What is the Glycemic Index?

By Lizzie Streit, MS, RDN, LD 

The glycemic index (GI) of a food is a measure of how much it impacts blood sugar. You may be familiar with the term or have even heard that a low-GI diet helps with weight loss. 

For some, the GI of foods can be helpful in planning meals and maintaining a healthy diet. However, there’s a lot to consider when evaluating the GI of a food. 

This article provides an overview of the GI and its pros and cons. 

What is the glycemic index? 

The GI of a food is a number from 0 to 100 that’s assigned based on the food’s ability to increase blood sugar. Low-GI (55 or less) foods do not increase blood sugar as much as medium- (56 to 69) or high-GI (70 or higher) foods. 

The breakdown of carbohydrates into simple sugars during digestion is what causes blood sugar to rise. Foods that contain a lot of refined carbohydrates and sugars tend to have high GIs, while those with protein, fat, and/or fiber in addition to carbohydrates have lower GIs. Foods that contain no or very few carbs, such as meat and nuts, do not have GIs. 

Keep in mind that the GI of foods depends on how they’re prepared. For example, foods that have been cooked or processed in other ways will have different GIs than their raw counterparts. The ripeness of a fruit or vegetable can also make a difference. 

Here are some examples of the approximate GIs of common foods: 

  • Apple (Golden Delicious, raw): 39 

  • Bagel (white): 69 

  • Brown rice (steamed): 50 

  • Carrots (raw and diced): 35 

  • Ice Cream (half vanilla, half chocolate): 80 

  • Milk (skim): 32 

  • Spaghetti (white, boiled 15 minutes): 41 

  • Watermelon (raw): 72 

  • White potato (boiled): 54 

  • Yogurt (low-fat, strawberry): 61 

The GI of some foods may actually fall into a range and be slightly higher or lower than what’s listed here. This resource from Harvard shows ranges for the GIs of common foods. 

To determine the GI of foods, researchers give healthy volunteers a test food that contains 50 grams of carbohydrates and a control food (such as white bread or pure glucose) with 50 grams of carbohydrates. The volunteers eat the foods on different days and get their blood drawn to be analyzed for changes in blood sugar concentrations.  

The researchers then calculate the GI of the test food by dividing a measure of blood sugar after the test food by the same measure after the control food and multiplying by 100. The resulting GI indicates how much the test food raises blood sugar compared to the control food.  

For example, sweet corn has a GI of around 52 when compared to pure glucose. This means that a person’s blood sugar response to sweet corn is about 52% of the response to pure glucose. 

You may be thinking this sounds complicated. But don’t worry, you don’t have to do the math! The University of Sydney keeps an online database of GIs that you can use to easily look up foods.  

How helpful is GI? 

GI may sound like an easy way to categorize foods as “good” or “bad” for blood sugar. However, it doesn’t take into consideration the serving size of a food. Recall how GI is calculated—by using servings of the test and control foods that each contain 50 grams of carbohydrates. But what if the serving you typically eat only has 15 grams of carbs? 

This is where a related measure, known as the glycemic load (GL), comes into play. Unlike the GI, the GL of a food does consider serving size. To calculate the GL, you multiply the GI by the amount of carbohydrates in a typical serving of the food and divide that by 100. A high GL is greater than or equal to 20, an intermediate GL ranges from 11 to 19, and a low GL is less than or equal to 10. 

Mango is a good example of a food that has an intermediate GI but low GL. Most people would consider mango a healthy food, but they may be confused if they see that the GI of mango could be close to 60. However, after considering that a typical serving size of mango has 15 grams of carbs, the GL is calculated as 9 (low). Watermelon is in a similar boat. Its GI is somewhere between 72 and 80, whereas its GL is between 4 and 5. 

Finally, GI and GL are far from perfect measures. While they are helpful in identifying how a certain food will generally affect blood sugar, they aren’t specific to you. Certain foods may impact your blood sugar levels differently than they do other people’s. What else you eat with a food also make a difference in how it affects blood sugar. 

How can you use GI? 

Now that you know about GI and GL, you may be interested in using the numbers to advise your food choices. 

Several observational studies have reported a link between diets with high-GI/GL foods and an increased risk of heart disease, stroke, gallstones, and type 2 diabetes. These results have led to theories that eating a low-GI/GL diet is better for health. Some studies also suggest that low-GI/GL diets may be more effective than other diets for weight loss, but the results are mixed. 

Choosing foods with low-GIs and -GLs may indeed help those with diabetes who want to keep their blood sugars in check. If you find this type of eating to be easy and enjoyable, then it may be a good option for you. However, it’s not the only beneficial style of eating for blood sugar management. 

A balanced and varied diet that includes better-for-you carbohydrates, like fruits, non-starchy veggies, and whole grains, is likely pretty similar to what you would eat on a low-GI/GL diet. Almost all healthy eating patterns already recommend cutting back on starchy and sugary foods, like white bread, cakes, candy, and baked foods with white flour. 

At the end of the day, stick with carbohydrate sources that are high in fiber and take longer to digest than sweets and sodas. Remember to keep an eye on portion size, and pair them with proteins and healthy fats. But if you are a number-driven person who finds the GI helpful (and won’t obsess over it), you may benefit from checking out the GI or GL of your foods. 

Note from Healthy For Life Meals: Our fresh meal delivery service includes menus that are nutritionally balanced and limit carbohydrates that spike blood sugar. Best of all, we do all the work in measuring nutritional content for you! Choose your plan today, and enjoy our meal prep service already next week! your plate. 

Stef Keegan