Getting to Know the Glycemic Index

By Susan Learner Barr, M.S., R.D.

In type 1 diabetes (T1D), to have good control of your blood-glucose levels, you need to learn how to count carbohydrates (CHO). That’s right, counting CHO is pretty much T1D “Diet 101.” First you learn that CHO are the main source of energy for your body—they are broken down into glucose, which is the essential fuel for your brain, and the universal fuel for most of your organs and tissues. But not all CHO are created equal. You digest different types of CHO at different rates—with varying effects on your blood glucose. Yet a person with T1D always needs to be super-diligent about all the CHO that he or she consumes.

So why bring the glycemic index (GI) of a food into the mix? Isn’t figuring out what to eat or not eat with T1D complicated enough? The answer is certainly “yes,” but keeping an eye on the GI of a food might be another valuable tool for better management of your blood glucose. In a study looking at the effect of a low-GI diet versus a standard diet on blood-glucose levels and at the nutritional quality of diets in children with T1D, researchers at the NIH found that a lower-GI diet was associated with improved nutrition and healthier daytime blood-glucose levels (as measured by continuous glucose monitors). Although this was a pilot study conducted in a small group of participants, the results, published in the Journal of the American Dietetics Association (now the Journal of the Academy of Nutrition and Dietetics) suggest that improving your GI “IQ” may be worth the effort.

Glycemic index: What’s it all about?

The “glycemic index” is a concept created by David Jenkins, Ph.D., an expert nutrition scientist and researcher at the University of Toronto. He and his coworkers introduced this concept in the early 1980s. Not only did they develop the classification of foods based on GI, but they also conducted studies examining the effects of a low-GI diet on insulin levels, heart disease risk factors, inflammation, and weight loss. Their work spawned a number of GI-based weight-loss plans, and the popularity of the GI has taken hold in countries such as Australia, where many foods are labeled with their GI value.

The GI is a rating system that ranks CHO-containing foods according to their capacity to increase blood glucose. Foods with a low GI cause less of a spike in post-meal (also referred to as “postprandial”) blood glucose than those with a high GI. The standard used to determine the GI of a food is a portion that contains 50 grams of CHO, which is less than two ounces. The reference food that is used is 50 grams of CHO provided by glucose (simple sugar), which is assigned a value of 100, based on mathematical calculations. This is the reference against which a food is measured: foods with a “low” GI have a range of 55 or lower and do not raise blood-glucose levels very high or for a long period of time. Those foods with a “medium” GI have a range of 56 to 69 and have a moderate effect on raising blood-glucose levels. Those foods with a “high” GI have a range of 70 and above, and raise blood-glucose levels higher for a longer period of time. For example, a small plain bagel, with a GI of 72, is considered a high-GI food. (Check out our list of the GI values for some commonly eaten foods.)

Using the GI to your advantage

Now that the math is squared away, it’s time to think about making the GI work for you. Your goal is to use the GI to help support better management and control of your blood glucose. Choosing foods that have a lower GI may offer the advantage of: 1) adding healthier (in general) foods to your diet; 2) lowering post-meal hyperglycemia; and 3) reducing the risk for hypoglycemia after your body has processed a meal or food. Some well-known nutrition considerations come into play here, including fiber and food processing, as well as some surprising ones, such as food preparation technique, acidity, and ripeness.

Tips and tricks for using the GI

You may wonder why you don’t see steak or olive oil on many GI food lists. That’s because GI is based on CHO, and meats and fats have little or no CHO. In addition, many more singular foods, such as types of fruit or brands of cereal, are found on GI lists, rather than “mixed” dishes such as macaroni and cheese or pizza, simply because a mixed dish is difficult to standardize due to variability in recipes. On the subject of mixing, however, it’s good to know that adding a low-GI food, such as beans, to a high-GI food, such as rice, lowers the overall GI. The same goes for adding low-GI nuts to a snack that contains higher-GI foods such as chips. Fiber is your friend, too. A more fiber-rich fruit such as a pear has a lower GI. Less processing of foods often means a lower GI, too. So if you like oatmeal, choosing the steel-cut variety, instead of the rolled variety, is the lowest-GI option for this type of cooked cereal. Other low-GI tips include cooking pasta “al dente” (which means “to the tooth” or “to the bite”) and adding an extra splash of vinegar or lemon juice to a dish for the beneficial acidity (acid delays the emptying of food from the stomach, which lowers the GI).

For a person with T1D, using the GI for choosing foods and planning meals is an enhancement, but not a replacement for counting CHO. But you can use the GI to your advantage for making smarter CHO choices. Keep the GI in mind when you are shopping in the supermarket or ordering at a restaurant. Check that your selections include some—or even a majority of—lower-GI foods. The GI can be a good-sense guide in managing blood-glucose levels and keeping you in sync with more-nutritious eating overall. Bottom line: “going low”—choosing lower-GI foods—can be a healthy habit to develop. No confusion about that!