The effects of pre-exercise glycemic index food on running capacity

Journal Title (Medline/Pubmed accepted abbreviation): Int J Sports Med
Year: 2011
Volume: 32
Page number: 666-671
doi : 10.1055/s-0031-1277180

Summary of background and research design:

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.

Summary of research findings:
  • Time to exhaustion was 23% longer for the low-GI group than for the placebo group (90.0 and 73.0 min, respectively; P < .05).
    • No difference was observed between high GI and placebo (81.8 and 73.0 min, respectively).
    • The difference between high and low GI diets was not significant.
    • No order effects on exercise performance were observed between first, second, and third trials.
  • VO2max was similar in all conditions during the first 30 minutes of running.
    • VO2max was higher in high GI vs placebo from 45 minutes to exhaustion (P < .01).
    • There were no differences among conditions for respiratory exchange ratios or HR.
  • Serum glucose levels rose 15 minutes after food ingestion and then lowered to varying degrees during the first 15 minutes of exercise before rising again.
    • At 15 minutes after food ingestion, glucose levels in the high-GI group were highest (P < .05 for comparisons with both low GI and placebo).
    • At exhaustion, glucose levels in the low-GI group were the highest (P < .05 for both high GI and placebo).
  • Insulin response was only different between the groups at 15 minutes after food ingestion (high-GI group higher than placebo group; P < .001).
    • Glycerol response was only different from 60 minutes to exhaustion (placebo group higher than low-GI group; P < .05).
    • Carbohydrate oxidation was only different at 30 and 45 minutes of exercise (high-GI group higher than low-GI group at 30 and 45 min of exercise [P < .05] and higher than placebo at 30 min [P < .05]).
    • Lactate levels and fat oxidation were not different between groups.

Interpretation of findings/Key practice applications:

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.
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