What is the Glycaemic Index

The glycaemic index is a measure of the ability of a food to raise blood sugar levels after it is eaten. The index compares the blood sugar response to a particular food with the body’s reaction to pure glucose, which is given the value of 100. For example, if a food raises blood sugar only half as much as pure glucose, that food is given a glycaemic index of 50. The portion size used to test the glycaemic index of various foods is the amount that contains 50 grams of carbohydrate. Some research has used white bread instead of glucose as the standard of comparison for determining the glycaemic index of foods. The glycaemic index of a food is governed by several factors, such as the form of carbohydrate it contains, the amount and form of fibre it contains, how much processing and cooking it has been subjected to, and the presence of other substances such as protein and fat.

Glycaemic load is a related measurement calculated by multiplying the glycaemic index of a food by the amount of carbohydrate contained in a typical serving of that food. Glycaemic load may be more reliable than glycaemic index as a predictor of how a food will affect the blood sugar level. That is because some foods with a high glycaemic index (such as carrots) contain such a small amount of carbohydrate in a normal serving that they would not be expected to raise the blood sugar level very much. Carrot juice, on the other hand, which contains a relatively large amount of carbohydrate, would produce a substantial increase in the blood sugar level.

How do people use the glycaemic index?

People most often use the glycaemic index to choose carbohydrate-containing foods that will only minimally raise their blood sugar levels, with the intent of preventing health problems associated with either high blood sugar or the body’s reaction to rising blood sugar. These health problems may include weight gain, diabetes, the insulin resistance syndrome, hypoglycaemia, and heart disease. Foods with a glycaemic index of 55 and below are considered ideal for those trying to consume low-glycaemic-index foods.

Athletes may choose high-glycaemic-index foods after intense exercise, in order to rapidly replenish depleted carbohydrate stores.

What do the advocates say?

The underlying premise for advocating eating low-glycaemic-index foods is that high-glycaemic-index foods cause a rapid elevation in blood sugar that the body attempts to balance by producing a large amount of insulin. Advocates claim that human physiology is not designed to tolerate these rapid and prolonged elevations in blood sugar and insulin caused by the prevalence of modern, high-glycaemic-index foods in the diet. As human civilization has evolved, primitive stone-age diets that featured naturally occurring, low-carbohydrate foods have been replaced, first by unprocessed but higher-carbohydrate agricultural foods such as whole grains and legumes, and more recently by highly processed, low-fibre flours and other starchy foods, plus an increasing amount of sweets. This trend towards higher-glycaemic-index foods in the diet is therefore deemed unnatural and hazardous to the healthy functioning of the body.

Research suggests that repeated overproduction of insulin could lead to insulin resistance, in which cells that normally respond to insulin become less sensitive to its effects. Excessive high-glycaemic-index foods, high insulin levels, and insulin resistance have been associated with many health concerns, including obesity, type 2 diabetes, heart disease, and some cancers. Changing to a low-glycaemic-index diet has been shown in most studies to reduce insulin resistance, help control appetite, improve weight loss results, enhance blood sugar control in diabetics, lower blood levels of total and LDL ("bad") cholesterol, and raise blood levels of HDL ("good") cholesterol.

What do the critics say?

Critics say that the way the glycaemic index is measured (one food at a time in quantities that contain a standard amount of carbohydrate) does not resemble the way people usually eat (many items are eaten together in varying portion sizes, often mixing high-carbohydrate with low-carbohydrate foods). They also criticize the complexity the glycaemic index concept and the fact that eating large amounts of some low-glycaemic-index foods, such as ice cream, would not be unhealthful due to the detrimental effects of other components of these foods, such as animal fats. In the case of type 2 diabetes, critics point out that the glycaemic index only measures the short-term effects of foods on blood sugar, whereas studies measuring longer-term effects of high-glycaemic-index foods in people with diabetes have found inconsistent results.

In answer to these criticisms, advocates point to the many studies linking diets containing high-glycaemic-index foods to common and serious health problems. They insist that the diet can be made more healthful by integrating the glycaemic index with other health concepts, such as lowering animal fat consumption, to achieve the best results.

What should I avoid when I want to eat low-glycaemic-index foods?

It is not necessary to completely avoid high-glycaemic-index foods. When these foods are combined in a meal with low-glycaemic-index foods, protein foods, or fat, then the overall glycaemic effect is reduced. Of course, to lower the overall glycaemic index of the diet, low-glycaemic-index foods should be emphasized as much as possible. The basic rules are to reduce intake of concentrated sugars and most potatoes, increase consumption of legumes and most vegetables and fruits, and choose grain products made by traditional methods (for example, pasta, stone-ground flour products, old-fashioned oatmeal) rather than those produced with modern technology (highly refined flour products, low-fibre flaked breakfast cereals, quick-cooking starches, etc.).

The following foods rank highest on the glycaemic index. These foods should be avoided or kept to a minimum by those wishing to consume a low-glycaemic-index diet

Bread, cereal,and rice to avoid:

• Rice
• Rice cakes
• Most breads, breakfast cereals, snacks, and desserts made with refined flour products

Other starchy foods to avoid:

• Potatoes (except new potatoes, sweet potatoes, and yams)

Fats, oils, and sweets to avoid:

• Soft drinks, including sweetened fruit drinks and most sports drinks
• Most cakes and pies
• Candy and candy bars
• Granola bars and most sports bars

Vegetables and fruits to avoid:

• Raisins
• Overripe bananas

Best bets

Bread, cereal, rice, and pasta:

• Whole wheat and whole grain breads
• Breads containing whole, intact grains and seeds (millet, linseed, etc)
• Brown rice, basmati rice
• Barley, buckwheat
• Whole grain cereals, muesli
• Whole wheat pita, chapatis
• Oatmeal

Other starchy foods:

• Legumes and legume products (hummus, baked beans, lentil soup, etc.)
• Bakery products made with whole grains, bran, whole fruit pieces, and/or nuts

Dairy products and dairy substitutes:

• Unsweetened milk and milk products
• Unsweetened yoghurt
• Soya beverages

Vegetables and fruits:

• Most vegetables and vegetable juices