Obesity Epidemiology

MEASUREMENTS OF ADIPOSITY AND BODY COMPOSITION 61In 1994, the National Institutes of Health organized a conference to assess theclinical and research applicability of BIA methods. 40 The consensus panel concludedthat BIA, measured at a single frequency, provided a reliable estimate of TBW undermost conditions. However, lack of a standardized methodology limited its clinical utilitybecause estimates could be affected by numerous variables, including body position,hydration status, consumption of food and beverages, ambient air and skin temperature,recent physical activity, and conductance of the examining table. Since 1994, there havebeen two significant advances in BIA technology and modeling. 41 First, the originalseries resistance model has been replaced with a parallel resistance model that allowsseparate estimates of intracellular water (ICW) and extracellular water (ECW). Second,multifrequency and segmental BIA technologies were developed to provide more accuratemeasurement of body composition than single-frequency BIA. 42Recently, Sun et al. 43 compared multifrequency BIA and DXA estimates of percentbody fat among 591 healthy subjects. They found correlations between BIA and DXA of0.88 for the whole population, 0.78 for men and 0.85 for women. The mean percent bodyfat determined by BIA (32.89% ± 8.00%) was significantly lower than that measured byDXA (34.72% ± 8.66%). BIA overestimated percent body fat by 3.03% and 4.40% whenthe percent body fat was 33% inwomen, respectively. The study concluded that BIA is a good alternative for estimatingpercent body fat in subjects within a normal body fat range, but tends to overestimate itin lean subjects and underestimate it in obese subjects.Use of standardized equipment, prediction equations, and body composition informationare essential for clinical use of BIA. 44 Estimates of body fat in morbidly obeseindividuals should be considered with caution because BIA tends to underestimatepercent body fat and overestimate FFM in this population. 45 Also, shape and size ofvarious body parts are known to affect BIA measurements, with smaller cross-sectionalareas (such as legs and arms) contributing the most to whole-body resistance. Thiscould possibly affect measurements of percent body fat in different ethnic groupsbecause of differences in body structure. Because BIA equipment is comparativelyinexpensive, portable, and simple to operate, it can be used in relatively large epidemiologicstudies. For example, NHANES III (1988-1994) included BIA measurementsfor 17,000 subjects aged 12 years or older. 46 However, a recent analysis showed thatcorrelations between BIA-derived percent body fat and markers of cardiovascular disease(e.g., blood cholesterol, triglycerides, and blood pressure) were no stronger thanthose for BMI. 47Investigators from the Malmö Diet and Cancer Study 48 collected BIA data from aSwedish cohort of 10,902 men and 16,814 women aged 45 to 73 years. They found asomewhat stronger association between BIA-estimated percent of body fat at baselineand mortality than for BMI. However, WHR was an even stronger predictor of mortalityindependent of body fat, especially in women. Bigaard et al. 49 obtained BIA estimatesof body fat and lean body mass from a Danish cohort of 27,178 men and 29,875 women50 to 64 years old. Reliability and validity of the BIA method were referenced against afour-compartment model with whole-body potassium counting and the dilution method.Sex-specific equations developed in that study were used to estimate FFM. 50 The FFMindex (FFMI) was calculated as FFM divided by height squared, and the BFM index(BFMI) was calculated as BMI minus FFMI. BMI was strongly correlated with bothBFMI and FFMI, but the correlation was stronger for BFMI than for FFMI. The studyfound a U-shaped association between BMI and all-cause mortality, a slight J-shapedassociation between BFMI and mortality, and a reverse J-shaped association between