What Is the Glycemic Index?

The glycemic index (GI) is a relative ranking of carbohydrates in foods according to how they affect blood sugar. When you have type 2 diabetes, one of the best ways to control your glucose levels is to eat foods that don't cause major blood sugar (glucose) spikes.

Knowing the glycemic index of the carbohydrates you eat can help you fine-tune your meals to keep your blood sugar within a normal range.

What Is the Glycemic Index?

The GI is a rating system where foods are ranked on a scale of 1 to 100 based on how much they raise blood sugar.

Processed foods such as candy, breads, cake, and cookies have a high GI, while whole foods such as unrefined grains, non-starchy vegetables, and fruits tend to have a lower GI.

Carbohydrates with a low GI value are digested, absorbed, and metabolized more slowly than their high-GI counterparts. They typically cause a lower and slower rise in blood glucose and, subsequently, insulin levels as well.

Gl and Carb Count of Common Foods

How the Glycemic Index Is Measured

The index values are created by a rigorous testing process. Ten or more people each eat 50 grams of the same digestible carbohydrate (the test food), then researchers measure each individual's glucose response two hours after consumption, plot the points on a graph, and measure the area under the curve (AUC) of their glucose response.

At a separate date, the same 10 people consume 50 grams of pure glucose (the reference food), and researchers again measure each person's glucose response AUC two hours after consumption.

The GI value of the test food is then calculated by dividing the glucose AUC for the test food by that of the reference food for each person. The final GI value is an average of those 10 numbers.

Glycemic Index Values

The GI values can be broken down into three ranges. Remember that a low GI is a food that won't raise your blood sugar as much as a food with a medium or high GI.

• Low GI: 55 or less
• Medium GI: 56 to 69
• High GI: 70 to 100

For example, rice milk (a processed food without any fiber) has a high GI of 86, while brown rice (plenty of fiber) has a medium GI of 66.

Glycemic Index vs. Glycemic Load

Critics of the GI system state that the index doesn't take into account how much food is being eaten or its other nutritional qualities (or lack thereof), such as protein, fat, vitamins, minerals, and antioxidants. As the GI looks strictly at the carb count, basing a diet around these numbers means you would be ignoring a lot of other helpful information to determine the true health value of a food.

To counteract the quantity issue, researchers developed the glycemic load (GL) measurement, which accounts for the quantity of the food being eaten. The glycemic load looks at both the quality and the quantity of the carb.

Glycemic load is calculated by multiplying the GI value by the number of carbohydrates (in grams), then dividing that number by 100.

For example, an apple has a GI of 40 and contains 15 grams of carbs. (40 x 15)/100 = 6, so the glycemic load of an apple is 6.

Glycemic Load Values

Like GI values, GL values can also be broken down into three ranges:

• Low GL: 10 or less
• Medium GL: 11 to 19
• High GL: 20 or more

Benefits of Referring to the Glycemic Index

Since it's the carbohydrates in food that raise blood sugar, understanding GI can help you figure out which foods are best for glucose management.

Among the benefits of following the GI list when planning your meals:

• It helps you be more mindful of your carb choices without fully restricting or severely limiting your intake.
• If you aim for a low-GI diet, you'll naturally be focusing on whole grains, fruits, vegetables, and legumes, as opposed to the higher-GI end of the spectrum, which includes more processed foods.
• Depending on your health goals, following a GI-based diet might mean you'll be able to rely less on standard dieting measures, such as calorie counting or regimented portion control.
• Simply being more mindful of your carb choices rather than severely limiting them can also be more sustainable in the long run, as compared to more restrictive diets.

Where the Glycemic Index Falls Short

The GI of foods can actually change depending on a number of factors, which can make the measure unreliable in certain cases.

The composition of a meal can change the effect of blood sugar rise. For example, eating an apple on its own may result in a different blood glucose response than if you ate it with some peanut butter. Protein and fat can delay carbohydrate metabolism and, therefore, result in a slower blood sugar rise. 

But this brings us to a broader point: The glycemic index is still just a list of numbers. How a food specifically affects someone's unique makeup and blood sugar varies by individual.

The Best Way to Test a Food's Impact

The American Diabetes Association states that carbohydrate amount (grams of carbohydrates) and available insulin may be the most important factors influencing blood sugar response after eating and should be considered when developing an eating plan. 

The most reliable way to assess how your body is affected by certain foods is to test your blood sugar two hours after a meal.

For most people, an ideal blood sugar result is less than 180mg/dL two hours after the start of a meal. If you are not sure of what your target blood sugar should be, discuss it with your physician.

A Word From Verywell

Referencing the GI of foods can useful, but it shouldn't be the only tool you use to help control blood sugars.

The glycemic index should be used as an adjunct to carb counting and lifestyle changes, such as eating an overall balanced diet, practicing good portion control, and exercising regularly.