BODY-COMPOSITION STUDY IN THE ELDERLY - COMPARISON BETWEEN TRITIUM DILUTION METHOD AND DUAL-PHOTON ABSORPTIOMETRY

Background. The methods commonly employed in body composition study in elderly persons derive fat-free mass (FFM) by measures of total body water (TBW); these methods assume FFM hydration constant (TBW/FFM = 0. 732). The aim of our study was to verify the accuracy of the TBW metho d in FFM estimation and to study the variability of fat-free mass hydr ation (TBW/FFM) in healthy elderly subjects. Methods. We assessed FFM in 27 healthy old subjects (76 +/- 7 yrs) by dual photon absorptiometr y (DPA) and by TBW measured by (H2O)-H-3. Furthermore, to verify the a ccuracy in FFM estimation by our methods, we measured resting metaboli c rate (RMR) by indirect calorimetry. Results. FFM hydration was 71.9 +/- 4.9 (range 63.6-80.4%), a range larger than that reported by direc t chemical analysis in adults. The FFM hydration measured in our subje cts was not different from the 73.2 value currently used in healthy ad ults. FFM values derived by TBW (assuming TBW/FFM = 0.732) were not si gnificantly different from FFM measured by DPA. We correlated FFM with resting metabolic rate and found a similar correlation between RMR an d FFM-DPA (r = .89, p < .001) and FFM-TBW (r = .85, p < .001). Conclus ions. Both methods employed in our study have proved to be accurate in estimating FFM in elderly persons; particularly, the value of 0.732 a ssigned to FFM hydration to derive fat-free mass in adults can be empl oyed also in body composition study of the elderly.