BACKGROUND: Few data exist on the metabolic responses to mixed meals with different glycemic indexes and their effects on substrate metabolism during exercise in women. OBJECTIVE: We examined the effects of preexercise mixed meals providing carbohydrates with high (HGI) or low glycemic index (LGI) on substrate utilization during rest and exercise in women. DESIGN: Eight healthy, active, eumenorrheic women [aged 18.6 +/- 0.9 y; body mass: 59.9 +/- 7.1 kg; maximal oxygen uptake (VO(2)max): 48.7 +/- 1.1 mL . kg(-1) . min(-1)] completed 2 trials. On each occasion, subjects were provided with a test breakfast 3 h before performing a 60-min run at 65% VO(2)max on a motorized treadmill. Both breakfasts provided 2 g carbohydrate/kg body mass and were isoenergetic. The calculated GIs of the meals were 78 (HGI) and 44 (LGI). RESULTS: Peak plasma glucose and serum insulin concentrations were greater after the HGI breakfast than after the LGI breakfast (P < 0.05). No significant differences in substrate oxidation were reported throughout the postprandial period. During exercise, the estimated rate of fat oxidation was greater in the LGI trial than in the HGI trial (P < 0.05). Similarly, plasma free fatty acid and glycerol concentrations were higher throughout exercise in the LGI trial (P < 0.05). No significant differences in plasma glucose or serum insulin were observed during exercise. CONCLUSION: Altering the GI of the carbohydrate within a meal significantly changes the postprandial hyperglycemic and hyperinsulinemic responses in women. A LGI preexercise meal resulted in a higher rate of fat oxidation during exercise than did an HGI meal.
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