What Is the Glycemic Index? How Food Affects Your Blood Sugar

What Is the Glycemic Index?

The glycemic index (GI) is a numerical scale that ranks carbohydrate-containing foods by how quickly and how much they raise blood glucose (blood sugar) levels after eating, compared to a reference food — typically pure glucose (GI = 100) or white bread (GI = 100 in some systems).

Developed by Dr. David Jenkins and colleagues at the University of Toronto in 1981, the GI was originally designed as a tool to help people with diabetes choose foods that produced smaller blood sugar fluctuations. It has since become widely used in nutrition science and popular diet culture.

How GI Is Measured

To determine a food's GI, researchers give 10 or more healthy volunteers a serving of the test food containing 50 grams of digestible carbohydrates. Blood glucose is measured every 15–30 minutes for 2 hours after eating. The resulting area under the blood glucose curve is compared to the same person's response to 50 grams of glucose (or white bread) consumed on another day. The GI is the ratio, expressed as a percentage.

GI Categories

• Low GI (55 or less): Lentils (29), chickpeas (28), most non-starchy vegetables, whole oats (55), most fruits (apples 36, oranges 43), whole-grain pasta (42), milk (31)
• Medium GI (56–69): Brown rice (65), whole wheat bread (69), popcorn (65), pineapple (59)
• High GI (70 or above): White bread (75), white rice (72), watermelon (72), instant oatmeal (83), glucose (100), baked potato (78), corn flakes (81)

Notably, table sugar (sucrose) has a GI of about 65 — lower than some starchy foods like baked potatoes or rice cakes — because sucrose is half fructose, which doesn't directly raise blood glucose.

Factors That Affect GI

The same food's GI can vary significantly based on:

• Processing: Finely milled flour has higher GI than coarse whole grain; instant oats higher than steel-cut oats. Processing breaks down starch structure, making it digest faster.
• Cooking method: Al dente pasta has lower GI than overcooked; cooled cooked potatoes have lower GI than hot ones (retrograde starch formation)
• Ripeness: Ripe bananas have higher GI than unripe ones
• Fat and protein content: Reduce GI by slowing stomach emptying — adding butter to bread lowers GI
• Acid content: Vinegar and lemon juice significantly reduce GI of a meal

The Limitation: Glycemic Load

GI has a major practical limitation: it's based on a standard 50g carbohydrate serving, not typical portion sizes. Watermelon has a high GI of 72, but a typical serving has very few carbohydrates — you'd need to eat enormous amounts to significantly raise blood sugar.

Glycemic Load (GL) corrects for this by accounting for both GI and the amount of carbohydrates per typical serving:

GL = (GI × grams of carbohydrate per serving) ÷ 100

• GL ≤ 10 = low
• GL 11–19 = medium
• GL ≥ 20 = high

Watermelon: GI 72, but a 120g serving has only 6g carbs, giving GL = (72 × 6) ÷ 100 = 4.3 (low). GL is generally considered a more practical measure than GI alone.

Does a Low-GI Diet Improve Health?

The evidence is mixed but generally supportive for specific populations:

• Diabetes management: Low-GI diets modestly improve HbA1c (average blood sugar), fasting glucose, and insulin sensitivity in people with type 1 and type 2 diabetes. Major diabetes organizations recognize low-GI eating as one valid dietary pattern.
• Weight management: Evidence is modest and inconsistent. Low-GI foods are often more satiating, but overall calorie balance matters more than GI for weight loss.
• Cardiovascular disease: A 2021 LANCET study found high-GI diets associated with increased risk of cardiovascular events, even in people without overweight — suggesting GI matters beyond just blood sugar for metabolic health.

Important caveat: GI focuses on individual foods in isolation, but people eat mixed meals where the combined GI differs from any single component. Dietary pattern, food quality, fiber, and overall nutrition matter far more than optimizing for GI alone.