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Age-related Differences in Substrate Oxidation but not Exercise Oxygen Transients During 24 Hours in a Whole-room Calorimeter

Previous studies of age-related differences in substrate oxidation over 24 hours and during exercise have provided divergent results, leaving it unclear how and if substrate oxidation is influenced with aging. The purpose of this study was to evaluate age-related differences in substrate oxidation during distinct periods of a 24-hr stay in a room calorimeter: over the entire 24 hours, during activities of daily living (ADLs), and during a 30-min treadmill walk (30MTW, at 1.3 m·s-1). Further, oxygen consumption transients were quantified in response to the 30MTW. Gas exchange measures were obtained during an incremental treadmill test in 9 young (28 + 3 yr) and 6 older (71 + 5 yr) healthy males to quantify VO2peak. On a separate visit, oxygen consumption (VO2) and carbon dioxide production (VCO2) were measured throughout a 24-hr stay in a whole-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. Oxygen consumption on- (oxygen deficit, mL) and off- (rate constant for VO2 decay) transients were determined in response to the 30MTW. As expected, VO2peak (ml∙kg-1 LM∙min-1) was ~24% lower in older compared to young (p=0.021). EE (kJ∙kg LM-1) did not differ significantly by group for any period (p>0.099). Compared with young, older oxidized less CHO over 24 hr (p0.053). In contrast, during the 30MTW CHO oxidation did not differ by group (p>0.148), but older oxidized more fat per LM than young (p=0.002). The contrast in substrate selection between the lower-intensity ADLs (~16 and 20% VO2peak in young and older) and the 30MTW (~28 and 39% VO2peak in young and older) is consistent with a crossover from predominantly fat to CHO at a relatively higher VO2peak (relative to LM and %peak) in this group of relatively fit older males. Notably, when expressed as a percent of total substrate oxidation, older oxidized relatively more fat and less CHO over 24 hr (p=0.012), ADLs (p