Nutrition glossary
What does glycemic index mean?
The glycemic index (GI) is a 0 to 100 scale that ranks carbohydrate foods by how quickly they raise your blood sugar compared with pure glucose, which scores 100. Low-GI foods raise it slowly and gently; high-GI foods raise it fast.
How GI is measured
Researchers give a small group of healthy volunteers a portion of a food containing a set amount of digestible carbohydrate, usually 50 g, and track their blood sugar for two hours. They compare that curve with the curve from 50 g of pure glucose. If the food produces a curve about half the size, it gets a GI of about 50.
Only foods with a meaningful amount of carbohydrate have a GI. Meat, fish, eggs, oils and most cheese have almost none, so they don't get a score. You may see it spelled "glycaemic index" in the UK and Australia; it's the same thing.
Low, medium and high GI foods
GI values vary with the variety, ripeness, processing and cooking of a food, so different tables give slightly different numbers. These are typical, rounded figures:
| Band | GI score | Example foods |
|---|---|---|
| Low | 55 or less | Lentils, chickpeas, apples, oranges, milk, plain yogurt, pasta cooked al dente, rolled oat porridge |
| Medium | 56 to 69 | Basmati rice, brown rice, sweet potato, pineapple, honey |
| High | 70 or more | White and most wholemeal sliced bread, cornflakes, instant oats, rice cakes, baked or mashed white potato, watermelon |
A few surprises stand out. Wholemeal sliced bread often scores close to white bread because the flour is finely milled. Instant oats score higher than rolled or steel-cut oats, even though they are the same grain. And pasta has a lower GI than bread made from similar flour, because of the way the dense dough is structured.
Glycemic load: the portion matters too
GI tells you how fast the carbohydrate in a food hits your blood, but not how much carbohydrate you are actually eating. Glycemic load (GL) fixes that by factoring in the portion:
GL = GI × grams of carbohydrate in the portion ÷ 100
Watermelon is the classic example. Its GI is high, around 76, but a 120 g slice has only about 6 g of carbohydrate, so its GL is around 5, which is low. A large bowl of cornflakes, by contrast, is high on both. As a rough guide, a GL of 10 or less is low, 11 to 19 is medium and 20 or more is high.
Where GI falls short
- You rarely eat foods alone. Adding fat, protein, fibre or acid (like vinegar) to a meal slows the rise in blood sugar. Toast with peanut butter behaves differently from dry toast.
- Low GI doesn't mean healthy. Crisps, chocolate and some ice creams have a lowish GI because of their fat content. Potatoes and watermelon score high but are perfectly good foods.
- People respond differently. The same food can cause quite different blood sugar rises in two people, or in you on two different days.
- Cooking changes the number. Soft, overcooked pasta and very ripe bananas score higher than al dente pasta and firmer bananas.
Using GI day to day
You don't need to look up numbers. The general pattern is enough: choose beans, lentils, whole fruit, oats and less-processed grains more often, and pair starchy foods with protein, vegetables or a bit of healthy fat. Those habits lower the GI of your meals and fit with the Mediterranean and DASH diets.
If you have diabetes, GI can be one useful tool, but the total amount of carbohydrate still matters most. Your diabetes team can help you use it alongside your medicines.
Common questions
What is a good glycemic index number?
A GI of 55 or less counts as low, 56 to 69 as medium and 70 or more as high. Most people who follow a low-GI approach try to build meals mainly around foods in the low band.
Is a low-GI diet good for weight loss?
It can help some people feel fuller for longer, but studies show mixed results. Weight loss still comes down to an overall calorie deficit, and low-GI eating is one way that may make that easier.
Do bananas have a high glycemic index?
Not usually. A typical banana sits in the low to medium range, around 50. Riper, spotty bananas score higher than slightly green ones because more of their starch has turned to sugar.
Related
Read about net carbs and added sugar to see how carbs are counted on labels, and macronutrients for how carbs fit with protein and fat. For whole eating plans, see low-carb diets, the DASH diet and healthy eating basics.