Introduction to the Glycemic Index

What is GI and Why Does It Matter?

The amount of carbohydrate in a food is a key factor affecting blood glucose. However, there are other factors which also affect the blood glucose pattern following a meal or snack, including the quality of the carbs. The quality of the carb affects the rate at which it is digested; how quickly a food is broken down, in turn, affects how quickly blood glucose will rise when you eat that food. The reverse is also true: foods which digest slowly can help prevent blood sugar from rising significantly.

For example, foods with a high-fat content (such as hash browns) or a high-fibre content (which includes less-refined grains, such as those in whole grain bread) will be digested more slowly than similar foods with a low-fat or low-fibre content / highly-refined grain (such as mashed potatoes or white bread). This differing effect on blood glucose due to differing rates of digestion relates to the concept of Glycemic Index (GI).

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What is the Glycemic Index (GI)?

A food’s glycemic index (GI) value describes how quickly (or slowly) that food will be digested: A food with a low GI value will be digested more slowly than a food with a high GI value; thus a low GI food (such as oatmeal) will raise blood sugar slowly, while a high GI food (such as Cheerios) will raise blood sugar quickly. In short:

The carb content of a food determines how much it will raise blood sugar.
The Glycemic Index of a food describes how quickly it will raise blood sugar.

Tips from the Trenches

“High Glycemic” vs. “High Glycemic Index”

The word “glycemic’ is often tossed around, and sometimes incorrectly. When people say a food is “high glycemic” that’s another way for saying it’s high carb (has a fair bit of carbohydrate content, which will be converted to sugar through the process of digestion, and so will raise the amount of sugar in the blood).

On the other hand, to refer to a food that digests quickly (and so has the potential to quickly raise blood sugar causing BG spikes) the term “high glycemic INDEX” applies.

Why Does Glycemic Index Matter? (Part 1)

To start with, the concept of Glycemic Index is very important for matching insulin action to the rate of digestion of a given food, by timing insulin delivery to hold blood glucose as steady as possible after a meal or snack. To avoid post-meal blood glucose spikes, insulin needs to be delivered at a different time for high glycemic foods than for low glycemic foods.

In addition, a food’s Glycemic Index (GI) value is relevant in terms of choosing effective treatments for low blood glucose. A fast-acting sugar will bring low blood glucose up more quickly than a slower-acting sugar. In other words, foods with a high glycemic value (such as Sweet Tarts®, Rockets® candy, and Gatorade®) will raise blood glucose faster than a food with a lower GI value (such as fruit juice or cola). This is not to say that lower GI foods cannot be used as low treatments, only that the delayed effect on blood glucose should be taken into account before providing a second low treatment for the same episode.

More information on low treatments and type 1 diabetes:
Treating Low Blood Glucose with High GI Foods

low glycemic index slow digestion 

Further reading about the Glycemic Index:
Glycemic Index Explained


The above information was reviewed for content accuracy by clinical staff of the Alberta Children’s Hospital Diabetes Clinic.

This material has been developed from sources that we believe are accurate, however, as the field of medicine (in particular as it applies to diabetes) is rapidly evolving, the information should not be relied upon, as it is designed for informational purposes only. It should not be used in place of medical advice, instruction and/or treatment. If you have specific questions, please consult your doctor or appropriate health care professional. 

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