DXA was not superiorto sagittal diameter (measured as the horizontal distance between the abdomen and thelower back).Many studies have evaluated the relationship between DXA estimates of percent bodyfat and fat distribution and metabolic and cardiovascular risk factors. 29 In general, DXAestimates of body fatness correlate well with measures of insulin resistance, glucose intolerance,and blood lipids. However, the correlations between DXA measures and adversecardiovascular risk factors in adults and children are no higher than those providedby simpler anthropometric measures, such as BMI, WC, and skinfold thicknesses. 37,38Whether DXA estimates of body fat predict long-term risk of chronic disease or mortalityhas yet to be determined.Body Composition Field MethodsBioelectrical Impedance AnalysisBIA estimates body composition by measuring the impedance or resistance to a smallelectrical current (typically 800 µA, 50 kHz) passed across body tissues (i.e., betweentwo detection electrodes attached to the right ankle and the right wrist of a subject). Themethod is based on the principle that resistance to an applied alternating electrical currentis a function of tissue composition: the greater the lean body mass or water contentof a person, the faster the current will pass through; the greater the fatty tissue, thegreater the resistance to the current. 39Substantial technological advances in BIA have been developed over the past two tothree decades, and many commercial BIA systems are now available to estimate bodycomposition in children and adults. Simpler systems based on a single frequency havegradually been replaced by those based on multiple frequencies, with more complexmethods for estimating body fat, FFM, skeletal muscle, body water, and water distribution.Numerous prediction equations have been developed to estimate percent body fatand FFM. 39 In developing these equations, researchers typically use TBW (measured bythe reference method of isotope dilution) and FFM (measured by DXA or underwaterweighing) as dependent variables, with measured resistance or closely related impedanceas the predictor variables. The electrical measurements are usually adjusted for height.To improve statistical predictability, models typically include age, sex, race, weight, andother anthropometric measures. As with other prediction equations for body fat, thosefor BIA tend to be population specific.