What is the glycemic index?

The glycemic index (GI) is a 0–100 scale that shows how quickly blood sugar rises after eating a carbohydrate food, compared with pure glucose, which scores 100. A low GI means a slower, gentler rise; a high GI means a fast rise and a higher peak.

How it is measured

The method was introduced by David Jenkins and colleagues in 1981 and is now standardized as ISO 26642. In short:

1. At least 10 healthy people eat, after an overnight fast, a portion of the food containing 50 g of available carbohydrate (or 25 g when that portion would be unrealistically large).
2. Their blood glucose is measured over 2 hours and the area under the curve is calculated.
3. On another day, the same people drink 50 g of pure glucose, the reference.
4. GI is the food's area divided by the glucose area, times 100, averaged across participants.

So GI compares carbohydrate quality using equal amounts of carbohydrate, not real portions. That is what glycemic load is for.

Low, medium and high GI

What makes a food high or low GI

Two foods with the same amount of carbohydrate can have very different GIs. These are the factors that matter most, with examples from the glycemic index chart:

• Type of starch. Amylose is digested more slowly than amylopectin, which is why higher-amylose basmati rice (60) sits well below sticky rice (90).
• Processing. The more a grain is milled, puffed or pre-cooked, the faster it is digested: steel-cut oats (52) versus instant oats (82).
• Cooking. Heat and water gelatinize starch. Al dente pasta has a lower GI than overcooked pasta, and boiled sweet potato (46) is far lower than baked sweet potato (88).
• Cooling. As cooked starch cools, part of it turns into resistant starch. Cooked-then-cooled potatoes (49) have a much lower GI than hot boiled potatoes (73).
• Ripeness. A slightly green banana (48) has a lower GI than a very ripe one.
• Soluble fiber, fat, protein and acidity. All of them slow stomach emptying or digestion. Barley, rich in beta-glucan, has one of the lowest GIs among grains: barley (30).
• Type of sugar. Fructose barely raises blood glucose: fructose (24) versus glucose (100).

The limits of the glycemic index

GI is useful, but it needs context:

• Low GI does not mean healthy. Potato chips (56) and milk chocolate (45) have a lower GI than boiled potatoes because fat slows absorption. They are still foods to eat in moderation.
• It ignores portion size. Glycemic load does not.
• Values vary. The same food can test differently depending on variety, origin and lab. On this site every food shows the range across all studies.
• Meals are mixed. What you eat alongside changes the effect: rice with beans, vegetables or protein produces a smaller response than rice alone.
• People respond differently. Studies with continuous glucose monitors have found that responses to the same food can vary widely between people.

How to use it day to day

Use GI to choose between similar options: a grainy bread instead of white bread, pasta instead of jasmine rice, rolled oats instead of cornflakes, whole fruit instead of juice. Then watch the portion and build the plate with protein, vegetables and healthy fats. This guide has concrete ways to lower the impact of any meal.

If you have diabetes or take glucose-lowering medication, talk to your doctor or dietitian before changing your diet.

Sources

1. Jenkins DJ, Wolever TM, Taylor RH, et al. Glycemic index of foods: a physiological basis for carbohydrate exchange. Am J Clin Nutr. 1981;34(3):362–366.
2. ISO 26642:2010. Food products — Determination of the glycaemic index (GI) and recommendation for food classification.
3. Atkinson FS, Brand-Miller JC, Foster-Powell K, Buyken AE, Goletzke J. International tables of glycemic index and glycemic load values 2021: a systematic review. Am J Clin Nutr. 2021;114(5):1625–1632.
4. Zeevi D, Korem T, Zmora N, et al. Personalized nutrition by prediction of glycemic responses. Cell. 2015;163(5):1079–1094.