Effect of low- and high-glycemic-index meals on metabolism and performance during high-intensity, intermittent exercise
Journal Title (Medline/Pubmed accepted abbreviation): Int J Sports Nutr Exerc Metab
Year: 2010
Volume: 20
Number: 6
Page numbers: 447-456
doi (if applicable): N/A

Summary of Background and Research Design

Background:Ingestion of a carbohydrate-rich meal several hours before an endurance sport allows for optimal levels of liver and muscle glycogen while exercising. Carbohydrates are the preferred source of fuel for the muscle tissue, and sufficient endogenous carbohydrate stores may delay the onset of fatigue during prolonged exercise. However, the ideal preexercise meal to provide the most advantageous carbohydrate levels has not been established. As insulin has a key role in inhibiting skeletal muscle fat oxidation (alternative fuel source for muscles), a low–glycemic-index (GI) meal may be preferred. This meal would result in lower insulin levels compared with a high-GI meal and may promote immediate fat oxidation, preserving the endogenous carbohydrates for the exercise period. Clinical evidence, thus far, has produced conflicting results regarding the value of low- vs high-GI pre-exercise meals for athletes.

Hypothesis/purpose of study:The hypothesis was that both high- and low-GI meals would improve exercise performance compared with fasting, but that the low-GI meal would be superior to the high-GI meal.

Subjects:16 male athletes (age, 22.8 ± 3.2 yr; peak treadmill speed [Vmax], 17.9 ± 1.7 km/hr) who had experience playing soccer before this study participated in the study. Only 2 men had muscle biopsies; therefore, 3 additional males were recruited for the study and had muscle biopsies only (same protocol, but other metabolic and performance data not obtained).

Experimental design:Randomized, single-blind, counter-balanced, cross-over study

Treatments and protocol:Volunteers completed 5 visits over 6 weeks: baseline assessment, preliminary testing, and 3 randomized experimental exercise trials 7 days apart. Each experimental trial had a different meal condition: low GI (~26), high GI (~76), and fasted control. The meals were balanced for energy, carbohydrate, fat, and protein content, and exercise commenced 2 hours after the subjects completed eating the test meal. The exercise trial consisted of two 45?minute sessions of high-intensity intermittent running on a treadmill (15-min blocks with a total of 6 walking, 6 jogging, 3 running, and 8 sprint intervals with blood drawn after each block) separated by 15 minutes of rest. The last 15-minute block of the second session assessed exercise performance and consisted of five 1-minute sprints with 2.5-minute recoveries in-between. Blood samples for carbohydrate and fat oxidation were collected at baseline, 15, 30, 60, and 120 minutes following meal ingestion, and every 15 minutes during exercise. Expired-gas volumes were assessed during the first, third, and fifth 15-minute blocks. Muscle biopsies (vastus lateralis) were obtained before the last 15-minute block of exercise for determination of muscle glycogen.

Summary of research findings:
  • There was a significant effect for total sprint distance (last exercise block), insulin levels, and free fatty acids by meal condition (P < .01, P < .001, and P < .001, respectively).
    • Total sprint distance was significantly longer in the low- and high-GI test compared with the control test (P = .01 and P = .04, respectively).
    • Insulin levels were higher and free fatty acids were lower in both the low- and high-GI tests compared with the control test.
  • Muscle glycogen stores were higher in both the low- and high-GI tests compared with the control test (P < .05 for both). However, there were no differences between the low- and high-GI test meals.
  • Rate of fat oxidation was lower in both the low-GI (third 15-min block) and high-GI (second 15-min block) tests compared with the control test (P = .01 and P = .005, respectively).
  • During exercise, mean oxygen uptake, respiratory-exchange ratio, glucose levels, and lactate levels were not different between meal conditions.
    • There was a main effect of time for respiratory-exchange ratio and lactate levels during sprints (P < .001 for both).
  • Ratings of perceived exertion (RPE) during exercise were significantly lower for the low-GI meal vs control. There were no differences, though, between low- and high-GI meals.

Interpretation of findings/Key practice applications:

Both low- and high-GI preexercise meals enhanced performance and glycogen availability compared with fasting during high-intensity intermittent exercise. There did not appear to be any differences in sprint performance between the 2 GI meals, although a low-GI meal presumably increases fat oxidation and decreases reliance on muscle glycogen for energy allowing for longer endurance. Although baseline pre-exercise muscle glycogen levels were not reported, the higher, albeit not statistically significant, muscle glycogen levels in the low- versus high-GI condition are notable. In this study, fat oxidation was decreased in both low- and high-GI tests compared with fasting. This finding has been observed in a number of studies and generally reflects the lack of any carbohydrate substrate in the fasting group. 
One key point of note is that the low- and high-GI meals were fed approximately 2 hours prior to the start of exercise. As can be observed in Figure 3, the expected alterations in blood glucose from each meal had already fully occurred by the time exercise started. Thus, it is perhaps not surprising that significant effects on performance did not occur. It would have been interesting to see a shorter time lag (eg, 30-45 min) between the meal and the beginning of exercise. As another potential explanation for these findings, the carbohydrate-rich meal may have provided enough stores to make the differences in GI negligible. In addition, carbohydrates were not provided during exercise as in other studies and could have resulted in different findings. Nonetheless, soccer players may be advised to ingest a pregame carbohydrate-rich meal regardless of GI index.
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