The glycaemic index (GI) of a carbohydrate-containing food tells you what effect eating that food will have on your blood glucose (also called blood sugar) level. Foods with a low GI (e.g. red lentils at 21) tend to break down slowly during digestion and make your blood glucose level rise more slowly and steadily than foods with a high GI (e.g. white bread at about 70). Pure glucose is given a GI of 100. Choosing low-GI foods can benefit people with diabetes and those who are trying to control their weight.
To understand the concept of GI, it helps to be familiar with how our bodies digest carbohydrate and use the energy that is released. Carbohydrate from food is mostly digested in the small intestine — during this process various sugars are released and absorbed into the blood. The major sugar is glucose, and GI is an indication of how long it takes the glucose from a food to enter the blood.
After glucose is absorbed into the blood, your body releases a hormone called insulin that helps glucose to be taken up by the cells. People with diabetes don’t produce enough insulin or their bodies don’t respond properly to insulin. This can mean that they have trouble regulating the amount of glucose in their blood, which can cause health problems in the long and short term. So someone with diabetes could find a tool like GI useful to predict how a certain food will affect their blood glucose level.
The GI of a food is measured by comparing its effects on blood glucose level with the effects of a set amount of pure glucose. Pure glucose is assigned a GI of 100; a food that releases glucose half as quickly as pure glucose has a GI of 50, while one that releases glucose 20 per cent faster than pure glucose would have a GI of 120.
GI measures only foods containing carbohydrates — meat, fish and many vegetables don’t have a GI value as they contain very little carbohydrate.
|The GI of carbohydrate-containing foods|
(GI of 55 or less)
(GI of 56-69)
(GI of 70 or more)
Before the GI system was developed, nutritionists classified carbohydrates into 2 types: ‘complex’ (mainly starches, such as those contained in bread, rice, potatoes and pasta) and ‘simple’ (mainly sugars, such as those in milk, honey and fruits). Complex carbohydrates were thought preferable for people with diabetes as they were considered to take longer to digest and absorb than simple carbohydrates and therefore less likely to cause a sudden spike in blood glucose level.
However, the concept of GI has changed this assumption somewhat, because some foods with predominantly simple carbohydrates (e.g. milk) have a lower GI than some foods containing mainly complex carbohydrates (e.g. potatoes). Foods with lower GI are considered to be absorbed more slowly than those with higher GI, even if they contain predominantly simple carbohydrates.
The GI of a food is affected by many factors, including:
Individual foods are not always eaten by themselves; instead they are often consumed as part of a mixed meal. Many factors influence the GI of a meal, including the type of starch, the amount of protein and fat, and the quantity and type of fibre. Nevertheless, including low-GI foods in a meal tends to reduce the GI of that meal.
When it comes to planning meals it’s important to think about how much carbohydrate a particular food contains, as well as how quickly it is absorbed. This is where the concept of glycaemic load (GL) comes in. To calculate the GL, you multiply the GI by the amount of available carbohydrate in a serve (in grams) and divide the total by 100. The GL is highest for foods that provide a lot of carbohydrate. For example:
Apple has a GI of 40 and contains 15 g of carbohydrate per serve. The GL of apple is therefore (40 x 15) / 100 = 6.
Watermelon has a GI of 72 and contains 6 g of carbohydrate per serve. The GL of watermelon is therefore (72 x 6) / 100 = 4.
Considering GI alone, you might think watermelon was a poor choice as it has a relatively high GI. However, because it contains little carbohydrate, it will not affect your blood glucose level much — we can tell this from the low GL.
Eating predominantly high-GI foods — especially if your diet is lacking in fibre from grains — may increase your risk of type 2 diabetes. Although more research needs to be done to reach a firm conclusion, Diabetes Australia recommends eating a diet with moderate amounts of carbohydrate and including high fibre foods that also have a low GI. Grains, vegetables, legumes (e.g. peas, beans and lentils) and fruit are particularly recommended.
There is evidence that for people who already have diabetes, eating a low-GI diet can improve blood glucose control.
Some studies have found that high-GI carbohydrate diets may be associated with an increased risk of heart attack, while diets with more low-GI carbohydrates may be associated with a reduced risk. But there is not yet enough evidence that low-GI diets reduce the risk factors for developing heart disease — again, more research needs to be done.
Some experts recommend a low-GI diet for women affected by polycystic ovarian syndrome (PCOS). This is because some women with PCOS do not respond properly to insulin and therefore have trouble regulating their blood glucose level.
Because low-GI foods release their energy slowly, they may prolong the time until you feel hungry again after eating. In terms of weight loss, studies have shown that overweight and obese people lose more weight on low-GI diets than on other weight-loss diets. The low-GI approach also produced a greater improvement in the balance of cholesterol and other blood fats in overweight and obese people than other weight-loss diets.
If you want to use the GI/GL concept to improve your diet, this can be done in 2 ways.
Food manufacturers are being encouraged to label their products with GI information under the GI Symbol Program (run by the University of Sydney, Diabetes Australia and the Juvenile Diabetes Research Foundation). Foods labelled with the GI symbol may be high, medium or low GI, but they must meet specific nutritional criteria — these differ according to the food — and they must have had their GI measured using an approved method.
The GI of a certain food is only part of the overall nutritional nature of that food. It does not give the complete picture. For example, despite their low GI, corn chips are high in saturated fat so should be eaten only in moderation. Conversely, high-GI foods such as rice can make an important contribution to our diet. Perhaps the best approach is to bear GI in mind as part of a balanced diet — the extra effort may well be worthwhile.
Last Reviewed: 17 May 2010