What Is the Glycemic Index?

Growing up, you were probably told to limit sweets. That meant not overdoing it on Halloween candy, for example, or having that extra bowl of sugary breakfast cereal.

While adult-you might be tempted to throw caution to the wind and eat whatever you want, maintaining a healthy diet is more important than ever.

One way of looking at a healthy diet is considering where foods fall on the glycemic index.

“The glycemic index is used to classify foods that contain carbohydrates, their potential for raising blood sugar and how quickly they raise your blood sugar,” says registered dietitian Anthony DiMarino, RD, LD.

DiMarino explains how the glycemic index can help you choose healthy foods.

What is the glycemic index?

The glycemic index measures how quickly foods that contain carbohydrates can increase your blood sugar. The scale is 0 to 100. Foods can fall into one of three categories:

• Low glycemic index: 1 to 55
• Medium glycemic index: 56 to 69
• High glycemic index: 70 and higher

Essentially, the glycemic index shows that not everything that contains the exact same amount of grams of carbs will behave the same way in your body. You can measure how your body responds to a cup of orange juice versus how it reacts to other food items: a banana, 25 grams of candy or 25 grams of whole-grain bread.

The reason for these differences comes from how carbs react in your body. Simple carbohydrates — say, the sugars found in soda and sweet desserts — are broken down faster than the more complex carbohydrates found in some vegetables and whole-grain foods. As a result, your blood sugar spikes and then falls rapidly. Over time, doctors believe these constant surges and retreats play a part in people becoming insulin resistant.

Glycemic index chart

When it comes to how to calculate glycemic index numbers, doctors have measured only certain foods or classes of foods (foods like meats, fats and oil don’t contain carbs).

To determine glycemic index numbers, doctors observe how healthy people’s blood sugar levels change after ingesting carb-containing foods. By measuring subsequent blood sugar levels — and comparing it to a baseline — they can determine where a food falls on a scale of zero to 100, where 100 represents pure glucose and zero is a food with no sugar at all.

Use this glycemic index food chart as a starting point.

Low-glycemic index foods

Foods with a number that’s 55 or less include:

• Apple
• Banana
• Barley
• Broccoli
• Carrots
• Cashews
• Chickpeas
• Dark chocolate
• Eggplant
• Lemons
• Lettuce
• Limes
• Mango
• Oranges
• Peanuts
• Pears
• Peppers
• Plain yogurt
• Skim milk
• Strawberries
• Tomatoes
• Whole milk

Medium-glycemic index foods

Foods that have a range of 56-69 on the glycemic index include:

• Brown rice
• Couscous
• Pineapple
• Popcorn
• Potato chips
• Soda
• Sweet potato

High-glycemic index foods

Foods that have a glycemic index number of 70 or more include:

• Cereal
• Cheeseburger
• Corn chips
• Doughnuts
• French fries
• Fried chicken
• Mashed potatoes
• Pizza
• Pretzels
• Sports drink
• White bread

The University of Sydney has a useful glycemic index of foods database you can search.

Why calculating the index isn’t exact

DiMarinocautions that determining the glycemic index ratings for individual foods is an imperfect science. Factors like how food is processed or prepared plays a role.

For example, one study of instant oatmeal shows that it had a glycemic index of 79, which is the high range. Yet steel-cut rolled oats have a glycemic index of 55, which puts it in a low glycemic index.

Different versions of the same food will also vary due to ingredients. For example, yogurt that’s marketed for kids is going to taste delicious because it has a lot of sugar or artificial sweeteners, like high fructose corn syrup — versus plain Greek, regular fat yogurt.

“They might have very similar amounts of carbs, but the glycemic index is very different because of the number of proteins and fats and the quality and quantity of the sugar that’s in there,” says DiMarino.

How much you eat of a certain carb-heavy food can also have an effect. For example, a small sip or two of soda won’t have as big an impact on your body as a large portion of whole-grain rice — even though the latter is generally perceived as healthier.

As with many diets, DiMarinostresses that “moderation” is key, as is striking a balance between eating low and high glycemic index foods.

“We’re all human,” he says. “It’s very difficult for most of us to try to eliminate any one kind of food if we like it. So, it’s all about having portions that are reasonable and not going overboard.”