Don't believe the hype: Using glycemic index to optimize endurance performance

Try to eat 50 to 100 grams of high-GI foods, like a bagel, immediately after exercise.
With all the hype about protein being the most vital nutrient for athletes, many athletes are beginning to question the role of carbohydrates in their diet. But they shouldn't!

Put down that bacon and let me say, right from the start, carbohydrates play the essential role in the athlete's diet because they are the key source of energy during training and competition.

Eating adequate carbohydrates before, during and after exercise forms the nutritional basis that prevents premature fatigue and speeds recovery following training/competition.


The glycemic index: What's it all about?

Previously, carbohydrates where classified as either (a) complex (e.g. breads, cereals, vegetables, etc., which were digested slowly and caused little fluctuation in blood-glucose levels) or (b) simple (e.g. candy, soft drinks, etc., which caused blood-glucose levels to rise and fall dramatically and produced an insulin response).

Current research, however, indicates that the reality is a little more nuanced. In fact, some "complex" carbohydrate foods actually behave more like what were previously labeled "simple" carbohydrates. It's now understood that the make-up of a given type of carbohydrate is less important than the glycemic response it provokes, and foods have subsequently been categorized according to what has been termed the glycemic index, or GI for short.

This newer understanding of carbohydrates has had ramifications in a wide variety of areas such as weight loss, diabetic control and athletic performance.

So what exactly is the GI?

It's the measure of the rate at which the blood-glucose level rises after a given food has been consumed. Consequently, foods are labeled low, medium- or high-GI foods.

The GI for a particular food is determined by comparing the blood-glucose response produced by that food against pure glucose, which has been assigned a GI rating of 100 and is used as the benchmark for all comparisons. Various authorities have generated reference documents detailing the GI responses of various foods and drinks so meals can be created that are either low or high GI, depending on a person's needs.

For example, a diabetic would be looking for slow-release energy over an extended period of time, so a low glycemic-index food would be the optimal choice. Conversely, an athlete wishing to rapidly replenish muscle-glycogen stores after training or competition would be better served by initially choosing a high-GI food or combination of foods to achieve this effect.

When foods with various GIs are combined, the total GI of the meal will depend upon the amount of each of these foods and their individual GI values. For example, a Nutella (low-GI) sandwich on white bread (high-GI) will take on characteristics of both carbohydrate sources.

Back in 1993, Australian Institute of Sport Dietitian Dr. Louise Burke headed up a study that found high-GI foods produced greater blood-glucose and insulin responses and consequently enhanced muscle-glycogen storage (by some 30 percent) after extended exercise in well-trained cyclists when compared to low-GI foods. The implication is that if you're trying to maximize performance in challenging workouts on a daily basis, some of your nutrition should come from high-GI foods to accelerate recovery.

However, it's not practical to plan all of your eating habits around the glycemic index; if you tried to do so, you'd spend all day calculating what to eat and when. A more practical approach is to ensure you ingest 50 to 100 grams of high-GI foods immediately after exercise and then focus your general diet on low- to moderate-GI foods that are rich not only in carbohydrates, but also in vitamins, minerals and fiber (e.g. whole grains, fruit and vegetables).

On the flip side, consuming foods with a low- to moderate-GI (such as beans, legumes and whole grains) is also important as these foods allow a slow release of glucose into the bloodstream and sustain energy over extended periods of time.

Glycemic Index of Foods


Low (55 or less) Medium (56-69) High
Skim milk
Plain yogurt
Soy beverage
Apple/plum/orange
Sweet potato
Converted or parboiled rice
Al dente (firm) pasta
Lentils/kidney/baked beans
Banana
Pineapple
Raisins
Brown rice
Couscous
Basmati rice
Whole wheat bread
Rye bread
Watermelon
Dried dates
Baked white potato
Instant rice
Bagel, white
Table sugar (sucrose)

Reference
Burke, L.M. et al.: "Muscle Glycogen Storage After Prolonged Exercise: Effect of the GI on Carbohydrate Feeding." J. App. Physiol.1993; 75: 1019-1023.

Australia's Rod Cedaro is a former pro triathlete and an ACC Accredited Level III triathlon coach.


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