The glycemic load, which is based on the idea that a high-glycemic food eaten in small quantities produces a blood sugar response that's similar to the response produced by low-glycemic foods, is a much more useful tool for your day-to-day use. It allows you to have more food choices than the glycemic index does alone.

That's good news because no one wants to be too restricted in what he or she can eat. But to create the glycemic load, researchers first had to come up with the glycemic index.

The glycemic index concept was developed in 1981 by two University of Toronto researchers, Dr. Thomas Wolever and Dr. David Jenkins. Their research compared the effect of 25 grams of carbohydrates (just picture two slices of bread if you're not familiar with the metric system) to that of 50 grams of carbohydrates (picture four slices of bread) to see whether the smaller amount created a lower-glycemic response in the human body based on the lower quantity of carbohydrates.

However, with the amount of carbohydrates varying so much in different foods (for instance, some fruits and vegetables have only 5 grams of carbohydrates whereas starches have up to 15 grams), 50 grams of carbohydrates (the standard amount used for glycemic index testing) doesn't always depict the portion size a person may typically eat.

To account for this variation, in 1997, Harvard University's Dr. Walter Willet created the glycemic load, which calculates the quality and quantity of carbohydrates at a meal. The fact that the glycemic load takes portion size into account is quite helpful because the average person is far less likely to eat 50 grams of a particular food in one sitting.

Looking at portion sizes and carbohydrate grams can give you a better understanding of the glycemic load. Although foods vary, the following table breaks down the average amount of carbohydrates in each carbohydrate-containing food group based on a particular portion size.

Average Carbohydrate Grams in Four Food Groups
Food GroupCarbohydrate GramsPortion Size
Starches 15 1/2 cup pasta, 1 slice bread, 1/3 cup white rice
Fruits 15 1 small piece
Dairy products 12 1 cup milk, 1 cup light yogurt
Nonstarchy vegetables 5 1/2 cup cooked, 1 cup raw

As you can see, the amount of carbohydrates in a serving of a particular food depends as much on the portion size as it does on the food itself. So consuming 50 grams of carbohydrates (which is definitely more than one serving) will have a dramatic impact on your blood sugar.

Take carrots, for example. Carrots have a high glycemic index when cooked (41 to be exact), yet they're considered a nonstarchy vegetable. To consume 50 grams of carbohydrates in carrots, you'd have to eat 5 cups! Because the amount of carbohydrates in carrots is so low compared to their average portion size, the glycemic load of carrots is low as well.

On the other hand, a serving of instant white rice, another high-glycemic food with a glycemic index of 72, has around 15 grams of carbohydrates per 1/3-cup serving. To eat 50 grams of carbohydrates in instant white rice, you'd have to eat slightly more than 1 cup of rice — a fairly typical portion size for most people. This portion size means the glycemic load for instant white rice doesn't change much from the food's glycemic index.

The glycemic index compares the potential of foods with equal amounts of carbohydrates to raise blood sugar. The purpose of the glycemic load is to have a usable indicator of the glycemic index that takes portion size into account.

Although adding glycemic load to the mix may cause the glycemic index of some foods, such as white rice, to remain the same, it opens up the door for enjoying more foods that may have a high glycemic index but a low glycemic load based on different portion sizes.