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The glycemic index (GI) was developed as a research tool to examine how a person’s glucose (blood sugar) level is affected by foods that contain carbohydrates. The glycemic index ranks food with a value based on how quickly that food is converted to sugar in the blood and how fast the glucose level returns to normal. Foods with a high GI score are digested and absorbed by the body more quickly, causing a rapid rise in blood glucose. Foods with a low GI score are converted to sugar more slowly and do not cause a rapid spike in blood glucose. 

A scale of 0 to 100 is used to rank the foods. A GI of less than 55 is considered low, a GI over 70 is considered high, and everything in between is considered intermediate or moderate.

In general, refined grains and sugars have a high GI, legumes and unprocessed or whole grains have a moderate GI, and fruits and vegetables have a low GI. As with fats, scientists categorized foods as “good” and “bad” carbohydrates. In theory, some people, especially those with diabetes, should choose more of the low-glycemic foods at meals to keep glucose levels stable.

However, using the glycemic index for meal planning is a complicated process, and may not adequately control glucose. GI values have many variables. First, a GI score is not necessarily connected with how nutritious a particular food is. The score is ranked solely for the rate at which it raises glucose levels. In addition, GI scores of a food vary depending on how it is prepared and its interaction with other foods. The GI of a specific food may also vary from individual to individual. Many foods do not have a known GI, which can make it difficult to plan a meal using the glycemic index as a guide.

Although some scientists believe that a low-glycemic diet may help control glucose and fat (lipids), the results are inconclusive. There is not enough evidence of long-term benefits to recommend use of the glycemic index for dietary control. The glycemic index can be used in meal planning but should not be the only consideration when making food choices. Foods may have a different GI from day to day, based on the individual’s glucose reading and any insulin resistance (e.g., in people with diabetes).