How to Use the Glycemic Index

Medically Reviewed by Zilpah Sheikh, MD on February 05, 2024
7 min read

Glycemic index (GI) is a number that gives you an idea about how fast your body converts the carbohydrates in a food into glucose. The GI is a way to rank foods containing carbs on a scale from 1 to 100 based on how much they affect your blood sugar levels. Two foods with the same amount of carbohydrates can have different GI numbers. The smaller the number, the less impact the food has on your blood sugar. 

You can group foods into three categories based on their GI: low, medium, and high. A score of 100 means that a food has a big effect on your blood sugar, while a score of 1 indicates little effect. GI can help you make food choices. For example, following a low-GI diet can help you avoid spikes in your blood sugar levels.

  • 1 to 55 = Low 
  • 56 to 69 = Medium
  • 70 to 100 = High 

The reason some foods make blood sugar shoot up fast is that simple carbohydrates in them, such as refined sugars and white flour, are easier for your body to change into glucose, the sugar your body uses for energy. In contrast, carbs such as those in vegetables and whole grains are digested more slowly. Foods that are close to how they're found in nature tend to have a lower glycemic index than refined and processed foods. If you eat a lot of those high-GI carbs, you may have a harder time controlling your blood sugar.

The reason this measure depends on carbs and not fats or lipids is that carbohydrates contain sugars, starches, and fiber. When your body breaks down sugar or starches, they turn into glucose. This sugar is the main energy source for your cells.

You need glucose, but not too much. Your body makes hormones to control glucose levels. These hormones include insulin and glucagon. Insulin moves glucose from your blood into your other organs. Glucagon releases glucose stored in your liver when you need more blood sugar. Your body should normally keep glucose levels at a healthy level. But this can change if you have diabetes or don't respond to insulin the way you should. 

You can also control how much glucose enters your bloodstream in the first place by changing how you eat. The glycemic index gives you a way to tell carbs that will turn into glucose slower from the "faster" carbs. You can use it to fine-tune your eating habits and help keep your blood sugar steadier.

Low-GI vs. high-GI foods

Look for the glycemic index on the labels of packaged foods. You can also find GI lists for common foods online. For example, Oregon State University has one with more than 100. Or ask your dietitian or nutrition counselor.

Examples of low-GI foods include:

  • Green vegetables
  • Many fruits, such as apples, pears, and oranges
  • Raw carrots
  • Kidney beans
  • Chickpeas 
  • Lentils
  • Skim milk
  • Peanuts
  • Cashews

Examples of medium GI foods include:

  • Corn
  • Bananas
  • Pineapple
  • Raisins
  • Cherries
  • Oat cereal
  • Brown rice
  • Honey or maple syrup
  • Multigrain, whole-grain wheat or rye bread

Examples of high-GI foods include:

  • White rice
  • White bread
  • Potatoes
  • Puffed rice cakes
  • Doughnuts
  • Watermelon
  • Jelly beans
  • Cornflakes

A food's GI index is a starting point on paper. But it could be different on your plate, depending on how you prepare and store your food. It also depends on how ripe it is and what you eat it with.

Preparation. Fat, fiber, and acid (such as lemon juice or vinegar) lower the glycemic index. The longer you cook starches such as pasta, the higher their glycemic index will be.

Food storage. If you keep starchy foods in the refrigerator after cooking, it can make them have more resistant starch. When you eat foods with resistant starch, you can't digest them as easily, which lowers the GI. Cook and cool potatoes, bread, pasta, or rice to lower their GI.

Ripeness. The glycemic index of fruits such as bananas goes up as they get sweeter and more ripe

Other foods eaten at the same time. You can bring down the overall glycemic index of a meal by combining a high-glycemic index food with foods that have lower ones. Adding fiber, fat, protein, or vinegar can change the way your body breaks down foods. For example, adding protein to rice will lower the GI. Adding ingredients such as pickles or olive oil can help, too.

