Glycemic Index

THE GLYCEMIC INDEX

 What is the Glycemic Index?

The Glycemic Index (GI) refers to carbohydrate containing foods according to the effect they have on blood glucose levels after they have been eaten and digested. In other words, it is a measure of the rate at which blood glucose levels are increased after eating a carbohydrate food, such as sugar or a carbohydrate containing food, such as bread.

Not all carbohydrates react similarly after ingestion. Some are absorbed and digested very quickly, like glucose, corn flakes, baked or mashed potato and certain types of rice - these fast release carbohydrates are referred to as high Glycemic Index foods. Other carbohydrates are absorbed very slowly, like lentils, soy beans, kidney beans and certain fresh fruits such as peaches, pears, grapefruit and cherries - these slow release carbohydrates are referred to as low Glycemic Index foods. In between there are many other foods which are absorbed at intermediate rates and these foods are classified as Intermediate Glycemic Index foods.

In the past, it was assumed that complex carbohydrates or starches, such as potatoes, mealie meal and bread, were digested and absorbed slowly, resulting in only a slight rise in blood glucose levels. Simple sugars, on the other hand, were believed to be digested and absorbed quickly, producing an undesirable large and rapid rise in blood glucose. We now know that these assumptions were not always correct (1), and the general public, as well as those with diabetes, no longer needs to avoid sugar altogether, provided it is used correctly, i.e. not more than 10g (2 teaspoons) of sugar with a normal meal. In fact, recent evidence shows that table sugar has a slightly more favourable effect on the blood glucose of normal and diabetic individuals than do potatoes, bread and a few other starches, if eaten alone (2).

Due to the fact that low GI foods sustain blood glucose levels at a nutritionally acceptable constant level, many long term health benefits are achieved, and this is why health professionals recommend lower GI foods.

There are many proven benefits of using the glycemic index in nutrition - decreased risk of cardiovascular disease (3), better diabetes management (4) and more successful weight management (5).

Glucose (the end product of carbohydrate digestion) is the body's fuel. Insulin facilitates the uptake of this glucose and it's delivery to the body cells. If too much glucose is continuously made available, as is often the case when individuals have bad eating habits and furthermore do not exercise, the pancreas organ, which has to continuously produce insulin, can become under active and no longer able to cope with the situation. Blood glucose levels become uncontrollable and the individual becomes diabetic.

The methodology for the testing of GI in South Africa has not been standardized. This is one of the areas of disagreement about GI which has to be resolved. One method measures the blood glucose response (BGR) of foods under controlled conditions on a group of at least 8-12 people. This is done on three different occasions on every person and the average BGR value for each person and the group is established for each food tested. The same tests are repeated using pure glucose as a standard in order to calibrate and rate all results which are then known as glycemic index or GI values.

The blood glucose response to glucose itself is very quick and causes the greatest and most rapid rise in blood glucose of all foods. It is, therefore, given the maximum value of 100 which sets the upper limit - all other carbohydrate foods have GI values ranging from this value down to the twenties.

Often the GI of a given food is not what one would expect. For example, the GI of South African brown bread is 81, whereas the average GI of sweetened low fat yogurt is only 33. For this reason all carbohydrate containing foods need to be tested in order to determine their GI. Guessing the GI of a food, would clearly not be very accurate. The GI of over 800 foods has been determined worldwide, including in South Africa and testing is continuously being carried out.

Some GI testing is done using white bread as a standard, instead of glucose. The consequence of this is that GI values seem to differ from one testing laboratory to another. But in reality the values are the same if one multiplies the glucose value by 1.43 or the bread value by 0.7.

What affects the glycemic index of a food product?

• Particle Size - Intact grains such as wheat or maize have much lower GI values than flours made from the same grains;

• Fibre - Foods containing soluble fibre, such as rolled oats and legumes, have a GI lowering effect, because they delay the digestive process;

• Fat & Protein - A mixed meal containing small amounts of fat and protein tends to lower the blood sugar response of a meal, as both fat and protein slow down the digestion of carbohydrate (2). The GI of maize meal porridge (high GI), for example, will be lowered significantly when served with milk;

• Cooking & Processing Methods - Cooking and processing normally change the structure of food resulting in easier absorption by the body;

• Product Temperature - when some cooked starches cool down, a more resistant starch develops, causing a GI lowering effect;

• Metabolism - Exercise, which increases metabolism, influences the glycemic response of a food in an individual;

• Portion Size - The amount of food consumed during a meal will affect the glycemic load, but the GI (rate of absorption) remains the same;

• Food acidity - Acidity delays the emptying of the stomach and thereby lowers the GI of a food.

Can GI be used for body mass or obesity control?

Together with potion control, the glycemic index is a useful nutritional tool that can be used to regulate carbohydrate intake. However, balanced nutrition should also be kept in mind, such as a lower fat diet coupled with exercise.

It must be borne in mind that high glucose levels result in high insulin levels, which in turn result in fat deposits in the cells. Thus the GI should be part of a weight management programme. For this reason FACS recommends that professional advice from a registered dietitian be sought.

Is labeling for GI mandatory

Current labeling regulations do not make provision for GI labeling. Voluntary GI information can be provided on labels, with reference to the testing methodology. This information should only be used as a guideline.

How should the GI be used ?

From a practical point of view, the GI should be seen as a nutritional tool to be used in helping one to follow a healthy diet. The inclusion of the low-GI concept into the general diet means increasing the consumption of fruits, vegetables and legumes and limiting the intake of refined sugars. In other words, GI value can assist in selecting food that is high in fibre, micronutrients and antioxidants and low in energy - which is the basis of a healthy diet.

Are GI values for South African foods available?

Because food products and formulations can differ from country to country, extensive work on the GI values of local foods has been done. The latest update appeared in 2005 and is entitled "The South African Glycemic Index and Load Guide" by Gabi Steenkamp and Liesbet Delport. This publication lists the GI of nearly 500 food products, local as well as overseas.

What is the FACS message?

Although some disagreement exists about the glycemic index concept, it can be used as a useful guideline for improving eating habits as there is no question that positive changes to our diets can bring about real health benefits. It is, however, our belief that the overall nutritional balance of our diets and the balance of lifestyle factors, must always be used in conjunction with new nutritional tools.

ACKNOWLEDGEMENT

FACS wishes to thank the Association for Dietetics in South Africa for their assistance in compiling this information leaflet.