Age-Related Differences in Carbohydrate and Fat Oxidation Over 24 Hours and During Daily Activities in Humans

The effects of older age on the oxidation of energy substrates (carbohydrates, fats) over 24 hr and during activities of daily living (ADL) may have substantial implications for metabolic health. To examine this issue, we evaluated age-related differences in substrate oxidation for 24 hr and periods of sleep, ADLs, and a 30-min treadmill walk (30MTW, 1.3m{middle dot}s^-1) in 17 young (29 {+/-} 5 yr, mean {+/-} SD, 6F) and 16 older (72 {+/-} 5, 8F) healthy active adults. Oxygen consumption (&VdotO2) and carbon dioxide production were measured using a 32,500 L room calorimeter to quantify substrate oxidation, which was normalized to body mass (kJ{middle dot}kg^-1) and total energy expenditure (%). Separately, &VdotO2peak was measured during an incremental treadmill test to exhaustion. &VdotO2peak (ml{middle dot}kg^-1{middle dot}min^-1) was lower in older adults compared with young adults. Older adults oxidized fewer carbohydrates than young over 24 hr (kJ{middle dot}kg^-1, %), sleep (kJ{middle dot}kg^-1), and ADLs (kJ{middle dot}kg^-1, %); and fat oxidation was greater in older than young over 24 hr (kJ{middle dot}kg^-1, %) and ADLs (kJ{middle dot}kg^-1, %). Carbohydrate oxidation did not differ by group during the 30MTW (kJ{middle dot}kg^-1, %), but fat oxidation was greater in older (kJ{middle dot}kg^-1) despite their walking at a greater relative intensity. These results indicate that older adults derived oxidative energy differently than young over 24 hr and during daily tasks. The lower carbohydrate and greater fat contribution to daily energy requirements may be meaningful in the development of metabolic disease in older adults.