FAT AND CARBOHYDRATE-METABOLISM DURING EXERCISE IN ELDERLY AND YOUNG SUBJECTS

We evaluated the effect of aging on fat and carbohydrate metabolism du ring moderate intensity exercise. Glycerol, free fatty acid (FFA), and glucose rate of appearance (R(a)) in plasma and substrate oxidation w ere determined during 60 min of cycle ergometer exercise in six elderl y (73 +/- 2 yr) and six young adults (26 +/- 2 yr) matched by gender a nd lean body mass. The elderly group was studied during exercise perfo rmed at 56 +/- 3% of maximum oxygen uptake, whereas the young adults w ere studied during exercise performed at the same absolute and at a si milar relative intensity as the elderly subjects. Mean fat oxidation d uring exercise was 25-35% lower in the elderly subjects than in the yo ung adults exercising at either the same absolute or similar relative intensities (P < 0.05). Mean carbohydrate oxidation in the elderly gro up was 35% higher than the young adults exercising at the same absolut e intensity (P < 0.001) but 40% lower than the young adults exercising at the same relative intensity (P < 0.001). Average FFA R(a) in the e lderly subjects was 35% higher than in the young adults exercising at the same absolute intensity (P < 0.05) but 35% lower than the young ad ults exercising at a similar relative intensity (P < 0.05). We conclud e that fat oxidation is decreased while carbohydrate oxidation is incr eased during moderate intensity exercise in elderly men and women. The shift in substrate oxidation was caused by age-related changes in ske letal muscle respiratory capacity because lipolytic rates and FFA avai lability were not rate limiting in the older subjects.