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?

clipboard 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

Glycaemic Index  Fruits / Vegetables Carbohydrates  Others
Low Fruits: Apples, pears, oranges, bananas, cherries, grapefruit, plums, prunes, tomatoesVegetables: Beans, peas, cabbage, lettuce, carrots, broccoli, spinach, mushrooms, onions Bread: Whole wheat, multi-grain, rye, oatmealWhole grain crackers, cooked pasta, oatmeal, brown rice Dairy products: Low fat yoghurt, whole/low fat/skim milkEggs, meats
Medium Fruits: Mangoes, pineapples, apricots, sultanas, melonVegetables: Potatoes (boiled, new, tinned), beetroot Wholemeal bread, oat bran, muesli, crumpets, hamburger buns, white rice Popcorn, Mars bars, honey
High Fruits: WatermelonVegetables: Mashed potatoes, parsnips White bread, bagels, Coco pops, rice cakes, cornflakes, puffed wheat, steamed white rice Chips/french fries, jellybeans

 

 


Glycaemic index and metabolism

After we consume carbohydrate containing foods, the body signals the pancreas to secrete a hormone called insulin to break it down. Insulin acts to lower the body’s sugar levels. When the blood sugar levels decrease to a particular level, the brain is sent a signal and you become hungry again.

If high GI foods are continually consumed, the rapid breakdown results in sharp decreases in your blood sugar levels, and you feel full for shorter periods of time. You can see that this sets up a continual cycle, causing weight gain and the development of diseases such as high cholesterol and blood pressure.

If you have diabetes, your insulin levels are either low or you are resistant to the effects of insulin. This allows blood sugar levels to increase – with high GI foods, the rapid surge in blood sugar levels is more difficult to control with medications or injectable insulin. After a period of time, prolonged high blood sugar levels leads to damage to many organs of the body, such as the blood vessels and nerves.

If you eat foods that are low in GI, blood sugar levels can be sustained at a lower level over a longer period of time. There is less demand on your pancreas if it is still producing insulin and you feel satisfied for longer. If your body does not produce any more insulin, medications and injectable insulin can control the levels more steadily. The sugars in the blood can be directed towards the cells to be used as energy rather than being stored as fat.


Glycaemic index and meals

During a mixed meal, the effects of carbohydrate containing foods is altered – this is because there are effects from protein and fats on the overall blood sugar level response. Many studies have been performed, which show that although the fat and protein levels affect the overall blood sugar level response, they do not affect the differences in GI that various foods have. For example, if each meal contains the same amount of protein and fat, foods with a high GI still lead to a higher blood sugar level response than low GI meals.


Glycaemic index and medical conditions

vegesConsuming foods that have a high GI at least doubles your risk of developing heart disease and type 2 diabetes later in life. These foods are also more often processed and are less healthy, resulting in increased chances of developing high blood pressure and high cholesterol levels. Glucose control is very important in helping prevent particular diseases, especially if you have diabetes. High GI foods make it harder to gain good control of blood sugar levels.

Studies have been done which show that in patients with diabetes, persistently elevated blood sugar levels is associated with an increased risk of disease of the arteries, disease of the heart and major vessels. In the long term, poor control of blood sugar levels in diabetic patients leads to both heart and blood vessel disease, kidney failure, nerve damage, eye problems and heart disease. Gaining optimal control over blood sugar levels is therefore essential, to ensure that these long term consequences are avoided.

Type 2 diabetes is often seen in association with the metabolic syndrome, a combination of risk factors such as increased weight (obesity), high blood pressure, high cholesterol and fat levels, and resistance to insulin. A diet with many high GI foods often leads to worsening of these risk factors, contributing to events such as fat build up on the inside of arteries (atherosclerosis) and high blood pressure levels.


How to incorporate low GI foods into your lifestyle

Some tips for maintaining a healthy diet and incorporating the GI index into your daily routine include:

  • Follow the dietary guidelines for Australians, trying to incorporate a variety of foods into your eating plan.
  • Try and use low GI foods instead of high GI foods when possible.
  • Try and have at least 3 low GI foods throughout the day, especially during mealtimes.
  • Replacing carbohydrates with protein or fat.
  • Try a different low GI food each day, and try to incorporate the ones you like into your weekly food routine.

More information

Nutrition For more information on nutrition, including information on types and composition of food, nutrition and people, conditions related to nutrition, and diets and recipes, as well as some useful videos and tools, see Nutrition.

References

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