Table provides the GI of a list of foods. The sugars found in most sports drinks are glucose, fructose, and sucrose. Sucrose is a disaccharide composed of glucose and fructose. Fructose is absorbed more slowly when compared with glucose and therefore does not create the rapid swings in blood sugar levels that may occur when ingesting high-GI foods. Ingesting a low-GI carbohydrate about 45 min prior to exercise may result in a more stable blood sugar level during the early stages of prolonged exercise.51 However, when either fructose or glucose is ingested immediately before or during exercise, there appears to be no difference in blood sugar and carbohydrate metabolism. Furthermore, neither fructose nor glucose is better at sparing muscle glycogen. One drawback to the slow absorption of fructose is its tendency to draw water into the intestines. This can result in cramping and diarrhea. Therefore, athletes should be cautious when consuming fructose for the first time. A number of studies have evaluated the effects of ingesting various forms of carbohydrate on performance. In one study, cyclists who ingested a low-GI meal 30 min prior to exercise had lower blood glucose and insulin levels than subjects ingesting a high-GI meal. In addition, cycling time to exhaustion was increased by 59% after ingesting the low-GI meal. These findings suggest that ingesting a low-GI meal prior to exercise would be advantageous from a metabolic and performance standpoint. However, other studies have reported that blood glucose levels were maintained better during high-intensity exercise in subjects who ingested high-GI foods rather than low-GI foods with no significant effects on endurance performance. In support of this latter contention, research conducted in our lab evaluated the effects of ingesting different types of carbohydrate gels on endurance cycling performance.
In a double-blind, randomized, and crossover study design, subjects ingested either a placebo, a high-GI carbohydrate gel (sucrose) or a low- to moderate-GI gel (honey) prior to and during a cycling time trial lasting about 3 hours. Results revealed that ingestion of both carbohydrate gels improved performance with no adverse effects from ingesting the higher-GI gel. These findings indicate that it really doesn’t matter which type of carbohydrate is ingested prior to or during exercise. Although ingesting low-GI foods or drinks may result in a more controlled release of blood sugar into the blood prior to exercise, there just is not enough evidence to recommend that athletes will improve their performance during prolonged exercise when they ingest low-GI foods or drinks.
For people trying to lose weight or promote health, it makes sense to consume low-GI carbohydrates in the diet instead of high-GI carbohydrates. In this regard, the metabolic effects of carbohydrates differ based on the GI. Consumption of high-GI foods causes a greater increase in insulin, carbohydrate storage, and carbohydrate utilization in comparison with consuming low-GI carbohydrates. High-GI diets have been associated with obesity, diabetes, and heart disease. Short-term intervention trials suggest that simply replacing high-GI foods with low-GI foods promotes weight loss and improves insulin sensitivity.
For this reason, a growing number of obesity researchers recommend that people consume a moderate- to high-carbohydrate diet consisting primarily of low-GI foods. Athletes should also consume the majority of carbohydrates in their diet in the form of low- to moderate-GI foods, except that carbohydrate intake during and following exercise should consist of high-GI foods. For the athlete, however, some additional points need to be made. First, research has shown that exercise training improves insulin sensitivity and helps individuals manage body weight. Therefore, people who engage in habitual exercise training may not benefit as much from a low-GI diet as sedentary individuals or diabetics.
It is also possible that the potential negative effects on health of consuming high-GI foods may be of less concern in trained individuals. Second, research has indicated that ingesting high-GI foods with protein after exercise is important to enhance protein and glycogen synthesis. The increased protein and glycogen synthesis is believed to be due in part to an increase in insulin levels. Therefore, it is our view that healthy athletes should not restrict intake of high-GI carbohydrates following exercise.
There is also evidence that ingesting a low- to moderate-GI carbohydrate prior to exercise may improve carbohydrate availability and reduce protein degradation during exercise. In addition, most sports drinks and sports gels are high-GI carbohydrates. Research has consistently shown that ingestion of sports drinks or sport gels during exercise may enhance prolonged exercise performance. Based on this research, we suggest the following dietary guidelines for athletes in consideration of the GI:
For people trying to lose weight or promote health, it makes sense to consume low-GI carbohydrates in the diet instead of high-GI carbohydrates. In this regard, the metabolic effects of carbohydrates differ based on the GI. Consumption of high-GI foods causes a greater increase in insulin, carbohydrate storage, and carbohydrate utilization in comparison with consuming low-GI carbohydrates. High-GI diets have been associated with obesity, diabetes, and heart disease. Short-term intervention trials suggest that simply replacing high-GI foods with low-GI foods promotes weight loss and improves insulin sensitivity.
For this reason, a growing number of obesity researchers recommend that people consume a moderate- to high-carbohydrate diet consisting primarily of low-GI foods. Athletes should also consume the majority of carbohydrates in their diet in the form of low- to moderate-GI foods, except that carbohydrate intake during and following exercise should consist of high-GI foods. For the athlete, however, some additional points need to be made. First, research has shown that exercise training improves insulin sensitivity and helps individuals manage body weight. Therefore, people who engage in habitual exercise training may not benefit as much from a low-GI diet as sedentary individuals or diabetics. It is also possible that the potential negative effects on health of consuming high-GI foods may be of less concern in trained individuals. Second, research has indicated that ingesting high-GI foods with protein after exercise is important to enhance protein and glycogen synthesis. The increased protein and glycogen synthesis is believed to be due in part to an increase in insulin levels. Therefore, it is our view that healthy athletes should not restrict intake of high-GI carbohydrates following exercise.
There is also evidence that ingesting a low- to moderate-GI carbohydrate prior to exercise may improve carbohydrate availability and reduce protein degradation during exercise. In addition, most sports drinks and sports gels are high-GI carbohydrates. Research has consistently shown that ingestion of sports drinks or sport gels during exercise may enhance prolonged exercise performance. Based on this research, we suggest the following dietary guidelines for athletes in consideration of the GI:
