Glycemic Index and Exercise

Our bodies have a limited capacity to store carbohydrate, which is why they ideally should make up a majority of the calories (55-65%) in the diet. This is important especially for athletes (recreational or otherwise) who utilize and deplete muscle glycogen on a daily basis. Since we know carbs are important, the question becomes... what kind of carbohydrate should one eat?

Not all carbohydrates are created equal. Generally speaking whole grains and soluble fibers like those found in most fruits are considered complex carbohydrate - they consist of long strands of glucose molecules and take a relatively long time to digest. Jellies, syrups, and white processed breads are considered simple carbohydrates - they consist of only one or two glucose molecules and digest very quickly (hence the "sugar high" of candy, soda, and the like).

To make it more difficult (why can't anything in nutrition just be simple??- pun intended), carbohydrates can also be classified according to their Glycemic Index (GI). The GI of a food is the magnitude of the increase in blood sugar following ingestion. Basically, researchers measure the blood glucose response to pure sugar and assign that a value of 100 - then compare other foods and their blood sugar responses - and assign them a number accordingly.

So, technically you can have a simple carbohydrate that has a low GI and visa versa. A good example is fructose - about as simple as they come - which only has a GI of 23. To add another layer of complexity one has to consider that many things can exert an influence on the GI of a food. Adding fat and/or protein can lower the GI of a high GI food. Initial glycogen stored may also affect the GI of a food.

Who cares about all this GI stuff anyway??

Those interested in exercise metabolism look at consumption of low vs. high GI foods before exercise to see if there is any effect on performance. The data from recent studies suggest that consumption of lower GI foods 30-60 minutes prior to exercise has the following effects:

• minimizes the hypoglycemia which occurs at the beginning of exercise
• increases the concentration of fatty acids in the blood
• increases fat oxidation thereby reducing reliance on carbohydrate as fuel

The idea is that these effects may enhance performance by perhaps prolonging the onset of fatigue during endurance exercise. Enhanced performance, however, has not been proven in these studies. The GI of carbohydrate is not as important during exercise because the insulin response is blunted during exercise. After exercise, it is actually beneficial to consume high GI foods to enhance muscle glycogen resynthesis, which relies on the presence of insulin.

Another possible application of GI is in the dietary treatment of diabetic patients. A diet with mostly low GI foods may produce a more favorable blood glucose profile throughout the day (i.e., large swings in glucose could potentially be avoided).

Following is a list of high (GI > 70), moderate (GI 50-70), and low (GI <50) GI foods:

For more information, please contact the Nutrition Office at 621-4550.

Written by: Paige Holm, R.D.

Reference: Gatorade Sports Science Institute "Sports Science Exchange" and Louise Burke "Food for Sports Performance"