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Intro to Glycemic Index

As we discussed in An Introduction to Carb Counting, the amount of carbohydrate in a food is the main factor affecting blood glucose. However, there are other factors which also affect the blood glucose pattern following a meal or snack.

glycemic index and rate of digestion

One factor is rate of digestion; how quickly a food is broken down will, in turn, change how that food affects blood glucose. For example, foods with a high-fat content (such as french fries) or a high-fibre content / less-refined grain (such as 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).


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.


timing insulin action to rate of digestion

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.

coming up quickly from a low blood sugar
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, a food 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.
(For more information on Treating Low Blood Glucose with High GI Foods, click here.)

(For next steps regarding the Glycemic Index, see Level 2: 
Glycemic Index Explained.)

Any questions? Comments? Feel free to Contact Us.

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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