Understanding the Glycemic Index

When it comes to caloric sweeteners, the most important rule of thumb is moderation (the amount you consume), even for those with diabetes. You may have heard or read about diets using foods that have a low glycemic index (GI), and wondered what that was all about.

The glycemic index (GI) was first introduced in the 1980s as a way to help people with diabetes gain better control of their blood sugar by evaluating different types of carbohydrates, and how quickly they impact blood sugar levels. Carbohydrates are classified as low, medium or high GI. Since that time, many popular fad diets such as the Sugar Buster’s Diet, The Zone Diet, and the Paleo Diet, include the concepts of the Glycemic Index, although at the end of the day, they also control calories, which is why they promote weight loss.

Using the glycemic index alone to choose foods doesn’t guarantee a balanced diet, but some research suggests using the GI can be beneficial, not only to those with diabetes, but also those at risk for heart disease or with polycystic ovary syndrome.

While I don’t recommend fad diets, the GI is a tool that may help you include healthy foods in your diet, and help control hunger. However, keep in mind that there may be times when it is desirable to have a rapid increase in blood sugar (nutrition for athletes or treating a low blood sugar incident in diabetes, for instance).

Let’s take a look at some popular diets that use the concept of GI:

The Sugar Buster’s diet is high in fiber, and allows most fruits, vegetables, protein, and whole grains. It encourages foods with a low GI, and limits foods such as refined sugar and white flour (white bread), white rice and white potatoes. It works because it does limit calories, and does keep blood sugar levels steady, helping control hunger. Eating food high in fiber also keeps you full longer. While this diet is not backed up by research, the general premise to cut back on sugar, and choose healthy fruits and vegetables is the basis for many well-supported dietary plans such as DASH Diet, the Mediterranean Diet and the MIND diet.

The Zone Diet claims to help you “burn fat in your sleep”. While that may not happen exactly, this diet also uses the principles of glycemic index (it’s only 40 percent carbohydrate with remaining calories as 30 percent protein and 30 percent fat). It discourages high GI fruits and vegetables, and focuses on low-fat protein and low GI vegetables and grains. Of course, the Zone also restricts calories, allowing about 1200 for women and 1500 for men, therefore promoting weight loss.

Those who are choosing a Paleo style diet are also using some GI principles, eliminating refined sugars and potatoes, but it also eliminates healthy foods like dairy products, grains, legumes (peanuts and beans) and canola oil. The Paleo diet focuses on lots of vegetables and protein, as well as nuts, seeds and other fats such as olive or coconut oils.

In context with the GI, it’s perhaps more important to understand the Glycemic Load (GL) of various carbohydrate foods. While the GI is measured at per 100 grams, the glycemic load indicates how a typical serving of a food raises blood sugar levels.

Key Points About the Glycemic Index:

  • The glycemic effect of a food is measured by evaluating how fast 100 grams of a food is likely to raise blood sugar. Foods rapidly digested and absorbed have a higher GI (rated 70 or higher), and foods more slowly digested have a medium (rated 56-69) or lower GI (rated 55 and lower).
  • The GI only applies to carbohydrate foods (fruits, vegetables, grains, lentils, legumes, dairy and sugars), and not all carbohydrates have been tested.
  • The GI of different sugars varies (fructose is 25, honey is 50, sucrose is 65, glucose is 100)
  • The GI doesn’t make a food “bad” or “good” it simply helps you control the intake of  various carbohydrates which increase blood sugar at different rates.

Key Points about Glycemic Load

  • The GL is also classified as either low, medium or high (low is 10 or less, medium is 11-19, and high is 20 or higher). Since the GL is based on serving sizes, it’s often more useful.
  • While many popular diets exclude sugars, common sweeteners have a low to medium GL. For instance, the Glycemic Load of 1 tablespoon of honey is 9.
  • Alternatively while the glycemic load of most fruits and vegetables is low, some do have a medium GL (bananas, peaches canned in syrup, potatoes, dried cranberries or dates), while others may have a high glycemic index, and low GL (carrots, for instance, have a GI of 92 and a GL of 5). Does that mean you should avoid those foods? Absolutely not, you’ll just want to balance them in your diet with protein, healthy fat, and low GL foods.

While the glycemic index can be a helpful tool, keep in mind that choosing foods solely based on a low GI doesn’t always result in long-term healthy diets or lifestyles. Eating a variety of foods in moderate portions, and participating in regular physical activity, is still the best plan.


Rosanne RustRosanne Rust MS, RDN, LDN is a registered, licensed dietitian-nutritionist with over 25 years experience. As a Nutrition Communications Consultant  she delivers clear messages helping you understand the science of nutrition so you can enjoy eating for better health. Rosanne is the co-author of several books, including DASH Diet For Dummies® and the The Glycemic Index Cookbook For Dummies®. A wife, and mother of 3 boys, she practices what she preaches, enjoying regular exercise, good food and festive entertaining. Follow her on Twitter @RustNutrition.