The glycemic index is theoretically based on how 50 grams of carbohydrates (not counting fiber) in a food will affect blood sugar levels. For example, white bread is a high-glycemic-index carb and supposedly causes a rapid spike in blood sugar, while beans are considered a low-glycemic-index carb and cause a more gradual increase in blood sugar levels.
The glycemic index was initially developed to help people with diabetes regulate their blood glucose. But people with diabetes generally eat foods in combinations (for example, a sandwich with bread, turkey and tomato); this alters the glycemic index of the meal. Athletes, however, commonly eat foods solo (a banana, a bagel). Hence, exercise scientists became curious about the possibility that quick or slow carbs might impact exercise performance because they affect blood glucose in different ways. Could athletes use this ranking system to determine what to eat before, during and after exercise?
Theory vs. science
- Low-glycemic-index foods (apples, yogurt, lentils, beans) provide a slow release of glucose into the blood stream. Could they help endurance athletes by providing sustained energy during long bouts of exercise?
- High-glycemic-index foods (sports drinks, jelly beans, bagel) quickly elevate blood sugar. Are they best to consume immediately after exercise to rapidly refuel the muscles and, thereby, enhance subsequent performance?
According to Kathy Beals PhD RD, athletes can disregard all the hype about the glycemic index and simply enjoy fruits, vegetables and whole grains without fretting about their glycemic effect. Beals spoke at the yearly conference sponsored by SCAN, the Sports and Cardiovascular Nutrition Dietary Practice Group of the American Dietetic Association, and claimed that too many factors influence a food's glycemic effect. Factors include where the food was grown, the amount eaten, fiber content, added butter, the way the food is prepared (mashed, baked, boiled), and if the food is eaten hot or cold.
To make the glycemic index even less meaningful, each of us has a differing daily glycemic response that can vary 43 percent on any given day. Among a group of subjects, the response can vary by 18 percent. (1)