APPLICATION OF BIOELECTRICAL-IMPEDANCE ANALYSIS TO ELDERLY POPULATIONS

Background. Bioelectrical impedance analysis (BIA) can potentially be used to estimate body composition in large populations studied at mult iple sites. However, it is not clear whether age-specific BIA equation s are necessary for accurate application of BIA to research on elderly subjects. Methods. We compared a published equation designed to predi ct fat-free mass (FFM) that had been derived in a young healthy popula tion (mean age 27 y; mean BMI 23.9 kg/m(2)), with equations that we de veloped for the elderly by using data from 455 participants in the Fra mingham Heart Study (78 y; 27.3 kg/m(2)), using dual-energy x-ray abso rptiometry (DXA) as a reference technique. The BIA equations were then compared in an independent sample of 283 participants in the New Mexi co Aging Process Study (76 y, 25.5 kg/m(2)), who also underwent BIA an d DXA. Results. When the young-population equation was applied to Fram ingham, it caused an overestimation of FFM in heavier subjects that wa s eliminated by use of the age-specific equation. However, when the tw o equations were tested in the New Mexico population, the published eq uation gave estimates of FFM that were closer to DXA than the Framingh am equations did. Conclusions. The accuracy of a BIA equation depends on the body composition of the population and the validation method ra ther than on age per se. Application of BIA to elderly populations req uires uniform validation procedures in the actual study population, ra ther than reliance on age-specific equations.