• How a food is prepared. For example, pasta cooked al dente has a lower glycemic index than pasta that is cooked until fully soft.
• Acidity. Acidic foods, such as sourdough bread, tend to have a lower glycemic index than similar, less-acidic foods. In addition, adding lemon juice or vinegar to a dish, such as a salad, will lower its glycemic index.
• Presence of sucrose. Table sugar (sucrose) is half glucose, which has a high glycemic index, and half fructose, which has a low glycemic index. As a result, the glycemic index of table sugar is intermediate, and foods that are high in sugar may similarly have an intermediate glycemic index.
• Fiber. Some types of fiber lower a food’s glycemic index, but some do not. Insoluble fiber, the type found in wheat, has little effect on glycemic index, so whole wheat bread has a glycemic index similar to that of white bread. Soluble fiber, on the other hand, lowers glycemic index. Foods high in soluble fiber such as oats, barley, and legumes (dried beans and lentils) have low glycemic index values.
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 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, PhD, 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.