To make matters more complicated, external factors can also alter how much a given food raises blood glucose, such as stress, time of day, prior food intake, and blood glucose level at the time of the meal; level of physical fitness; and having diabetes. (People with diabetes tend to have higher blood glucose levels in response to foods compared with people who don’t have diabetes.) All of this potential for variability has led some people to doubt the usefulness of the glycemic index in meal planning. On the other hand, studies have shown that the combination of the carbohydrate content of a meal and the source of carbohydrate in the meal accounts for 90% of a person’s blood glucose response to the meal. Paying attention to amount and type of carbohydrate, therefore, is clearly important.
What about portion size? The glycemic index of a food remains the same no matter how much is eaten. But eating a large amount of a carbohydrate-containing food will raise your blood glucose level more than eating a small amount of that food. That’s where glycemic load comes in.
Glycemic load is calculated by multiplying a food’s glycemic index by the number of grams of carbohydrate in a serving of food and dividing the result by 100. Here’s how the equation looks:
Glycemic load = (glycemic index × grams of carbohydrate) ÷ 100.
To assess the glycemic load of a small, 4-ounce apple, therefore, the calculation would be: (40 × 15 grams of carbohydrate) ÷ 100 = 6.
Similarly, for a 10-ounce slice of watermelon, the calculation would be: (76 × 15 grams of carbohydrate) ÷ 100 = 11.
A glycemic load of 10 or lower is considered low, 11–19 intermediate, and over 20 is considered high. Since both amount of carbohydrate and glycemic index are used to determine glycemic load, there are two ways to lower the glycemic load of a snack or meal: Eat a smaller portion, or choose a food with a lower glycemic index.
Glycemic index and diabetes management
With all of the variables that can affect the glycemic index of a meal, the question remains whether glycemic index values are useful in helping people with diabetes maintain blood glucose control. Two recently published reports address this question.
In 2003, one of the best-known researchers of the glycemic index and author of several books on the topic, Jennie Brand-Miller, Ph.D., reviewed 14 experiments that studied the effects of following a low-GI diet on people with diabetes. (People with both Type 1 and Type 2 diabetes were represented in the studies.) Her results showed a 0.43 percentage-point average decrease in glycosylated hemoglobin (HbA1c) level, which is a measure of blood glucose control over the preceding 2–3 months. Less than a percentage point of improvement of HbA1c may sound paltry, but in fact, other research has shown that as little as a 1 percentage-point decrease in HbA1c is associated with a reduced risk of death, heart attack, and so-called microvascular complications such as diabetic neuropathy. What’s more, the improvement seen with the use of a low-glycemic-index diet is comparable to that seen with some prescription medicines that target high blood glucose after meals.
In 2004, James W. Anderson, M.D., Professor of Internal Medicine at the University of Kentucky Graduate Center for Nutritional Sciences, and his colleagues reviewed nine randomized control trials (considered the strongest experimental research) on low-glycemic-index diets. They found that the consensus among the nine trials is that low-glycemic-index diets are associated with lower blood glucose levels, lower LDL (“bad”) cholesterol levels, lower triglycerides, and higher HDL (“good”) cholesterol levels. As a result, they recommended that information about the glycemic index be included in diabetes education and in the Exchange Lists for Meal Planning.