The Glycemic Index Explained
What is the Glycemic Index?
You may already know that the glycemic index (GI) is a way of measuring how quickly a given type of food will raise blood sugar; it’s related to how quickly or slowly a food will digest. Here we’ll explain how quickly different GI levels raise blood glucose, what factors affect the glycemic index of foods, and whether “low GI” necessarily means “healthy”.
As background reading for this article:An Introduction to the Glycemic Index
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What Do Glycemic Index Numbers Mean?
The range of Glycemic Index (GI) values is 0-100. 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 results.
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 glycemic…
For example, Uncle Ben’s Converted Rice® is low glycemic, with a GI value of 38; white basmati rice is medium GI with a value of 58; sticky white rice is high glycemic, 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:
Low glycemic foods are healthy, right?
This is a good place to point out that “low glycemic” does not necessarily mean “better” or “healthier”; and healthy does not necessarily mean low GI.
A food’s GI value is not a judgment call about what you should be feeding your child – it is simply a description of how quickly that food will be digested. In the examples above you may notice that Frosted Flakes® are considered low glycemic, even though some parents may not consider them to be as “healthy” as Bran Flakes®, which are considered high glycemic. There are a number of factors (sometimes it feels like too many!) that parents can choose to either take into account or disregard in deciding which foods to make available for their children, including carb content, glycemic index, amount of refined sugar, fat content, use of chemicals and preservatives, amount of vitamins and minerals, taste, child preferences… the list goes on. Ultimately, your decision is based on the priorities and goals you have for your family; the “right” answer will be different for different families.
Precision of the Index Values
It is also important to note that citing a number like “38” or “87” seems to imply a precision that is, admittedly, a bit of an illusion.
There is a high degree of standardization and design rigor involved in the GI determination process, so we can have confidence in the results. However, these numbers were derived by testing groups of people and arriving at an average; how closely your child’s individual response matches the group average is a bit of an unknown.
This does not undermine the value of the Glycemic Index concept – it’s a great starting point (we would even say a revolutionary concept) – but it’s not a one-size-fits-all solution. Some individuals seem to show a response to certain foods that does not reflect the average. Some circumstances (large meals, food combinations) seem to alter how our body responds. So we need to take care in how we interpret the numbers, seeing them as important guidelines rather than as immutable constants. Is there really a difference between a food that has a GI of 69 and is considered moderate GI, and one that has a GI of 70 and is considered high GI? Not much. But knowing that a food has a GI of around 70, versus a GI around 30, is very valuable.
(For GI values for a variety of specific foods, we have found “The New Glucose Revolution” by Dr. Jennie Brand-Miller (primary author) to be very helpful.)
How Quickly Do Low vs High GI Foods Raise Blood Glucose?
As illustrated in the graph below,
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 several hours to appear.
What Factors Affect Glycemic Index?
There are a number of key characteristics of foods that affect whether the food has a high, medium, or low glycemic value.
1. The Physical Form of the Food
An intact fibrous coat, such as that on whole grains and legumes, acts as a physical barrier and slows down digestion, lowering a food’s GI value. Beans, barley, and whole grain bread are examples.
2. Cooking Methods
Cooking and processing increases the glycemic index of a food because it increases the swelling of the starch molecules in the food which in turn make it easier for your body to break it down. An example would be the longer you cook pasta, the softer it becomes, resulting in a higher GI.
3. Types of Starch
There are two types of starch in foods, amylose and amylopectin. The more amylose starch a food contains, the lower the glycemic index. Uncle Ben’s Converted Rice® is an example of a food that is very high in amylose starch, which is why it’s significantly lower GI compared to other types of rice.
4. Particle Size
The smaller the particle size, the easier it is for digestive enzymes to penetrate. This is why enriched wheat flour and whole wheat flour (which are highly processed, finely-milled flours) have a high GI value, while stone-ground flour and whole grain flour (which are less-refined and therefore have larger particles) have lower GI values.
