Virtual Medical Centre

Glycaemic Index (GI)

• What is the glycaemic index (GI)?
• How are carbohydrates divided into groups?
• How is the glycaemic index determined?
• Factors affecting the glycaemic index

What is the glycaemic index (GI)?

We have all heard of the claims that low glycaemic index (GI) foods are better for you and many food manufacturers now proudly claim that their food products have a low GI - what does all this really mean? The glycaemic index (GI) is obtained by measuring the effect that a carbohydrate containing food has on blood sugar levels, compared to the effect of the same amount of pure sugar, on blood sugar levels. The Dietary Guidelines for Australians recommend that carbohydrates should make up at least 55% of total daily calories.

In more recent times, the quality of these carbohydrates has been shown to also be important in blood sugar control. Even though different foods may contain the same amounts of carbohydrates, the effects on blood sugar control may be totally different. This has lead to the development of measures such as the GI of a food product. Research has shown that by consuming a diet with a lower GI and rich in healthy foods, people with diabetes can reduce their average blood sugar levels and reduce the risk of complications. Other studies have also shown that diets that have an increased amount of high GI foods are associated with an increased risk of developing heart disease, type 2 diabetes and disease of the arteries.  

How are carbohydrates divided into groups?

Carbohydrates can be classified as simple (ie sugars - mono and disaccharides) or complex (ie starches - polysaccharides). This depends on the number of sugar molecules they contain. It was originally thought that complex carbohydrates cause a smaller increase in blood sugar levels compared to simple carbohydrates. However, further research shows that different types of complex carbohydrates produced varying effects on blood sugar levels and the body's insulin response mechanisms. This means that complex carbohydrates may not necessarily produce a different response on blood sugar levels compared to simple carbohydrates. Rather it is the type of food and a combination of other factors (as discussed below) that are important in determining the GI.

How is the glycaemic index determined?

To determine the GI value of a food, 10 or more people are fed a portion of the test food containing 50 grams of carbohydrate. These patients are fasted the night before to ensure that accurate results are obtained.

Some blood is then taken by making a small prick on the finger, over the next two hours. These blood test results are then used to draw a blood glucose response graph. The area under this graph is then measured. These 10 people are then given an equal amount of pure glucose sugar and their two hour blood glucose levels are measured.

Finally, the food's GI value is calculated by dividing the area under the graph for the food tested over the area under the graph for the equal amount of pure sugar. The average GI from the 10 people becomes the final GI level for that food.

Factors affecting the glycaemic index

There are various factors that affect the GI of a food. These include the types of sugar and starches in the food, the way it is prepared, and it's fat and fibre content. How rapidly the food product is digested and absorbed is very important in determining the GI. Generally, foods with a low level of starch and high in fibre tend to have a lower GI level - ie grains, oats and barley.

Each carbohydrate containing food is compared to the affect that 50 grams of glucose has on blood sugar levels and ranked accordingly. Glucose occupies the number 100 on the GI. Each food is given a number that shows a rise in blood sugar levels, compared against glucose. The following ranges are usually applied to determine the GI of a particular food:

• Low GI - 55 or less.
• Medium GI - 56 to 69.
• High GI - 70 or more.

Foods with a low GI (less than 55) means that they cause a slower and lower rise in blood sugar levels. These include breads such as mixed-grain and fruit and oat breads, barley, pasta, noodles, beans, sweet potatoes, green peas and milk.

Foods with a high GI (greater than 70) means that they cause a faster and higher rise in blood sugar levels. High GI foods include white bread, steamed white rice, chips and coffee.

Limiting your intake of high GI foods will help bring down the average GI of your meal and prevent sudden increases in blood sugar levels. By consuming foods that have a low GI, not only do you benefit from a slower and lower rise in blood sugar levels, but often these foods are more healthy and nutritious. Particularly if you suffer from diabetes, research has shown that by eating a diet with a lower GI and rich in healthy, nutritious foods, diabetics can reduce their average blood sugar levels. This results in a lowered risk of complications in the long term, such as eye, kidney and nerve damage.

Table 1: List of foods in relation to their GI