Journal Title (Medline/Pubmed accepted abbreviation): J Int Soc Sports Nutr
Background: Carbohydrate (CHO) availability during exercise may be a key factor in preserving muscle glycogen levels via increased glucose uptake during prolonged exercise performance. Glycemic index (GI) ranks food according to the effect on blood glucose levels, and some clinical studies suggest that low GI foods increase CHO availability and exercise performance compared with high GI foods. However, overall clinical evidence is conflicting.
Hypothesis/Purpose: The purpose of this study was to evaluate the effects of food with various GI values on exercise performance, β-endorphin levels, and nutrient utilization during prolonged exercise.
Subjects: Eight untrained, healthy males participated in this study (mean age, 22.8 ± 3.6 y; height, 174.1 ± 4.2 cm; body mass, 75.1 ± 5.2 kg; body fat percentage, 10.6% ± 3.4%; maximal oxygen consumption [VO2max], 45.9 ± 6.4 mL/kg/min).
Experimental design: Randomized, cross-over
Treatments and protocol:
Each male maintained a constant diet and was randomized to either a high-GI (white bread + jam; GI = 70) or low-GI meal (dried apricots; GI = 30; 1.5 g CHO/kg body mass for each) plus 300 mL of water or 300 mL of water alone 30 minutes before an ergometer cycling test. The other 2 conditions were then analyzed, with a 7-day interval between testing. The cycling test consisted of 1 hour at 65% VO2max (determined before testing) and time to exhaustion at 90% VO2max (resistance increased and maintenance of 60 rpm). Expired air samples for carbon dioxide consumption [VCO2] and VO2 (to calculate total CHO and fat oxidation rates) and rating of perceived exertion (RPE) were collected every 10 minutes. Heart rate (HR) was monitored continuously. Blood samples for glucose, lactate, β-endorphin, and plasma volume changes (calculated from hematocrit and hemoglobin levels) were collected before the meal, at 15 and 30 minutes after the meal, every 20 minutes during exercise, and at exhaustion.
In this study, foods with different GIs did not influence exercise performance or CHO and fat oxidation during exercise, even though the glucose and insulin levels responded accordingly to the type of CHO ingested. However, the men in this study were not athletes, and accurate performance and metabolic changes may not be able to be measured in this population. In addition, the timing of low- or high-GI food intake and amount of CHO per kg of body weight may influence metabolic changes and, therefore, exercise performance. The 1.5-kg CHO/kg body mass in this study did not elicit a β-endorphin response.