A great example of this is whole wheat bread vs. whole grain bread. Because whole wheat bread contains flour that has been finely milled, it has nearly the same glycemic value as white bread whereas whole grain bread has a substantially lower glycemic value.
(Note: For a bread to be “whole grain”, the package must say exactly that; “whole wheat” bread is not whole grain unless it also states that it uses “whole grain” whole wheat.)
The presence of fibre, as well as the type of fibre that is present, affects the GI value of a food.
- Soluble fibre can be dissolved in water – it is the gel, gum and often jelly-like components of apples, oats and legumes. Soluble fibre slows down digestion and thereby lowers a food’s glycemic response. Examples of foods that are high in soluble fibre include: oatmeal, oat bran, nuts and seeds, legumes (beans, peas and lentils), apples, pears, strawberries and blueberries.
- On the other hand, insoluble fibre (commonly called “roughage”) is dry, bran-like, and cannot be dissolved in water. All cereal grains and products which retain the outer coat of the grain are sources of insoluble fibre, such as whole grain bread and All-Bran cereal. The presence of insoluble fibre in a given food does not directly affect the speed of digestion. Therefore, not all foods containing insoluble fibre are low GI; it will only lower the GI of a food when it exists in its original, intact form, such as in whole grains of wheat or when it’s coarsely milled. In its intact form, it creates a physical barrier to the digestive enzymes. Good sources of soluble fibre include whole grains, whole grain breads, barley, brown rice, wheat bran, seeds, and most vegetables.
The type of sugar a food contains will influence its glycemic index. When you are comparing foods, the ones containing glucose syrup will have the highest GI value, those containing sugar or sucrose will be next highest, and those containing fructose will be the lowest. For example, apple juice, which contains fructose, will be much lower GI (40) than Gatorade® which contains glucose (89). Honey ranks very close to sucrose at 64 (unless a pure floral honey is used, in which case the GI is 35).
The presence or absence of sugar in a food will also affect the glycemic index, regardless of the type of sugar present. Sugar restricts the swelling of starch in a food by binding with the water in the food, which will subsequently result in a lower GI value than the same type of food containing less or no sugar. Some cookies and breakfast cereals that contain sugar may have relatively low GI values. Frosted Flakes® are one example of this.
Acids in foods slow down stomach emptying, which decreases the rate at which the starch can be digested. Sourdough bread, vinegar, lemon juice, lime juice, salad dressings, and pickled vegetables, for example, all lower the GI values of the associated foods.
Fat slows down the rate of stomach emptying, which again decreases the rate at which the starch can be digested. For example, potato chips are much lower GI than boiled potatoes (54 compared to 88). There is one caution for foods high in saturated fats: even though they are lower GI, they will cause increased insulin resistance for hours afterwards, especially if eaten in large amounts (ex. pizza). For more information on dealing with foods high in saturated fat, see Advanced Carb Counting in the Nutrition section of this website.
9. Resistant Starch
As the name implies, this is starch that resists digestion. It is not absorbed and basically acts like insoluble fiber which also resists digestion, thereby making a food lower glycemic. Sources of this include firm, unripe bananas (you know that film you get on your teeth when you eat a greenish banana?) and also starchy foods that have been cooked and then cooled, such as potato salad, pasta salad or sushi.
10. Mixing High GI foods with Low GI foods
If you eat a meal that includes a high GI food (a potato, for example), as well as a low GI food (such as corn), the end result will be a meal that is medium GI.
One implication of this effect is that, when treating a low blood glucose, it is wise to give the high GI food time to work (15 min.) before eating a lot of other food (especially if those foods are low GI). For example, if your child has Skittles® for a low treatment and then gives in to that ravenous urge to eat everything in sight by having a peanut butter sandwich on whole grain bread, his blood sugar will rise more slowly than if he waits between the low treatment and the snack. This could lead to over treatment of the low: at 15 minutes he checks again, finds his blood glucose has risen only a point or two, so pops in another 10-15 grams of Skittles® only to find that later his blood glucose is sky high. The cause may be that extra (and unnecessary) dose of low treatment.
Why Does Glycemic Index Matter?
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
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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