What is the meaning of the “glycaemic index”?

The glycaemic index (GI) is a way of measuring the effects of food on blood sugar levels. After a carbohydrate-containing food is eaten there is a rise, and subsequent decrease, in blood sugar (glucose). This phenomenon is known as the glycaemic response. The glycaemic response after eating different carbohydrate-containing foods can be compared to the glycaemic response after consuming a standard food, such as white bread or glucose. The resulting measurement is called the glycaemic index of that food.

A ranking on a scale up to 100 rates the increase in blood sugar levels after eating a portion of food providing 50 grams of carbohydrate. When glucose is used as the standard, the basis of comparison is the response to a 50-gram portion of pure glucose (GI of 100). Foods that break down quickly during digestion, for example sticky rice, have the highest glycaemic indexes. They raise blood sugar levels higher and more quickly than foods with low GIs. Low GI foods, such as beans, break down more slowly, releasing glucose gradually into the bloodstream.

A number of factors influence the rate and duration of the glycaemic response. For example, the type and form of the carbohydrate eaten, the cooking and processing methods used, the time of day the carbohydrate is ingested or the amount of other nutrients in the food, such as fat or protein. In addition, individuals differ in their metabolism, which can also affect the glycaemic response.

Measuring how our blood sugar will react to various foods, or measuring the GI, has provided insights into the prevention and control of conditions such as obesity, diabetes and heart disease and has also revolutionised dietary advice for sport, for example which foods should be eaten in the hours before physical exertion.

The Glycaemic Index of Some Common Foods compared to glucose

Low GI foods (GI less than 55):

• Noodles and pasta;
• Lentils
• Apples/apple juice
• Pears
• Oranges/orange juice
• Grapes
• Low fat yoghurt
• Fruit bread
• Baked beans
• Chocolate

Intermediate GI foods (GI 55-70):

• Basmati rice
• Banana
• Rolled oats
• Soft drinks
• Sweet corn
• Pineapple
• White sugar

High GI foods (GI > 70):

• Bread (white or wholemeal)
• Baked potato
• Cornflakes
• French Fries
• Honey
• Mashed potatoes
• White rice (low amylose or “sticky rice”)