What is glycemic index and why should athletes and non-athletes care?
The carbohydrates we eat are converted into a simple sugar called glucose before they can be used within the body. We all know that eating too many sweets can make you fat, but the amount of sugar in the sweets is not the only important variable. The rate at which the carbohydrate in a particular meal is converted to glucose and absorbed into the blood varies from food to food, and is as important to health as the total amounts of sugar and carbohydrate present.
The concentration of glucose in the blood must be maintained within a specific narrow range, or all sorts of body processes go awry. Too much blood sugar (diabetes) and too little (hypoglycemia) are each associated with their own specific debilitating symptoms. In order to maintain blood glucose in the correct narrow range, the body uses two hormones that signal the muscles, fat tissue, and liver to take up or release glucose. After a carbohydrate-rich meal, glucose moves from the gut to the blood, raising blood glucose levels. To bring the levels back down, the pancreas releases insulin, which triggers glucose uptake. Between meals, when digestion is complete and blood glucose levels begin to drop, the pancreas releases glucagon. Increased glucagon levels in the blood trigger glucose release from the various places it has been stored. Working together, insulin and glucagon nudge blood glucose up or down—and keep it within the body’s narrow range of tolerance.
A food that is high in easily digested carbohydrates causes a large and rapid increase in blood glucose levels, which the body answers with a large and rapid release of insulin. Such a food is said to have a high glycemic index (High GI). Another food with a similar total carbohydrate content—but that is digested and enters the bloodstream more slowly—causes a smaller insulin response, and is said to have a low glycemic index (Low GI). There are two common glycemic index scales. One rates pure glucose as 100 and compares the glycemic response of other foods to glucose, in order to assign a score. On this scale, white bread scores a 70. The other glycemic index scale uses white bread as the reference point, giving it a score of 100.
While exercisers and athletes need blood glucose in addition to the carbohydrates stored in their muscles for fuel, a large dose of glucose and insulin is not the best thing—even for athletes in heavy training. High insulin levels drive glucose into muscle, but also into fat. Glucose stored in muscle is energy for the next training session, but glucose stored in fat just becomes more fat. Insulin also suppresses fat metabolism, so high glycemic index foods make fat loss doubly difficult by causing the body to store fat while also making the body less able to utilize it. This effect lasts several hours after a high glycemic index meal.
Insulin levels do not return to baseline the moment excess blood sugar has been cleared. After a high-GI meal, insulin remains high after blood glucose levels return to normal, and insulin suppresses glucagon release and continues to drive glucose into fat and muscle, resulting in below-normal blood sugar—which you may experience as lethargy, grumpiness, or hunger that will not clear for several hours or even days—unless you again consume a high-GI meal. In this sense, sugars, white flour, and other high glycemic index foods cause an increase in appetite and are, in effect, addictive.
High glycemic index foods consumed while you are not exercising impair recovery from glycogen-depleting exercise, and if consumed in large quantities in the few hours before exercise, decrease endurance in the next exercise session. During exercise, glucose is actually taken from the bloodstream into the muscle even in the absence of insulin. If you are an athlete in training, consume high-GI foods only during exercise and for a short period of time immediately after exercise. If you are seeking weight loss (and specifically fat loss) and you are not exercising, avoid high-GI foods at all times.
Disadvantages of high glycemic index foods consumed outside of exercise:
