What Is the Glycemic Index?

The glycemic index 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 spikes. Knowing the glycemic index of the carbohydrates you eat is one tool that can help you fine-tune your meals to keep your blood glucose within a normal range.

The glycemic index (GI) is a rating system where foods are ranked on a scale of one to 100 based on how much they raise blood glucose. Processed foods such as candy, breads, cake, and cookies have a high GI, while whole foods such as unrefined grains, starchy vegetables, and fruits tend to have a lower GI. Carbohydrates with a low GI value (55 or less) are digested, absorbed, and metabolized more slowly than their high-GI counterparts. They typically cause a lower and slower rise in blood glucose and, therefore usually, insulin levels.

The index values are created by a rigorous testing process: 10 or more people each eat 50 grams of the same digestible (available) 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 the glucose AUC for the reference food for each person, and the final GI value is an average of those 10 numbers.

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: 1 to 55
• 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 68.

Critics of the GI system state that the index doesn't take into account the quantity of the food being eaten or the other nutritional qualities (or lack thereof) in the food, 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'd be eschewing 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. It is calculated by multiplying the GI 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—this is considered a low GL food.

The GL values can also be broken down into three ranges.

• Low GL: 1 to 10
• Medium GL: 11 to 19
• High GL: 20 or more

Since it's the carbohydrates in food that raise blood sugar, the glycemic index can help when trying to figure out which foods are best for glucose management. Since all carbohydrates affect blood glucose levels differently, knowing which carbohydrates have a lower glycemic index can help you plan your meals more effectively.

Benefits of following the GI list when planning your meals include being 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 tedious 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.

The GI of foods can actually change depending on a number of factors, making it a fairly unreliable guide. The number listed in the chart isn't necessarily the same value as the food on your plate, depending on several conditions:

• Cooking time: For example, al dente pasta has a lower GI than longer-cooked pasta
• Preparation: Acids such as vinegar and lemon juice and fats such as olive oil can lower the GI of foods
• Ripeness: GI increases with riper fruit

In addition, 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, as opposed to eating an apple with peanut butter. Protein and fat can delay carbohydrate metabolism and therefore result in a slower blood sugar rise. 

Lastly, there is some evidence to suggest that the order in which you eat a meal may affect your blood sugar. Some studies suggest that eating protein and fat before carbohydrates may help to reduce post-meal blood sugars. 

But this brings us to a broader point: the glycemic index is still just a list of numbers. How a food specifically affects your unique makeup and blood sugar will vary by individual.

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 will be 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. 

Understanding the GI of foods can be a good tool to use in addition to carbohydrate counting to help control blood sugars, but it shouldn't be the only tool you use. Familiarize yourself with this chart and the GI concept, but note that the GI should be used as an adjunct to other lifestyle changes such as eating an overall balanced diet, practicing good portion control, and exercising regularly. 

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. For the most accurate results, test your blood sugar two hours after the start of a meal to see how your body specifically responds to certain foods.