The Glycemic Index Explained

What is the Glycemic Index?
Why Does it Matter for Type 1 Diabetes?

The glycemic index (GI) is a way of assessing how quickly a given type of food will raise blood sugar, which is related to how quickly or slowly a food will digest. Here we’ll explain what the GI numbers mean, why they matter for type 1 diabetes, how quickly blood glucose rises after eating high vs. low GI foods, and whether “low GI” necessarily means “healthy”.

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As background reading for this article:
An Introduction to the Glycemic Index

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According to the official website for the glycemic index and international GI database:

“The glycemic index (or GI) is a ranking of carbohydrates on a scale from 0 to 100 according to the extent to which they raise blood sugar (glucose) levels after eating. Foods with a high GI are those which are rapidly digested, absorbed and metabolised and result in marked fluctuations in blood sugar (glucose) levels. Low GI carbohydrates (are digested more slowly and) produce smaller fluctuations in your blood glucose.”

What Do Glycemic Index Numbers Mean?

A high number represents fast food digestion; high glycemic index foods will raise blood glucose quickly, resulting in higher post-meal blood glucose spikes.

A low number represents slow food digestion; low glycemic index foods will raise blood glucose slowly, resulting in “flatter” post-meal blood sugar graphs.

100 is the GI value of pure glucose, which raises blood glucose very quickly, and is the standard against which all other foods are compared. When you look at a glycemic index list, the number beside each food indicates how quickly that food will raise your child’s blood glucose. Further, values are broken down into three levels: low, medium and high GI

  • Low GI
  • Medium GI
  • High GI
  • 0 – 55
  • 56 – 69
  • 70 +



For example, Uncle Ben’s Converted Rice® is low on the glycemic index, with a GI value of 38; white basmati rice is medium GI with a value of 58; sticky white rice is high on the glycemic index, with a GI value of 98.
Similarly, Rice Krispies® and Bran Flakes are high GI breakfast cereals (at 87 and 74 respectively), while All Bran® (34) and Frosted Flakes® (55) are both considered low GI.

A few (semi-related) points about breakfast cereals…

Let me take you on a brief digression for a moment to say:

  • “Multi-grain” does not necessarily mean lower GI (or healthier) – it literally means they’ve included more than one grain; some of those grains may actually be high GI, such as rice. Also, any or all of the “multiple grains” may be highly processed, which will make them high GI.
  • Anything that is a flake or a puff is likely high GI. (With the exception of Frosted Flakes®, which are coated with a hard sugar shell that prevents the swelling of the starch and therefore slows down digestion.)

Tips from the Trenches

This one got me! I assumed that the natural-grain-without-extra-additives state of the puffed wheat I was buying for my son made it low GI – but puffed wheat has a GI value of 80 (high GI). This was contributing to post-snack blood sugar spikes without me even knowing it. Oh, well, live and learn. ~Michelle

How Quickly Do Low vs High GI Foods Raise Blood Glucose?

As illustrated in the graph below (from Gary Scheiner’s book Think Like a Pancreas),

Foods with a high glycemic index raise the blood sugar the fastest and blood sugar tends to peak in 30-60 minutes after eating.

Foods with a medium glycemic index result in a slower rise in blood sugar and blood sugar tends to peak 60-90 minutes after eating.

Foods with a low glycemic index cause the slowest rise in blood sugar, resulting in a modest blood sugar peak which takes more than 2 hours to appear.

Why Does GI Matter? (Part 2)

The key to managing blood glucose… matching insulin to food intake!

 
As is universally the case for type 1 diabetes, when eating any type of food the goal is to match insulin action with the rate of digestion of the food, so that there is the right amount of insulin, at the right time, to cover the food.

You may find that it works great for most meals to deliver the regular meal bolus at the usual time (about 15 minutes before the start of a meal), which in many cases provides enough insulin, at the right time, to minimize post-meal spikes, without providing so much insulin that blood glucose dips too low. The insulin action curve of today’s rapid-acting insulin most closely matches the rate of digestion of moderate GI foods, which is why you may find that things work out fine when your family is eating a moderate GI meal.

On the other hand, “the usual” bolus (in terms of amount and timing) when given with a low GI meal typically results in initial low blood glucose followed by high blood glucose later. Why would this be the case?

When your child eats these low glycemic foods, the insulin is getting in before the food is fully digested; there is too much insulin present right after the meal, when only a small portion of the food has digested, and not enough insulin
present later, when the glucose from the slowly-digested food has reached the bloodstream.

For this reason, it is important to be aware of the glycemic index of different foods, so that you have a possible explanation for unpredictable post-meal blood glucose patterns, and also so that adjustments can be made in the timing and structure of meal boluses, if you choose to do so. If you notice that your child’s blood glucose first drops low and then spikes a few hours later after eating certain foods (such as pasta, brown rice, beans/lentils, apples, pizza), this may* be due to the low Glycemic Index of these foods. (In contrast, a post-meal blood glucose pattern that includes a steady rise, with no initial drop in blood glucose, may be related to the insulin dose rather than the glycemic value of the food.)

*Note: there are other factors that may contribute to lows after meals, including the use of peaking background insulin such as N/NPH, too much meal insulin, too few carbs, too much basal insulin, and previous exercise, among others. Consult your child’s diabetes health care team to determine likely causes and solutions for blood glucose swings.

Interested in exploring strategies for incorporating the GI concept into your child’s diabetes management plan?
Choosing High/Low GI Foods
Adjusting for High/Low Glycemic Foods

Resources:

For more information ion the glycemic index and diabetes, check out the following:

The University of Sydney, Glycemic Index website: GlycemicIndex.com

Diabetes Canada article, The Glycemic Index

Glycemic Index list from Mendosa.com (helping Defeat Diabetes Since 1995)

References:

Dr. Jennie Brand-Miller and Thomas M.S. Wolever; The New Glucose Revolution: The Authoritative Guide to the Glycemic Index – The Dietary Solution for Lifelong Health

 

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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