Objective: Whole body protein turnover (PTO) and resting energy expenditure (REE) are both correlated to fat-free mass (FFM), in young and elderly subjects, and REE is positively correlated to PTO in young adults. Thus, the aim of this study was to compare the energetic cost of PTO in young (n=39, 23.4+/-3.1 y) and elderly (n=41, 67.5+/-3.6 y) healthy volunteers.
Measurements: REE (indirect calorimetry), PTO ((13)C-leucine isotopic dilution) and body composition (bioelectrical impedance analysis with age-specific equations) were measured in the postabsorptive state.
Results: Elderly subjects had a higher fatness (30.5+/-7.1 vs 18.2+/-5.5%, elderly vs young, P<0.001), a similar REE (0.97+/-0.13 vs 1.06+/-0.15 kcal min(-1)), and a lower PTO (1.28+/-0.22 vs 1.44+/-0.18 micromol kg(-1) min(-1), P<0.001). PTO, REE and FFM were significantly correlated and after adjustment for FFM, REE was positively correlated to PTO (r=0.61, P<0.001). The slope of this relationship was the same in both groups, while the adjusted mean REE was lower in elderly subjects (0.97+/-0.09 vs 1.05+/-0.07 kcal min(-1), P<0.01).
Conclusion: In comparison with young subjects, the energetic cost associated with PTO in elderly subjects is not different, but the proportion of REE not associated with PTO is lower.