Understanding the Glycemic Index

The term “glycemic index” may sound complicated, but it’s simply a measure of the rise in blood sugar after eating a food that contains carbohydrates. Over the years, the glycemic index has had its share of fans and foes and is still used by people trying to control their blood sugar or body weight.

Physicians David Jenkins and Thomas Wolever invented the glycemic index in 1981 by first plotting curves to show how a specific amount of glucose raised study participants’ blood sugar over two hours. Then they had participants consume the same amount of carbohydrate via foods and plotted the resulting rise in blood sugar. A food’s glycemic index is the percent by which it raises blood sugar compared to glucose and is expressed on a scale of 0 to 100. 

During the next several years, Jenkins and Wolever published additional studies with participants from various populations, including healthy adults and people with hyperlipidemia and diabetes. Glycemic index seemed to be a useful tool for people with diabetes to control blood sugar, as well as a way to lower cholesterol and triglycerides in people with high blood lipids. Additionally, Wolever found when healthy adults consumed low-glycemic index meals for dinner, they tended to experience lower glycemic responses to carbohydrates at breakfast the next day than they did when they had high-glycemic index meals at dinner. 

A main criticism of the glycemic index is that people usually consume a variety of foods in a single meal. Since a balanced meal typically contains a blend of foods with different glycemic indices, the glycemic index of an individual food in a total meal may be insignificant. Jenkins and Wolever addressed these concerns in a 1986 study showing how a meal’s glycemic index may be calculated. Research participants experienced expected rises in blood sugar based on calculated glycemic indices of the meals they ate. 

More recently, studies have addressed glycemic index in a variety of populations with differing results. In 2002, Jenkins authored a review paper promoting a low-glycemic index diet for people with diabetes and for reducing cardiovascular disease risk. For individuals with diabetes, the American Diabetes Association says the glycemic index may be used to help fine-tune blood sugar, but the amount of carbohydrate a food contains is more important than its glycemic index. The Academy of Nutrition and Dietetics’ Nutrition Care Manual includes information on glycemic index as an alternative to the ketogenic diet for people with epilepsy.  

Emerging evidence suggests a low-glycemic diet may help improve insulin sensitivity in children with obesity and elevated insulin. The 2013 International Carbohydrate Quality Consortium, convened by the Glycemic Index Foundation, asserts glycemic index is a valid strategy for controlling blood sugar in healthy adults and it can be used to prevent diabetes, heart disease and obesity. However, the Natural Medicines Database says there is insufficient evidence to rate claims for the glycemic index. Additionally, the 2015-2020 Dietary Guidelines for Americans do not mention glycemic index, leading one to reason it is not of concern for the average, healthy American. 

Using the glycemic index as a weight-loss strategy lacks strong evidence. When used in addition to caloric restriction, a low-glycemic index diet can produce weight loss. However, one of the diet’s modern selling points is users don’t have to count calories and can still lose weight, which is a claim yet to be supported. A 2015 study of 91 adults with obesity found a low-glycemic index diet had no effect on weight loss, fat mass, lean mass nor metabolic adaptation during a 17-week weight-loss period compared to an isocaloric high-glycemic index diet. But a small 2013 study found when participants consumed high-glycemic index meals, they experienced increased hunger and cravings. 

Skepticism about using the glycemic index as a dietary tool exists for several reasons. The glycemic index of a food tells nothing about its nutritional qualities. For instance, watermelon has a higher glycemic index than ice cream, but most would say watermelon is a more nutritious option and may be eaten by people with diabetes, without need for concern, in accordance with their carbohydrate patterns. Academy spokesperson Jim White, RDN, ACSMEP- C, says he sees clients who are afraid to eat nutritious high-glycemic index foods such as potatoes, melon or pineapple, but these foods should not be avoided. When it comes to healthy eating, following the key recommendations of the Dietary Guidelines while using a total diet approach and MyPlate are more realistic and applicable strategies for most people. 

Academy spokesperson Marina Chaparro, MPH, RDN, LD, CDE, finds the glycemic index to be too variable and impractical to recommend to her clients with diabetes. Both Chaparro and White suggest clients use glycemic load, rather than glycemic index, to take portion size into account. “I teach my clients to focus on wholesome food, learn how to count carbs and focus on the right portion size,” Chaparro says. 

Although the glycemic index is not perfect, it can be a useful tool in addition to evidence-based strategies for controlling blood sugar in people with or at risk for diabetes, as long as they are not unnecessarily restricting foods. Prediabetes and diabetes are not caused by eating high-glycemic foods; they are complex diseases with a variety of risk factors, including other dietary variables, activity level, age, race, sex and family history. A wellness plan including assessment of all these factors helps health care professionals develop individualized strategies for patients and clients. 

Taylor Wolfram, MS, RDN, LDN

Taylor Wolfram, MS, RDN, LDN, is a dietetics content manager at the Academy of Nutrition and Dietetics and associate editor of Food & Nutrition.