Effects of training duration on substrate turnover and oxidation during exercise

Phillips, S. M., H. J. Green, M. A. Tarnopolsky, G. J. F. Heigenhauser, R. E. Hill, and S. M. Grant. Effects of training duration on substrate turnover and oxidation during exercise. J. Appl. Physiol. 81(5):2182-2191, 1996.Adaptations in fat and carbohydrate metabolismafter a prolonged endurance training program were examined using stableisotope tracers of glucose([6,6-²H₂]glucose),glycerol([²H₅]glycerol),and palmitate([²H₂]palmitate).Active, but untrained, males exercised on a cycle for 2 h/day[60% pretraining peak O₂consumption (O_{2 peak}) = 44.3 ± 2.4 ml ^· kg^{1 ·} min¹]for a total of 31 days. Three cycle tests (90 min at 60% pretraining O_{2 peak}) wereadministered before training (PRE) and after 5 (5D) and 31 (31D) daysof training. Exercise increased the rate of glucose production(R_a) and utilization(R_d) as well as the rate oflipolysis (glycerol R_a) and freefatty acid turnover (FFA R_a/R_d).At 5D, training induced a 10% (P < 0.05) increase in total fat oxidation because of an increase inintramuscular triglyceride oxidation (+63%,P < 0.05) and a decreased glycogenoxidation (16%, P < 0.05).At 31D, total fat oxidation during exercise increased a further 58%(P < 0.01). The pattern of fatutilization during exercise at 31D showed a reduced reliance on plasmaFFA oxidation (FFA R_d) and agreater dependence on oxidation of intramuscular triglyceride, whichincreased more than twofold (P < 0.001). In addition, glucose R_aand R_d were reduced at all timepoints during exercise at 31D compared with PRE and 5D. We concludethat long-term training induces a progressive increase in fatutilization mediated by a greater oxidation of fats from intramuscularsources and a reduction in glucose oxidation. Initial changes arepresent as early as 5D and occur before increases in muscle maximalmitochondrial enzyme activity [S. M. Phillips, H. J. Green, M. A. Tarnopolsky, G. J. F. Heigenhauser, and S. M. Grant.Am. J. Physiol. 270 (Endocrinol. Metab. 33):E265-E272, 1996].