Endurance exercise has been shown to both raise and lower leucine oxidation in studies in rodents. We studied the effects of a 38 d endurance exercise training program upon leucine turnover during a 90 min exercise bout at 60 % VO_2peak in 6 males and 6 females. Subjects were studied at both the same absolute (ABS) and relative (REL) exercise intensities post-training. Pre (PRE)- and post-training measurements were taken for analysis of: L-[1-^13C]leucine turnover, muscle branched-chain oxoacid dehydrogenase activity (BCOAD), muscle glycogen, phosphocreatine and ATP utilization, and resting enzyme activity of citrate synthase (CS) and NADH-cytochrome c-oxidoreductase (complex I-III). We also determined total substrate oxidation by indirect calorimetry, and plasma lactate, glucose, and insulin concentrations. The exercise training resulted in a significant increase in both CS (P < 0.001) and complex I- III (P < 0.05) activities. Leucine oxidation increased during exercise for the pre-training trial (P < 0.001), however, there was no increase for either the post-training ABS or REL trial. Leucine oxidation was significantly lower for females at all time points (P < 0.01). Total BCOAD activity was also significantly increased when comparing the PRE to both ABS and REL trials (P < 0.001). The % activation of BCOAD was significantly increased from t=0 to t=90 in both the PRE and REL exercise trials with the increase in PRE being greater (P < 0.001 (PRE), and P < 0.05 (REL)). Exercise RER was lower for females vs. males (P< 0.05). In addition, the ABS trial was significantly lower than PRE and REL (P < 0.01). Plasma lactate was significantly lower at all time points for ABS vs. PRE (P < 0.001) and REL vs. PRE at t=30 min of exercise (P < 0.001). Resting muscle glycogen was higher for both ABS and REL vs. PRE (P < 0.001). In conclusion, we found that 38 d of endurance exercise training significantly attenuated both leucine oxidation and BCOAD activation during 90 min of endurance exercise at 60 % VO_2peak for both absolute and relative exercise intensities. In addition, females were also shown to oxidize a greater proportion of energy from lipid and a lesser amount from carbohydrates and proteins during exercise. / Thesis / Master of Science (MS)