What is the glycaemic index?

The glycaemic index (GI) is a measure of how quickly, and by how much, the carbohydrates in a food raise your blood glucose levels. This is important because, for most of us, carbohydrates like breads, cereals, fruit and vegetables are the basis of our food intake.

It used to be thought that the type of carbohydrate (simple or complex) determined the value of a food. The GI shows that the way the carbohydrate works in your body is more important.

Most foods with a low GI rating fill you up quickly and provide sustained energy. That's usually a good thing.

High GI foods are not likely to be so filling, so you eat more, and they give you a quick, short-lived energy hit. This is less desirable if you are watching your weight, because you can eat too much and still feel hungry again quite soon.

Low GI foods are also preferable for people with diabetes, because the slower absorption of carbohydrates avoids any sudden rise in blood glucose levels.

But sometimes you need that quick hit, and at these times high GI foods are preferable.

The nutrition advice that follows from this is not always simple. Among the complicating factors:

• Your blood glucose response will vary, depending on your energy needs at any given time. It also varies from person to person.
• High-fat foods seem to buck the trend. They tend to have a low GI rating, but may not fill you up quickly.
• The GI is not the only measure of a food's value.
• Glucose response to any food varies depending on how you prepare it and what you eat it with.

So how do you make sense of it all, and what is the value of the GI anyway?