Factors such as your age, how active you are, and how fast you digest food also affect how your body reacts to the carbs you eat. If you have a diabetes complication called gastroparesis, which delays your stomach from emptying, your body will absorb food much more slowly. When you break food down more slowly, it takes longer for glucose to enter your bloodstream.

If you're on a low-GI diet, you'll plan your meals based mainly on how much foods will affect your blood sugar. You will limit carbs in your diet to those with low GI scores, such as beans, green vegetables, fruits, and raw carrots. 

You might want to follow a low-GI diet if you:

  • Want to lose weight or avoid gaining weight
  • Want to eat healthy
  • Want to keep your blood sugar levels from going up quickly to manage diabetes
  • Want to lower your risk for diabetes or heart disease

Do low-GI diets work?

Studies have shown different results, but a low-GI diet may help you:

  • Lose weight
  • Lower blood pressure
  • Lower cholesterol
  • Improve or prevent diabetes
  • Prevent heart and vascular diseases

Some of the benefits of a low-GI diet may not come from the effect on blood sugar since many low-GI foods are healthy in other ways. Low-GI foods tend to have a lot of nutrients and fiber in them.

You should also remember that some nutritious foods can have a higher GI. GI is one way to make choices about healthy eating, but you should keep other factors in mind, too.

Glycemic load (GL) is another way to track glucose by considering both a food's GI and how much of it you're eating. You get GL by multiplying the amount of carbohydrates in your food by the GI and dividing that number by 100. A GL over 20 is considered high and a GL under 10 is low.

The glycemic index or load shouldn't be the only thing you consider when making choices about what to eat. The fact that a food has a low glycemic index doesn't mean it's super-healthy, or that you should eat a lot of it. Calories, vitamins, and minerals are still important.

For example, potato chips have a lower glycemic index than oatmeal and about the same as green peas. But oatmeal and green peas have more nutrients. Watermelon and doughnuts have similar GIs. But a serving of watermelon has a GL of 8. An average doughnut has a GL of 17. Many foods have a medium or high GI but a low to medium GL. Some examples include:

  • Parsnips
  • Watermelon
  • Banana
  • Pineapple

Portion sizes matter, too. The more of whatever kind of carbs you eat, the more they'll affect your blood sugar. That's what the glycemic load tells you. It's a number you may see along with the glycemic index in lists. Think of it as the glycemic index for a specific amount of that food.

Glycemic load helps you account for both the quantity and the quality of your carbs at the same time. 

Low-GL vs. high-GL foods

For a diet with a lower glycemic load, eat:

  • More whole grains, nuts, legumes, fruits, vegetables without starch, and other foods with a low glycemic index
  • Fewer foods with a high glycemic index, such as potatoes, white rice, and white bread
  • Less sugary foods, including candy, cookies, cakes, and sweet drinks

You can still eat foods with a high glycemic index or glycemic load. Just enjoy them in smaller portions, and offset them with nutritious, low-glycemic index foods when you do.

The glycemic index is a way to think about how healthy foods are based on their effect on blood sugar. Keeping your blood sugar more stable by eating low-GI foods can help when you have diabetes. It also can help you prevent diabetes and other health problems, including heart disease. But GI is only one way to think about your food choices. Talk to your doctor or a dietician about steps you can take to eat a healthier diet, including more low-GI foods.

  • What foods have a glycemic index score of 0?

Foods that don't have any carbohydrates in them have a zero GI. Examples are foods with protein and fat but no carbs, such as chicken, beef, fish, and eggs.

  • Do bananas have a high glycemic index?

Bananas have a GI of 55, which is considered medium. They also have a GL of 13. But remember that the GI of a specific banana will vary depending on lots of factors.

  • Does peanut butter have a low glycemic index?

Peanuts have a low GI of 18. The GI of your peanut butter will depend on how much added sugar or other ingredients are in it. Check the label to see what the GI is. Natural peanut butter containing only peanuts would have a low GI.

  • What is the glycemic index of an apple?

A medium-sized raw apple has a GI of 39, which is considered low. But the GI of an apple also will depend on what other foods you eat it with, how ripe it is, and whether you eat it cooked or raw.