As mentioned earlier, the glycemic index of a carbohydrate-containing food is a ranking of how much it raises blood glucose level compared to pure glucose, which is assigned a glycemic index value of 100. (For a list of the glycemic index value of hundreds of foods, go to www.glycemicindex.com.) According to the GI Group of the University of Sydney, in Australia, a glycemic index value of 70 or higher is considered high, 56–69 is considered medium, and 55 or lower is considered low.
To determine a food’s glycemic index value, researchers give 10 or more volunteers a portion of the food being tested that contains 50 grams of carbohydrate. The volunteers’ blood glucose levels are checked before they eat the food and periodically during the two hours after they eat it. On another occasion, those same volunteers consume a portion of glucose (or sometimes white bread is used) that contains 50 grams of carbohydrate, and the same blood glucose measurements are taken. The two sets of measurements are then compared. Ten or more volunteers are used in these tests, and their results are averaged, because each person responds to food slightly differently.
Not only does the glycemic index of a food vary slightly from person to person, but it also depends on whether the food is eaten in isolation or with other foods. Consuming a food along with protein, fat, or other carbohydrates that have a lower glycemic index effectively lowers its glycemic index value. Other things that can affect a food’s glycemic index value include the ripeness of fruits (underripe fruits have a lower glycemic index than ripe fruit) and how foods are cooked or otherwise processed.
In people with diabetes, the glycemic index value of a food is additionally affected by a person’s premeal blood glucose level. If a person’s blood glucose level is elevated, the glycemic index of a food is lower than normal, and if a person’s blood glucose is low, the glycemic index of the food goes up.
The glycemic load takes into consideration both the glycemic index of a food and the amount of carbohydrate in the portion of food eaten. The glycemic load is calculated by multiplying the glycemic index value by the number of grams of carbohydrate, then dividing by 100. Using cooked carrots as an example, 1 cup of cooked carrots provides about 10 grams of carbohydrate, and the glycemic index of cooked carrots is 49. That makes the glycemic load of a cup of cooked carrots about 5, since 49 × 10 ÷ 100 = 4.9.
In general, a serving of food with a glycemic load of 1–10 is considered to have a low glycemic load, 11–19 is a medium glycemic load, and 20 or higher is a high glycemic load. The table “Same Carbohydrate, Different Glycemic Load” shows the glycemic load for a variety of portions of foods.
A small study published in The American Journal of Clinical Nutrition in 2011 found that the glycemic load of a portion of a single food or of a meal (with a mix of foods) was a better predictor of after-meal blood glucose level than the carbohydrate content of the food portion or meal. However, the study subjects did not have diabetes, so it is not yet known whether the same would hold true for people with diabetes.
In the meantime, you may wish to experiment on yourself to see whether calculating the glycemic load of your meals and snacks helps you to anticipate your after-meal blood glucose levels better than carbohydrate counting alone.
A variety of other studies have looked at the effects of following a low-glycemic-index or low-glycemic-load diet on diabetes control and on heart disease risk factors, and they have generally shown positive effects.