Friday, March 28, 2008

Decoding the Glycemic Index

There has been a lot of interest in the glycemic index or GI of certain foods. The GI can be somewhat difficult to understand because it compares foods that seem to quite similar. The GI is simply a way of characterizing how different types of foods affect blood glucose. Eating foods that contain carbohydrates alone or in combination with fats and proteins cause blood glucose to rise to varying degrees. The GI is a measure of how high and how quickly blood glucose rises. Foods that cause blood glucose to rise very quickly have a high GI. Foods with a low GI cause smaller, more prolonged elevations in blood glucose. This article attempts to clarify some of the confusion surrounding the GI and makes a few recommendations for pre-match and recovery meals.

The GI of foods is a VERY important concept for diabetic patients and others who have difficulty controlling blood glucose. They must pay close attention to their diet. High GI foods can cause critical increases in blood glucose and lead to serious complications for the patient. Low GI foods help them maintain blood glucose within a healthy range. For healthy athletes the importance the GI is somewhat controversial. Early studies suggested that a low GI pre-match meal was most effective for performance. More recent studies, however, have questioned this idea.

The GI Explained

As a rule of thumb, a GI of less than 55 is considered low and a GI of greater than 70 is considered high. The GI for individual foods can be somewhat confusing. Food that seem to be similar in their carbohydrate content can have very different effects on blood glucose. For the most part, low to moderate GI foods are things like beans, milk, yogurt, most fruits (apples, bananas, oranges), pasta and nuts. High GI foods are items such as bagels, white bread, rice, sports drinks and many candies. However, it is often the combined GI of the entire meal that is important. For example, many cereals have a high GI. But, when they are eaten with milk, a low GI beverage, the meal GI is relatively moderate. Candies such a jelly beans are considered very high GI. But, chocolate bars that have a high fat content and are mixed with nuts may have a moderate-low GI. Also, sandwiches are made with high GI bread but the overall GI is reduced by including turkey or ham.

The effect of various foods on blood glucose is very important for the athlete. Eating very high GI foods (for example, candies such as jelly beans) shortly before a match can cause a rapid increase in blood glucose, but the effect can be short-lived. With a large and rapid blood glucose increase, insulin is released into the blood. In attempt to return blood glucose to normal levels, insulin promotes glucose uptake by various cells (particularly fat, liver and muscle). Many times, the insulin response overcompensates so that blood glucose drops to levels below normal. This so-called sugar rush and crash can have major effects on performance, including fatigue, lethargy and irritability. So it is important for the athlete to avoid foods with excessively high GI.

Pre-Match and Recovery Meals

The available research comparing low GI and high GI pre-exercise meals show mixed results. Some early studies showed show improvements in endurance performance with a low GI meal, but more recent research has shown little or no differences between the diets. One issue that is mentioned in some of these reports is that athletes who consume a high GI pre-match meal often feel hungry within a few hours. Thus, by the time kick-off rolls around, the player is ready to eat another meal. Thus, at this time, the GI of the pre-match meal is probably not as important as the carbohydrate content. Athletes should eat meals that are high in carbohydrate and low in fat. However, if feeling hungry adversely affects performance during the match, the player should opt for a meal that has a moderate-low GI.

As for recovery, immediately after the match, the preference is high GI foods such as bagels and sports drinks. This will cause a more rapid increase in blood glucose and movement of glucose into the muscle cells to replenish lost glycogen. A critical component of the recovery process is to replenish muscle glycogen as quickly as possible. Low GI foods may slightly delay this response. If the recovery period between matches is short (a few hours), a high GI recovery “snack” is even more important.

Recommendations

The GI of specific foods and meals give the athlete a measure of how blood glucose responds to various foods. As for the pre-match meal, the GI of the meal is probably not as critical as the carbohydrate content. As for recovery, a high GI snack and beverage immediately after the match is critical. This should then be followed by a recovery meal that is high in carbohydrates. A specific strategy for this can be found in the post, “Tournament Time – Nutritional Strategies”.