Longitudinal changes in adult fat-free mass: influence of body weight

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Abstract
Background: Almost all of the data on changes in body composition during aging are cross-sectional in nature. These show that fat-free mass (FFM) declines with age. Objective: The goal was to analyze results of assays done in the author's laboratory of the FFM of normal adults studied at intervals for ≥2 decades to ascertain longitudinal changes. Design:40K counting was used to estimate FFM in adult university personnel (15 men, 5 women) over periods ranging from 21 to 38 y. No advice was given about diet or exercise. Results: There was considerable variation in the change of FFM over time. Some subjects lost FFM as the years went by, whereas others actually gained FFM. Analysis of the data showed that change in body weight was a prime factor in determining the magnitude and direction of the FFM change (R2 = 0.54). Adults who maintained their weight lost an average of 1.5 kg FFM per decade and so gained an equal amount of fat; those who lost weight lost even more FFM, whereas those who gained weight either gained FFM or lost it more slowly than the others. Data from the literature confirmed this trend. Conclusions: FFM loss is not inevitable during adulthood—at least up to age 81 y, the oldest age yet studied. The magnitude and direction of the FFM change, be it positive or negative, is strongly influenced by change in body weight.