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The belief that high-carbohydrate diets enhance training capacity (mean power output) has been extrapolated from studies that have varied dietary carbohydrate over a few days and measured muscle glycogen but did not assess power output during training. We hypothesized that a high-carbohydrate (HI) diet (10 g.kg body mass-1.day-1) would promote greater muscle glycogen content and greater mean power output during training than a moderate-carbohydrate (MOD) diet (5 g.kg body mass-1.day-1) over 4 wk of intense twice-daily rowing training. Dietary protein intake was 2 g.kg body mass-1.day-1, and fat intake was adjusted to maintain body mass. Twelve male and 10 female collegiate rowers were randomly assigned to the treatment groups. Training was 40 min at 70% peak O2 consumption (VO2) (A.M.) and either three 2,500-m time trials to assess power output or interval training at 70-90% peak VO2 (P.M.). Mean daily training was 65 min at 70% peak VO2 and 38 min at greater than or equal to 90% peak VO2. Mean muscle glycogen content increased 65% in the HI group (P less than 0.05) but remained constant at 119 mmol/kg in the MOD group over the 4 wk. Mean power output in time trials increased 10.7 and 1.6% after 4 wk in the HI and MOD groups, respectively (P less than 0.05). We conclude that a diet with 10 g carbohydrate.kg body mass-1.day-1 promotes greater muscle glycogen content and greater power output during training than a diet containing 5 g carbohydrate.kg body mass-1.day-1 over 4 wk of intense twice-daily rowing training.(ABSTRACT TRUNCATED AT 250 WORDS)

In untrained subjects, strenuous exercise provokes the appearance of oxidant stress markers in blood and muscle. On the other hand, trained muscle is resistant to oxidant stress unless exercise challenges the muscle glycogen supply. It is not known whether chronic high-intensity exercise alters the susceptibility of skeletal muscle to oxidant stress, whether there are gender-related differences in markers of oxidant stress, or whether elevating muscle glycogen stores by increasing dietary carbohydrate can minimize any exercise-related oxidant stress. To address these issues, collegiate rowers (12 men, 11 women) were randomly assigned to a moderate-(MOD, 5 g/kg body wt) or high-carbohydrate (HI, 10 g/kg) diet in a double-blind design and underwent strenuous training for 4 wk. Training in the A.M. was 40 min at 70% maximal O2 consumption (VO2); in the P.M. it was either three 2,500-m time trials (to assess power output) or aerobic and lactate tolerance training. Total daily training time was 65 min at 70% maximal VO2 and 38 min at > or = 90% maximal VO2. Thrice-weekly morning blood samples were assayed for serum creatine kinase (CK), plasma thiobarbituric acid-reactive substances (TBARS), and serum beta-glucuronidase (beta-Gluc). Weekly muscle biopsies were obtained for analysis of glycogen and, when tissue sample quantity allowed, TBARS. HI rowers produced more power and improved power more (10.7 +/- 1.0 vs. 1.6 +/- 1.6%) over the 4 wk than did the MOD rowers. Preexercise muscle glycogen concentration was maintained at 119 mmol/kg in MOD but increased 65% in HI rowers (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

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Because muscle damage from eccentric exercise has been associated with alterations in muscle glycogen metabolism, this study determined the effects of exercise on the insulin and glucose responses to an oral glucose tolerance test (OGTT). In a repeated-measures design, 11 subjects undertook either no exercise, 2 min of isokinetic leg exercise, or 50 min of level or downhill running. No exercise was performed and diet was controlled during the 48 hrs after the treatments and before the OGTT. Ratings of muscle soreness and CK activity were significantly elevated 48 hrs after downhill running. Level running also increased CK activity but did not induce muscle soreness. Isokinetic exercise did not affect either one. Blood glucose responses to the OGTT were similar among the treatments. In contrast, the insulin responses to the OGTT following downhill running were significantly increased. These results suggest that eccentric exercise associated with downhill running that results in delayed muscle soreness is associated with the development of a mild insulin-resistant condition.