We evaluated the effect of aging on fat and carbohydrate metabolism during moderate intensity exercise. Six elderly (73*2 yr) men and women were studied during 60 min of cycle ergometer exercise performed at 5&6*4.5 % of maximum oxygen uptake. Six young adults (26*2 yr), matched by gender and lean body mass (LBM), were studied on two occasions: during exercise performed at the same absolute and at a similar relative intensity as the elderly subjects. Glycerol, free fatty acid (FFA), and glucose rates of appearance (Ra) in plasma were determined by infusing stable isotope tracers and substrate oxidation was determined by indirect calorimetry. Mean fat oxidation during exercise was lower in the elderly subjects (3.43+0.27 umol.min-1~kg LBM-1) than in the young adults exercising at either the same absolute (4.27+0.96 umol.min-1.kg LBM-1) (P <0.001) or similar relative (4.65+1.02 umol.min-1.kg LBM-1) (P<0.001) intensifies. Mean carbohydrate oxidation in the elderly group (86.2+6.7 umol.min-kg LBM-1) was higher than the young adults exercising at the same absolute intensity (63.4+ 8.2 umol.min-1.kg LBM-1) (p<0.001) but lower than the young adults exercising at the same relative intensity (131.9+ 13.1 umol.min.-1.kg LBM-1) (P<0.001). Average glycerol and FFA Ra's in the elderly subjects (5.09+0.51 and 9.67+0.50 umol.min-kg LBM-1, respectively) were higher than in the young adults exercising at the same absolute intensity (4.01+0.37 and 6.49+0.54 umol.min-1.kg LBM-1, respectively) (P<0.001) but lower than the young adults exercising at a similar relative intensity (6.42+1.06 and 12.87+2.46 umol.min-1kg LBM-1, respectively) (P<0.001). Average glucose Ra during exercise was similar in both groups during exercise performed at the same absolute intensity, but was 25 % lower in the elderly than in the young subjects during exercise at a similar relative intensity (P<0.001). We conclude that fat oxidation is decreased while carbohydrate oxidation is increased during moderate intensity exercise in elderly men and women. The shift in substrate oxidation was caused by age-related changes in skeletal muscle respiratory capacity because lipolytic rates and free fatty acid availability were not rate limiting in the older subjects.

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