Training-induced alterations of carbohydrate metabolism in women: women respond differently from men

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 O-2 consumption ((V) over dot O-2 (peak))] in healthy female subjects (n = 17; age 23.8 +/- 2.0 yr). Two pretraining trials (45 and 65% of (V) over dot O-2 (peak)) and two posttraining trials [same absolute workload (65% of old (V) over dot O-2 (peak)) and same relative workload (65% of new (V) over dot O-2 (peak))] were performed on nine subjects by using a primed-continuous infusion of [1-C-13]- and [6,6-H-2]glucose. Eight additional subjects were studied by using [6,6-H-2]glucose. Subjects were studied postabsorption for 90 min of rest and 1 h of cycling exercise. After training, subjects increased (V) over dot O-2 (peak) by 25.2 +/- 2.4%. Pretraining, the intensity effect on glucose kinetics was evident between 45 and 65% of VO2 (peak) with rates of appearance (R-a: 4.52 +/- 0.25 vs. 5.53 +/- 0.33 mg.kg(-1).min(-1)), disappearance (R-d: 4.46 +/- 0.25 vs. 5.54 +/- 0.33 mg.kg(-1).min(-1)), and oxidation (R-ox: 2.45 +/- 0.16 vs. 4.35 +/- 0.26 mg.kg(-1).min(-1)) of glucose being significantly greater (P less than or equal to 0.05) in the 65% than in the 45% trial. Training reduced R-a (4.7 +/- 0.30 mg.kg(-1).min(-1)), R-d (4.69 +/- 0.20 mg.kg(-1).min(-1)), and R-ox (3.54 +/- 0.50 mg.kg(-1).min(-1)) at the same absolute workload (P less than or equal to 0.05). When subjects were tested at the same relative workload, R-a, R-d, and R-ox 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.