Glycemic Index (GI)
generalA scale from 0-100 ranking how quickly a food raises blood sugar.
Definition
A scale from 0-100 ranking how quickly a food raises blood sugar. Low GI (under 55) foods cause gradual rises; high GI (over 70) foods cause rapid spikes. Useful for diabetics and weight management.
What Is the Glycemic Index?
The Glycemic Index (GI) is a numerical scale from 0 to 100 that ranks carbohydrate-containing foods according to how quickly they raise blood glucose levels compared to a reference food — either pure glucose or white bread. Foods are classified as:
- Low GI: 55 or below (e.g., lentils ~32, steel-cut oats ~55)
- Medium GI: 56–69 (e.g., white rice ~64, pineapple ~66)
- High GI: 70 or above (e.g., white bread ~75, instant oats ~79)
Developed by Dr. David Jenkins at the University of Toronto in 1981, the GI was originally designed as a tool for managing blood glucose in people with diabetes.
Glycemic Load: A More Complete Picture
GI alone does not account for the amount of carbohydrate in a typical serving. Glycemic Load (GL) corrects for this by multiplying the GI by the grams of carbohydrate per serving, then dividing by 100. A food can have a high GI but a low GL if the serving size is small — watermelon has a GI of ~76 but a GL of only ~8 per standard serving because it contains mostly water.
GL under 10 is considered low; 11–19 is medium; 20 or above is high.
Practical Applications and Limitations
GI values are affected by ripeness, cooking method, food processing, and the presence of fat, protein, and fiber — all of which slow gastric emptying and glucose absorption. Eating mixed meals further reduces GI relevance. Despite limitations, choosing lower-GI carbohydrate sources is associated with improved glycemic control, reduced hunger, and lower risk of type 2 diabetes in epidemiological studies. The GI is most useful as one tool among many rather than the sole criterion for food selection.
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Related Terms
Whole Grain
A grain that retains all three parts: bran (fiber, B vitamins), germ (nutrients, healthy fats), and endosperm (starch, protein).
Carbohydrate
A macronutrient and the body's primary energy source.
Net Carbs
The digestible carbohydrates that impact blood sugar, calculated as total carbohydrates minus dietary fiber (and sometimes sugar alcohols).
Glucose
The body's primary fuel molecule, a simple sugar (monosaccharide).
Sucrose
Common table sugar, a disaccharide made of glucose + fructose.
Maltose
A disaccharide made of two glucose molecules, produced when starch breaks down during digestion or fermentation.
Sugar Alcohol
A carbohydrate that provides fewer calories (1.5-3 kcal/g vs 4 kcal/g for sugar), does not spike blood sugar significantly, and does not cause tooth decay.
Glycemic Load (GL)
A measure combining glycemic index and carbohydrate quantity: GL = (GI × carb grams) / 100.
Insulin Resistance
A condition where cells fail to respond normally to insulin, requiring the pancreas to produce more insulin to maintain blood sugar.