What Is the Glycemic Index? A Simple Guide to GI and GL

The glycemic index (GI) is a scale from 0 to 100 that ranks carbohydrate-containing foods by how quickly they raise blood sugar compared with pure glucose, which scores 100. Foods scoring 55 or less are considered low GI, 56 to 69 medium, and 70 or more high. Because GI ignores how much you actually eat, a related measure called glycemic load is often more useful day to day.

How GI is measured

Researchers give volunteers a portion of a food containing a set amount of digestible carbohydrate, usually 50 grams, then track their blood sugar over the next two hours. They compare that curve with the response to the same amount of pure glucose, and the result becomes the food’s GI value.

Because testing involves small groups of people, and because foods vary by variety, ripeness and preparation, published GI values are averages rather than exact figures. The same food can appear with different numbers in different tables.

Low, medium and high GI foods

These groupings are typical, but values shift with cooking, ripeness and processing.

Watermelon is the classic example of why GI alone can mislead, which brings us to glycemic load.

Glycemic load: adding portion size

Glycemic load (GL) combines a food’s GI with the amount of carbohydrate in a typical serving:

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

A GL of 10 or less is considered low, 11 to 19 medium, and 20 or more high.

Watermelon has a high GI, in the 70s, but a cup of diced watermelon contains only about 11 to 12 grams of carbohydrate because it’s mostly water. That gives it a GL of roughly 8, which is low. A big bowl of many types of white rice, on the other hand, pairs a fairly high GI with a lot of carbohydrate, so its GL ends up high.

What changes a food’s GI

• Ripeness. A ripe, spotty banana usually has a higher GI than a slightly green one, because more of its starch has turned to sugar.
• Processing. Instant oats tend to score higher than steel-cut oats, since finer pieces digest faster.
• Cooking time. Pasta cooked al dente typically has a lower GI than soft, overcooked pasta.
• Cooling. Cooking and then cooling starchy foods like potatoes, rice or pasta forms some resistant starch, which digests more slowly.
• What you eat alongside it. Fat, protein, fiber and acidic ingredients like vinegar or lemon juice slow digestion and soften the blood sugar rise of a meal.

Should you eat by the glycemic index?

GI can be a helpful tool, especially for people working with their care team to manage blood sugar. Trading a high-GI food for a lower-GI cousin, such as steel-cut oats instead of a sugary flaked cereal, often brings more fiber and longer-lasting fullness as a bonus.

But GI has real limits:

• Low GI doesn’t mean healthy. Ice cream and some candy bars score low because their fat slows digestion.
• Most meals are mixed. You rarely eat a carbohydrate food by itself, and the rest of the plate changes the response.
• People respond differently. Blood sugar reactions to the same food can vary from one person to another.
• Many nutritious foods contain little or no carbohydrate, so GI doesn’t really apply to them, including meat, fish, eggs, oils and most leafy greens.

That’s why GI works best as one consideration among many, next to fiber, added sugar and overall food quality. Our guide to complex carbs explains why whole, fiber-rich carbohydrates tend to behave better than refined ones, and our dietary fiber explainer covers why fiber is such a useful ally.

Practical ways to use it

1. Choose intact or minimally processed grains such as barley, bulgur, steel-cut oats and breads with visible whole grains.
2. Make beans and lentils a regular part of your meals.
3. Pair carbohydrates with protein, healthy fat and vegetables instead of eating them solo.
4. Cook pasta al dente, and try leftover potatoes or rice cold in salads.
5. Keep portions of high-GI foods moderate rather than cutting them out completely.
6. If you have diabetes or prediabetes, ask your doctor, diabetes educator or registered dietitian how GI fits your plan.

If you also see “net carbs” on packages, our explainer on net carbs vs total carbs clears up what that number does and doesn’t tell you.

The bottom line

The glycemic index ranks carbohydrate foods by how quickly they raise blood sugar, from low (55 or less) to high (70 or more). Glycemic load adds portion size and is often the more practical number. Both are useful guides, but they work best alongside the bigger picture: fiber, whole foods, balanced meals and how your own body responds.