Abstract
We examined the hypothesis that glucose flux was directly related to relative exercise intensity both before and after a 12-wk cycle ergometer training program [5 days/wk, 1-h duration, 75% peak O2 consumption (V˙o 2 peak)] in healthy female subjects ( n = 17; age 23.8 ± 2.0 yr). Two pretraining trials (45 and 65% of V˙o 2 peak) and two posttraining trials [same absolute workload (65% of oldV˙o 2 peak) and same relative workload (65% of newV˙o 2 peak)] were performed on nine subjects by using a primed-continuous infusion of [1-13C]- and [6,6-2H]glucose. Eight additional subjects were studied by using [6,6-2H]glucose. Subjects were studied postabsorption for 90 min of rest and 1 h of cycling exercise. After training, subjects increasedV˙o 2 peak by 25.2 ± 2.4%. Pretraining, the intensity effect on glucose kinetics was evident between 45 and 65% ofV˙o 2 peak with rates of appearance (Ra: 4.52 ± 0.25 vs. 5.53 ± 0.33 mg ⋅ kg−1 ⋅ min−1), disappearance (Rd: 4.46 ± 0.25 vs. 5.54 ± 0.33 mg ⋅ kg−1 ⋅ min−1), and oxidation (Rox: 2.45 ± 0.16 vs. 4.35 ± 0.26 mg ⋅ kg−1 ⋅ min−1) of glucose being significantly greater ( P ≤ 0.05) in the 65% than in the 45% trial. Training reduced Ra (4.7 ± 0.30 mg ⋅ kg−1 ⋅ min−1), Rd (4.69 ± 0.20 mg ⋅ kg−1 ⋅ min−1), and Rox (3.54 ± 0.50 mg ⋅ kg−1 ⋅ min−1) at the same absolute workload ( P ≤ 0.05). When subjects were tested at the same relative workload, Ra, Rd, and Rox were not significantly different after training. However, at both workloads after training, there was a significant decrease in total carbohydrate oxidation as determined by the respiratory exchange ratio. These results show the following in young women: 1) glucose use is directly related to exercise intensity; 2) training decreases glucose flux for a given power output; 3) when expressed as relative exercise intensity, training does not affect the magnitude of blood glucose flux during exercise; but 4) training does reduce total carbohydrate oxidation.