Journal Title (Medline/Pubmed accepted abbreviation): Int J Sports Med
Page number: 666-671
doi : 10.1055/s-0031-1277180
Background: Plasma glucose levels reach a maximum concentration approximately 30 to 45 minutes after food ingestion, resulting in insulin secretion and rapid reduction in plasma glucose levels. Exercise during this critical period may further reduce plasma glucose levels and lead to hypoglycemia. Type of food ingested (low or high glycemic index [GI]) may also influence plasma glucose levels. However, clinical evidence for both timing of food ingestion and low or high GI is conflicting and has been primarily evaluated during cycling.
Hypothesis: Low-GI food will reduce glycemic and insulin responses and provide a lower, but longer-lasting, glucose availability that can enhance running capacity.
Subjects: Nine recreational runners without impaired glucose tolerance participated in this study (mean age, 26.0 ± 2.9 yr; height, 174.7 ± 3.1 cm; weight, 74.4 ± 2.1 kg; maximal oxygen consumption [VO2max ], 48.9 ± 1.9 mL/kg/min; and body fat, 14.2% ± 2.0%).
Experimental design: Randomized, blinded, cross-over
Treatments Protocol: Each runner maintained a constant diet and was randomized to a high- or low-GI meal (potatoes vs. lentils, respectively, 1 g carbohydrate/kg body mass for each) or placebo (0.05 g aspartame/kg body mass) as a 400-mL volume 15 minutes before a treadmill running test. The other 2 conditions were then analyzed, with a 7-day interval between testing. The treadmill test consisted of running 5 minutes at 60% VO2max (determined before testing), 45 minutes at 70% VO2max, and time to exhaustion at 80% VO2max (maintenance of designated speed). Expired air samples for carbon dioxide consumption (VCO 2) and VO2 (to calculate total carbohydrate and fat oxidation rates) were collected before the meal, before exercise, and every 15 minutes during exercise. Heart rate (HR) was monitored continuously. Blood samples for glucose, glycerol, lactate, and insulin were collected before the meal, before exercise, every 15 minutes during exercise, and at exhaustion.
In this study, low-GI foods ingested 15 minutes before exercise prevented hyperinsulinemia, maintained blood glucose concentrations, and resulted in higher endurance running capacity. Most likely, the short interval between low-GI food ingestion and exercise initiation normalizes the action of insulin and glucose, which may preserve carbohydrate energy and better mobilize glycerol for fat oxidation (if necessary). However, as both timing and GI value of food are important for physiologic responses and exercise performance, these factors should also be evaluated.