The glycemic index ranks foods on a scale of 0 to 100 based on how quickly they raise blood sugar levels. Foods that raise blood sugar quickly have a higher number, whereas foods that take longer to affect blood sugar levels have a lower number.

To measure the glycemic index of a food, a specific weight of the digestible carbohydrates in the food (usually 50 grams, which is about 4 tablespoons of sugar) is fed to at least ten different people who volunteer for the study. Their blood sugar levels are measured every 15 to 30 minutes over a two-hour period to develop a blood sugar response curve. The blood sugar response of each food is compared to that of a test food, typically table sugar (glucose), which is assigned the number 100. The responses for each test subject are averaged, resulting in the glycemic index number for that food. Every individual person may have a slightly different glycemic (blood sugar) response to foods, which is why the tests use a number of volunteers and average their results together.

The information on glycemic index (GI) lists is divided into three basic categories so you don’t have to get caught up in numbers and can instead focus on the primary goal of the glycemic index — choosing foods that keep your blood sugar levels more even, resulting in longer-lasting satiety (the feeling of fullness) and improved health. Here are the three categories:

  • GI of 55 or less = low

  • GI of 56 to 69 = medium

  • GI of 70 or more = high

A basic guideline is to keep your total daily glycemic load under 100. In this case, if you’re eating three meals per day and each meal has about the same glycemic load, you’ll go over the ideal total maximum of 100. To prevent yourself from going over, choose to balance a higher-glycemic meal with a lower-glycemic one or swap out a higher-glycemic food for a lower-glycemic one to reduce a meal’s overall glycemic load.