The GI's strengths

The GI concept is backed by both the World Health Organisation and the Food and Agricultural Organisation because it helps prevent coronary heart disease, diabetes and obesity.

Sustained energy levels

Low GI foods help keep your blood glucose levels constant. This keeps your energy levels up and avoids the feeling of sluggishness you might get after a quickly digested high GI meal.

Conversely, blood glucose levels drop after a bout of strenuous exercise. High GI foods will allow you to replenish your glucose stores and get you back on an even keel.

Diabetes management

Keeping blood glucose levels constant is especially important for people with diabetes. Understanding the GI values of foods can help explain changing levels, and may help match certain foods with medication.

But, when a person with diabetes has low blood glucose levels, which may result in a "hypo", a high GI food will bring them back to normal.

Sorting those carbohydrates

Until very recently, the main message about carbohydrates was that they should form the basis of our diet.

This isn't wrong in itself. But critics argue that it is wrong to treat all carbohydrates in the same way, and that it is misleading to say just that "simple" carbohydrates (sugars) are less valuable than "complex" carbohydrates (starches).

Potatoes are perhaps the touchstone for the new thinking. They are mostly starch, which gets broken down very quickly into blood glucose. In fact, a potato (especially if it's hot and mashed), can give your body a faster hit than a bowl of sugar.

Any decent nutritional theory has to be able to explain this - and what we should do about it. The GI does both these things. (The good news: you don't have to give up potatoes! See The GI of common foods.)

Weight loss

When it comes right down to it, if you eat more food energy than your body needs, you will get fat.

Therefore, reducing your overall food energy intake (while getting more exercise) is generally regarded as the central strategy in a good weight control programme. Eating low GI foods can help with this, because that should mean you're not likely to eat so much.

Some studies have also suggested that eating a low GI diet can lower blood cholesterol levels, lower the risk of heart disease and lower the risk of diabetes.

But that's not the whole story.

As Professor Jim Mann, head of human nutrition at Otago University, says, "The GI concept is a valuable tool but using it isn't as simple as just choosing low GI foods."