Age-related Differences In Substrate Oxidation During 24 Hours In A Whole-room Indirect Calorimeter

Previous studies of age-related differences in substrate oxidation over 24 hours and during exercise have provided divergent results, leaving it unclear whether substrate oxidation is influenced by aging. PURPOSE: Evaluate age-related differences in substrate oxidation during distinct periods of a 24-hr stay in a room calorimeter: over the full 24 hours, during activities of daily living (ADLs), and during a 30-min treadmill walk at 1.3 m·s-1 (30MTW). METHODS: Peak O2 uptake (VO2peak) was determined from an incremental treadmill test in 9 young (Y, 28 ± 3 yr) and 6 older (O, 71 ± 5 yr) healthy males. On a separate visit, oxygen consumption (VO2) and carbon dioxide production (VCO2) were measured throughout a 24-hr stay in a room calorimeter and used to determine substrate oxidation from carbohydrate (CHO) and fat for the selected periods. Energy expenditure (EE), CHO oxidation, and fat oxidation were normalized to lean mass (kJ·kg LM-1), determined by dual energy x-ray absorptiometry. Statistics: Group differences were assessed with unpaired t-tests. RESULTS: As expected, VO2peak (ml·kg-1 LM·min-1) was ~24% lower in O than Y (p = 0.021). EE (kJ·kg LM-1) did not differ by group for any period (p > 0.099). Compared with Y, O oxidized less CHO over 24 hr (p < 0.012) and during ADLs (p < 0.001) on an absolute basis and relative to LM, while fat oxidation did not differ by group during either of these periods (p > 0.053). In contrast, during the 30MTW CHO oxidation did not differ by group (p > 0.148), but O oxidized more fat per LM than Y (p = 0.002). The contrast in substrate selection between the lower-intensity ADLs (~16 and 20% VO2peak in Y and O) and the 30MTW (~28 and 39% VO2peak in Y and O) is consistent with a crossover from predominantly fat to CHO at a relatively higher VO2peak (relative to LM and %peak) in O. Notably, when expressed as a percent of total substrate oxidation, O oxidized relatively more fat and less CHO over 24 hr (fat: Y = 52 ± 8%, O = 63 ± 5%; p = 0.012), ADLs (fat: Y = 58 ± 8%, O = 76 ± 7%; p < 0.001), and the 30MTW (fat: Y = 39 ± 8%, O = 50 ± 8%; p = 0.027). CONCLUSION: Our results provide novel data regarding age-related differences in substrate use during activities that might occur over a 24-hr period in healthy males. Older males showed a consistent preference for fat as a substrate, which was reflected in a rightward shift of their “crossover point.”