Understanding the glycaemic index

The glycaemic index (GI) classifies foods according to how fast they release sugar (glucose) into the bloodstream.

High-GI foods release glucose quickly, causing a rapid rise in blood-glucose levels, to which the body reacts with insulin, which turns on fat storage. Low-GI foods, in contrast, release glucose steadily over several hours, so less insulin is required. Diets based on low-GI foods are therefore recommended for those with diabetes and insulin resistance, as well as for people with cardiovascular disease and certain digestive disorders.

GI value

This figure depends on various factors, including the type of carbohydrate the food contains, how it has been processed, and the presence of fat and dietary fibre. In general, low-GI foods contain more fibre, are less processed, and do not contain as much glucose as high-GI foods. However, there is not a simple correlation between complex carbohydrates and low-GI values. For example, wholemeal bread is a healthy carbohydrate, but it has a high-GI value, whereas fructose - the type of sugar found in fruits and frequently added to soft drinks in the form of corn syrup - has a low-GI value. However, this does not mean that products sweetened with corn syrup are healthy.

Glycaemic loading

This is a concept developed to deal with anomalies thrown up by the glycaemic index that cast doubt on its usefulness as a nutritional tool. For example, a well-known brand of chocolate fudge cake has a relatively low GI of 41, while a 100 per cent whole-grain loaf has a GI of 69 - though common sense tells us that the latter is healthier. To deal with such paradoxes, glycaemic load is calculated by multiplying the GI value of a food by the amount of carbohydrate in one serving. This gives a figure that reflects both the time the food takes to be broken down into glucose and also its carbohydrate content.

Conclusion

Our view is that while glycaemic loading is a better indicator of glycaemic response than glycaemic index alone, this is still only part of the story. Other aspects of the nutritional value of foods must also be considered, including vitamin and mineral content, phytochemicals, fibre, and protein. In terms of health benefits, the evidence is clear that eating only refined grain products can increase the risk of a variety of diseases, while eating whole grains has beneficial health effects.

Lisa Hark, PhD RD & Dr Darwin Deen

Nutrition for Life Copyright © 2005 Dorling Kindersley Text copyright © 2005 Lisa Hark and Darwin Deen

Posted 30.06.2010

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