What do soft drinks, baked potatoes, and watermelon have in common? What do milk, broccoli, and apples have in common? If you guessed that it has something to do with the “glycemic index,” you guessed right! But do you know why? What is the glycemic index, and why does it matter?
The glycemic index (GI) is a nutrition metric that can help individuals make smart food choices to promote a healthier life. The index was created by two physicians, David Jenkins and Thomas Wolever, in 1981 and has since become a popular tool in scientific, clinical, and household realms.
Glycemic Index Values
The glycemic index (GI) is a set of values, ranging from 0 to 100, assigned to foods based on how rapidly they raise blood sugar levels after they are eaten. Foods that have a low GI tend to raise blood sugar slowly and steadily. In contrast, foods with a high GI generate a rapid rise in blood sugar usually followed by a rapid decline. For example, soft drinks, baked potatoes, and watermelon are foods higher on the glycemic index whereas milk, broccoli, and apples are lower on the glycemic index.
- Low GI: 55 or less
- Medium GI: 56–69
- High GI: 70+
When high GI foods are eaten, they cause a fast rise in blood sugar, and a typical cycle occurs: The body has a quick burst of energy. The pancreas releases insulin, a hormone that regulates blood sugar, which stimulates the body’s fat storage mechanism (meaning, the body wants to keep fat). Within a few hours, the insulin depletes the blood sugar spike, causing blood sugar to drop quickly. Feelings of hunger follow, often accompanied by tiredness. The result is cravings for other high GI foods for a quick burst of energy.
This scenario can be beneficial in certain circumstances such as energy recovery after exercising or counteracting hypoglycemia (low blood sugar level). However, a diet mainly consisting of high GI foods can lead to weight gain because of the hunger-craving cycle it creates. A high GI diet has also been shown to contribute to the development of heart disease and diabetes.
In contrast, low GI foods promote a gradual and steady rise in blood sugar and prolonged energy. Low glycemic foods do not stimulate the fat storing mechanism. Also, eating low GI foods does not lead to a rapid blood sugar drop, which in turn results in fewer cravings for high GI foods. Diets where the average GI is low has been found to promote weight loss and prevent chronic health conditions such as diabetes, heart disease, and other health conditions that may arise from excess weight or obesity.
“Target GI” and Influences on GI Values
Generally accepted GI values are estimates; knowing the relative GI of foods makes it easier to plan a well-balanced diet in combination with healthy eating patterns and appropriate serving sizes. For people who already have certain health conditions, like diabetes, a GI-informed diet can help improve management of those conditions and prevent complications.
The average GI of the typical American diet is around 55–60. The Glycemic Index Foundation recommends an overall average dietary GI of 45 or less. However, achieving a “target GI” may be a bit like trying to a hit a moving target: many factors contribute to the “glycemic effect” (blood sugar level) foods produce in the body.
In a regular diet, individual foods and ingredients are usually combined into meals. When a meal contains foods with different GI values, the overall glycemic effect of that meal can be different than it would be in response to any individual food eaten by itself. Certain food characteristics and additional factors can alter GI values, such as:
- Recipe. For example, whole wheat bread produced by two different manufacturers can have entirely different proportions of ingredients that may either contribute to a higher or lower GI value of the bread.
- Fiber. Fiber content lowers GI value. Fiber provides natural resistance to digestive enzymes resulting in slower digestion and more gradual absorption of sugars.
- Variety. For example, there are many different varieties of apples. One apple type may be inherently sweet, which would contribute to a higher GI value, while another type may have more fiber, which would contribute to a lower GI value.
- Acidity. A food’s acidity lowers GI. For example, adding vinegar or lemon juice to certain foods results in a lower GI.
- Ripeness. Ripeness tends to increase GI. For example, ripe mangos or bananas have higher concentrations of sugars and hence higher GI values than their unripe counterparts.
- Cooking. Longer cooking time tends to modify GI. For example, pasta that is cooked longer tends to have higher GI than pasta cooked for less time (that is, cooked just until “al dente”).
- Processing. Processing or refinement also tends to change GI. For example, fruit juice is usually higher in GI than fruit itself. This usually happens because processing removes naturally occurring fibers, leading to decreased resistance to digestive enzymes and more rapid uptake of sugar into the bloodstream.
- Protein and fat. The presence of proteins and fats in a meal results in a lower glycemic effect.
All these factors may account for the differences in GI values that appear on different glycemic index charts as well as the overall average GI of a meal. The University of Sydney formed a GI research service in 1995 that established a specialized laboratory for reliable GI testing. The service’s website www.glycemicindex.com provides a search tool where GI for a wide variety of foods can be found and used in planning a GI-informed diet.
Ask your healthcare provider how a GI-informed diet may benefit you.
Beverly Merz, “Using the glycemic index to stave off holiday weight gain,” Harvard Women’s Health Watch, updated April 21, 2015.
Dennis Thompson, “Glycemic Index Foods,” MedicineNet.
Glycemic Index Foundation, “About Glycemic Index.”
Harvard Health Publishing, “Glycemic index for 60+ foods,” Harvard Medical School, updated January 6, 2020.
Taylor Wolfram, “Understanding the Glycemic Index,” Food & Nutrition, December 19, 2016.
University of Sydney, “Search for the Glycemic Index.”
Zawn Villines, “Glycemic index: Everything you need to know,” MedicalNewsToday, July 4, 2019.