Glycemic Index
Glycemic Index — The glycemic index (GI) is a numerical scale that ranks carbohydrate-containing foods by how quickly they raise blood glucose after consumption, relative to a reference food (usually pure glucose, set at 100). Foods are classified as low (≤55), medium (56-69), or high (≥70) GI based on standardized two-hour blood glucose response curves.
What is the glycemic index?
The glycemic index (GI) is a 0-100 scale that ranks foods by their effect on postprandial (after-eating) blood glucose. The methodology was developed by Dr. David Jenkins at the University of Toronto in 1981 and remains the international standard for comparing carbohydrate quality.
A food’s GI is determined experimentally: healthy volunteers consume a 50-gram available-carbohydrate portion of the test food, and blood glucose is measured at intervals over two hours. The area under the resulting curve is divided by the area under the curve for a reference food (pure glucose, GI = 100), and the ratio is multiplied by 100.
How are foods classified?
- Low GI: 55 or less (most legumes, many fruits, intact grains, most non-starchy vegetables)
- Medium GI: 56-69 (whole-wheat bread, brown rice, sweet corn)
- High GI: 70 or above (white bread, white rice, instant oatmeal, most refined cereals)
GI values are published by the University of Sydney’s Glycemic Index Database, which is the most-cited reference source.
Why glycemic index matters
GI is most clinically relevant for people managing diabetes, insulin resistance, or reactive hypoglycemia. For weight management in metabolically healthy individuals, total calorie intake and protein intake are far stronger determinants of outcomes than GI, though low-GI diets may modestly improve satiety.
GI has limitations. It is measured on isolated 50-gram carbohydrate portions, but real meals combine carbs with fat, protein, and fiber, all of which blunt the glucose response. For mixed meals, glycemic load is generally a more useful metric because it incorporates portion size. See also our entry on net carbs.