The glycemic index (GI) ranks carbohydrate foods by how quickly they raise blood sugar, on a scale up to 100. Foods with a GI of 55 or less are low, 56 to 69 medium, and 70 or more high, according to Diabetes UK. Low-GI foods include lentils, apples, carrots and coarse oats, while white bread, regular potatoes and sugary drinks are high. GI is a useful guide, but portion size and the whole meal matter more, and a low GI doesn’t automatically mean a food is healthy.
How the glycemic index works
When you eat carbohydrate, it’s broken down into glucose, which enters your blood. Some foods are digested quickly and cause a fast rise in blood sugar. Others are digested slowly and cause a gentler rise. The GI measures this by comparing a food with pure glucose. The University of Sydney’s glycemic index database defines low GI as 55 or less and high GI as 70 or more.
GI only applies to foods that contain carbohydrate. Meat, fish, eggs, oils and most cheese have little or no carbohydrate, so they don’t have a meaningful GI.
Low, medium and high GI foods
Examples from Diabetes UK:
| GI category | Range | Examples |
|---|---|---|
| Low | 55 or less | Strawberries, apples, carrots, lentils, rye bread, coarse oatmeal |
| Medium | 56 to 69 | Pineapple, sweet potato, basmati rice, chapatti, plain popcorn |
| High | 70 or more | White bread, regular potatoes, doughnuts, sweets, sugar-sweetened drinks |
Exact GI values vary between studies, brands, varieties and cooking methods, which is why the 2021 international GI tables, which list over 4,000 foods, separate more reliable values from less reliable ones. Treat categories as a guide rather than precise numbers.
What changes a food’s GI
Diabetes UK lists several factors:
- Cooking method: frying, boiling and baking can all change GI.
- Processing: milling and puréeing tend to raise it. Coarse oats have a lower GI than instant oats, for example.
- Ripeness and variety: riper fruit and some varieties of rice and potato have a higher GI.
- Fat, protein and fiber: these slow digestion, which lowers the GI of a meal.
That last point is why the GI of a single food can be misleading. You rarely eat foods alone. Adding beans to rice, or eating fruit with yogurt and nuts, changes how the meal affects your blood sugar.
The limits of GI
GI is a useful tool, but Diabetes UK points out some important limits:
- It ignores portion size. The amount of carbohydrate you eat has a bigger effect on blood sugar than GI alone. A small portion of a high-GI food can affect blood sugar less than a large portion of a low-GI one.
- Low GI isn’t the same as healthy. Chocolate has a low GI because of its fat, but that doesn’t make it a health food.
- High GI isn’t the same as unhealthy. Baked potatoes are high GI, but they provide fiber (especially the skin) and other nutrients.
Glycemic load (GL) tries to fix the portion problem by combining GI with the amount of carbohydrate in a typical serving. It appears alongside GI in the international GI and GL tables and is mainly used in research and diet planning.
Who might find GI useful?
According to Diabetes UK:
- Type 2 diabetes: choosing lower-GI foods can help manage long-term blood sugar levels (HbA1c).
- Gestational diabetes: swapping high-GI foods for lower-GI ones can help manage blood sugar.
- Type 1 diabetes: the evidence is weaker, but low-GI foods may help keep blood sugar steadier from day to day.
For people without diabetes, choosing more low-GI foods often means choosing more whole grains, beans, vegetables and fruit, which fits general healthy eating advice. If you have diabetes, ask your doctor for a referral to a registered dietitian who can help you plan meals.
Easy low-GI swaps
| Instead of | Try |
|---|---|
| White bread | Rye, sourdough or whole grain bread |
| Instant oats | Coarse or steel-cut oats |
| Regular potatoes | Sweet potato, or new potatoes with skins, in smaller portions |
| Sugary cereal | Plain oats or bran cereal |
| Sugary drinks | Water, milk or unsweetened tea |
| White rice on its own | Basmati or brown rice mixed with lentils or beans |
Building a lower-GI meal
- Start with vegetables, filling about half the plate.
- Add protein, such as fish, chicken, eggs, tofu or beans.
- Choose a whole-grain or legume-based carbohydrate in a moderate portion.
- Include some healthy fat, such as olive oil or nuts.
Our guide to the Mediterranean diet follows this pattern, and our guide to how much sugar per day explains how to cut back on the highest-GI foods: sweets and sugary drinks.
GI and keto
Very low-carbohydrate diets such as keto sidestep GI by cutting most carbohydrate out altogether. Our guide to keto and insulin resistance compares that approach with a lower-GI Mediterranean-style diet.
A worked example: why portion size matters
GI compares foods using the same amount of carbohydrate, not the same portion. Glycemic load adjusts for that, using this formula: GL = GI × grams of carbohydrate in the portion ÷ 100.
Say a food has a GI of 70 (high) and your portion contains 15 g of carbohydrate. Its GL is 70 × 15 ÷ 100 = 10.5. A food with a GI of 50 (low), eaten in a portion with 40 g of carbohydrate, has a GL of 50 × 40 ÷ 100 = 20. In this hypothetical example, the “low-GI” portion has the bigger overall effect, simply because there’s more of it. That’s why Diabetes UK says the amount of carbohydrate matters more than GI alone.
Common questions
Is brown rice lower GI than white rice?
Varieties differ a lot. Diabetes UK lists basmati rice as medium GI. Brown rice has more fiber, and mixing rice with beans or lentils, or eating it with vegetables and protein, lowers the effect of the whole meal.
Are low-GI foods better for weight loss?
Low-GI foods such as oats, beans and whole grains tend to be higher in fiber and more filling, which can help. But calories and portion sizes still matter, and some low-GI foods, such as chocolate, are high in calories.
Do I need to know the GI of every food?
No. For most people, choosing more whole grains, beans, vegetables and fruit, and fewer sugary drinks, sweets and refined starches, puts you on a lower-GI pattern without counting anything.
This article is general information, not medical advice. Talk to your doctor or a registered dietitian before making big changes to how you eat, especially if you have a health condition, take regular medication, or are pregnant or breastfeeding.
