Endurance Sports Nutrition Articles & Resources

Glycemic index is a way of ranking foods on the actual postprandial blood glucose response, compared with a reference food, either glucose or white bread. Moderate to high glycemic index foods and drinks are considered appropriate during prolonged exercise. High glycemic index carbohydrates are considered the best choice to enhance glycogen storage after exercise by promoting greater glucose and insulin responses. Unfortunately, these recommendations can be difficult to follow since the glycemic index of foods often used by athletes has not been measured. Therefore, a recent study in the Journal of the American Dietetic Association measured the glycemic index of three types of sports foods, sports drinks, energy bars, and meal-replacement drinks. This study used glucose as its reference food, with a defined glycemic response of 100. Five apparently healthy subjects volunteered to participate in this study. After an overnight fast, blood glucose was measured at 0, 15, 30, 45, 60, 90 and 120 minutes after the start of consuming the test food. Each subject completed three tests, with the mean result being used as the value to calculate the glycemic index.

The concentration of carbohydrates in the sport drinks ranged from 4 percent to 20 percent and the resulting volumes ingested to provide 50 g of carbohydrate ranged from L to L. GatorLode had the same glycemic index as glucose, 100, while Allsport had the lowest glycemic index among the drinks, 53. The glycemic index for the energy bars (Cliff bar, Power bar, PR-bar and MET-RX bar) did not differ significantly from glucose even though they varied widely in energy content based on the 50 g carbohydrate requirement for measuring glycemic index. The meal-replacement drinks (Ensure, MET-RX, Boost) showed the widest range in the peak glucose response but not in glycemic index, which suggest a possible shortcoming in the use of the glycemic index for athletes.

The energy contents of the sports drinks tested were relatively similar. The energy bars varied dramatically in energy content because of the different amounts of protein and fat. This produced much more variability in the glycemic response. The authors suggest that the meal-replacement drinks might not be ideal for use in athletes because the classification of foods according to their glycemic index does not consider the insulin response, which has large effects on metabolism and is functionally significant because it regulates glucose disposal. These results show that products with similar ingredients on their labels can produce a markedly different glycemic index.

Randall J. Gretebeck, Kimberlee A. Gretebeck, and Thomas J. Tittelbach. Glycemic index of popular sport drinks and energy foods. Journal of the American Dietetic Association 102(3): 415-417 (March 2002) [Address correspondence to: Randall J. Gretebeck, Division of Health, Physical Education, and Recreation, 263 Matthaei, College of Education, Wayne State University, Detroit, MI 48202.]