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

Exercise Physiology Laboratory, Department of Integrative Biology, University of California, Berkeley, California 94720-3140 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 ( O 2 peak )] in healthy female subjects ( n = 17; age 23.8 ± 2.0 yr). Two pretraining trials (45 and 65% of O 2 peak ) and two posttraining trials [same absolute workload (65% of old O 2 peak ) and same relative workload (65% of new O 2 peak )] were performed on nine subjects by using a primed-continuous infusion of [1- 13 C]- and [6,6- 2 H]glucose. Eight additional subjects were studied by using [6,6- 2 H]glucose. Subjects were studied postabsorption for 90 min of rest and 1 h of cycling exercise. After training, subjects increased O 2 peak by 25.2 ± 2.4%. Pretraining, the intensity effect on glucose kinetics was evident between 45 and 65% of O 2 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 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 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. stable isotopes; substrate utilization; exercise; glucose metabolism; menstrual cycle; crossover concept; glycogen; lactate