The Glycemic Index: characteristics of high vs low GI foods
The Glycemic Index (GI) tells how quickly a given carb-containing food will raise (spike) blood sugar. Here we'll explain the characteristics of high and low glycemic index foods, so that you can figure out if a given food is high, moderate or low GI. Then if you have type 1 diabetes, you can plan to give the right amount of insulin, at the right time.
The Glycemic Index - A quick review
The Glycemic Index (GI) scale categorizes carbohydrate-containing foods into three levels: high GI (70+ on the GI scale), medium GI (56-69) and low GI (0-55).
- High GI foods digest quickly, converting to sugar that will raise blood glucose quickly. This results in higher, longer post-meal blood glucose spikes.
- Low GI foods convert to sugar slowly, therefore raising blood glucose slowly and resulting in "flatter" post-meal blood glucose curves.
Factors that Make Foods High vs Low GI
In the ongoing battle to manage blood glucose and tame post-meal spikes, 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. To empower us to choose, we need to know the key factors that that impact whether a certain food has a high, medium, or low glycemic index value. Here are some of those factors:
1. Whole Foods vs. Processed
Processed foods are generally higher on the glycemic index than whole foods (what we call in our home "foods the way God made them"). And the more processing a food undergoes, the quicker its digestion. Why is this? 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 index value as white bread, whereas whole grain bread has a substantially lower glycemic index 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" wheat.)
2. Type of Sugar
The type of sugar a food contains will influence its glycemic index rating. If all else is equal, foods containing glucose (dextrose) will have the highest GI value and will act very quickly to raise blood sugar;* those containing fructose or galactose will be next; and those containing lactose and sucrose (table sugar) will act slower than the others. 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).
*This is why products with glucose or dextrose listed as one of the first 3 ingredients make great low treatments.
The presence or absence of sugar in a food will also affect the glycemic index value, regardless of the type of sugar present. If sugar is added to a food product, it 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.
3. Solids vs. Liquids
The liquid form of a given food will raise blood glucose faster than the solid form, of that same food. For example, a glass of orange juice will raise blood glucose higher and faster than a fresh orange will. This is in part due to...
4. Dietary Fibre
The presence of fibre, as well as the type of fibre that is present, affects how quickly a food will raise your child's blood sugar. As an added benefit in terms of managing blood sugar... dietary fibre keeps things moving through our gastrointestinal tract and makes us feel full longer.
- The carbohydrate molecules of soluble fibre can be dissolved in water, giving foods like apples and oatmeal that gel-like, gummy texture. 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, barley, nuts and seeds (including flax and chia seeds), legumes (beans, peas and lentils), apples, pears, bananas, strawberries and blueberries.
- 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 celery, dark green leafy vegetables, root vegetables, whole grains and whole grain breads, brown rice, wheat bran, whole grain seeds, seeds and nuts.
5. Fat Content
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: in addition to the low-GI-effect, they will also cause increased insulin resistance for hours afterwards, especially if eaten in large amounts (ex. pizza).
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.
7. Raw, Cooked & REALLY Cooked
A cooked starch more easily breaks down into glucose. Cooking foods, by boiling or steaming for example, increases the glycemic index of that 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. For example, al dente pasta will create a more gradual peak in blood sugar than over-cooked, mushy pasta; the longer you cook pasta, the softer it becomes, resulting in a higher GI.
8. Served Cold vs. Hot
The molecules of cold foods are more closely bonded together than when the same food is heated. As a result, it takes more energy to break those cold-food particles apart through the process of digestion. For this reason, a bowl of warm sushi rice will raise BG faster than an equal amount of rice made into sushi and chilled; hot boiled potatoes will raise BG faster than cold potato salad made with those same potatoes.
9. 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 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, which felt necessary at the time because of the slower digestion brought on by the low GI sandwich.
10. Physical Armour
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. Kidney beans, chick peas, barley, and whole grain bread are examples.
11. 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.
12. 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 on the glycemic index. 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.
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.
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