INTRODUCTION/PURPOSE: Fat metabolism and muscle adaptation was investigated in 6 older trained men (age: 61 ± 4 years; VO2max: 48 ± 2 mL kg min) following repeated prolonged exercise).

METHODS: 2706 km (1,681 miles) cycling was performed over 14 days and a blood sample and a muscle biopsy were obtained at rest after an overnight fast before and 30 hours after the completion of the cycling. VO2-max and maximal fat oxidation were measured using incremental exercise tests. Heart rate was continuously sampled during cycling to estimate exercise intensity.

RESULTS: The daily duration of exercise was 10 hours and 31 ± 37 min and the mean intensity was 53 ± 1 % of VO2max. Body weight remained unchanged. VO2max and maximal fat oxidation rate decreased by 6 ± 2 % (P = 0.04) and 32 ± 8 % (P < 0.01), respectively. The exercise intensity that elicits maximal fat oxidation was not significantly decreased. Plasma free fatty acid (FA) concentration decreased (P < 0.002) from 500 ± 77 to 160 ± 38 μmol L. Plasma glucose concentration as well as muscle glycogen, myoglobin and triacylglycerol content remained unchanged. Muscle citrate synthase (CS)- and ß-hydroxy-Acyl-CoA-dehydrogenase (HAD)- activity were unchanged, but the protein expression of HKII, GLUT4 and ATGL were significantly increased.

CONCLUSION: Overall the decreased maximal fat oxidation was probably due to lower exogenous plasma FA availability and the muscle adaptation pattern indicates an increased glucose transport capacity and an increased muscle lipolysis capacity supporting an increased contribution of exogenous glucose and endogenous fat during exercise.