What Is The Glycaemic Index?
The Glycaemic 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 Glycaemic 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 Glycaemic Index foods. In between there are many other foods which are absorbed at intermediate rates and these foods are classified as Intermediate Glycaemic Index foods.
In the past, it was assumed that complex carbohydrates or starches, such as potatoes, mealie meal (corn 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.
How Does GI Affect Health?
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, 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.
High blood insulin levels also encourage fat storage. Therefore it is clear that controlling blood glucose levels is useful in the management diabetes, overweight and heart disease. Because the body is a complex biological system, other non-related problems could also cause high insulin levels or damage to the pancreas.
How Is The GI Determined?
The methodology for the testing of GI has been standardized. Blood glucose response (BGR) of different foods under controlled conditions is monitored on a group of 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 the 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. There are a few exceptions of very highly processed foods that have a GI value higher than 100, which means they spike blood glucose levels even faster that pure glucose. In South Africa such an example would be flavoured instant oats.
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 thousands of foods has been determined worldwide, including South African foods and GI 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 Glycaemic Index Of A Food Product?
Particle Size - Intact grains such as whole wheat or maize on the cob (green mealies) 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 'mealie pap'), for example, will be lowered significantly when served with milk;
Cooking & Processing Methods - Cooking and processing normally changes the structure of food resulting in easier absorption by the body; the more a food is processed, the higher the GI as the food is absorbed faster.
Product Temperature - when some cooked starches cool down, a more resistant starch develops, causing a GI lowering effect, by slowing down the rate of digestion and thus absorption of the glucose in the starch;
Metabolism - Exercise, which increases metabolism, influences the glycemic response of a food in an individual. After exercise of longer than 90 minutes, all foods are digested more slowly and thus will have a lower glycemic response.
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?
The cornerstone of weight management is portion control. Together with the glycemic index they make useful nutritional tools that can be used to regulate the absorption of carbohydrates in meals to lessen the release of insulin and thereby lessen the fat storage effect. However, balanced nutrition must always be kept in mind, so that meals should consist of vegetables (half the plate), a small portion of meat, fish or chicken and a fistful of low GI starch. Coupled with regular exercise, weight management becomes a lifestyle.
Remember that high glucose levels result in high insulin levels, which in turn result in fat deposits in the cells. Thus lower GI foods 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 South African labeling regulations do not make provision for GI labeling. Voluntary GI information can be provided on labels by way of the GIFSA endorsement logo which has been accredited by the Dept of Health, as the GI testing done by them is in accordance with international standards. The food labelling regulations make no other provision for other GI information on a food label to date.
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 and starches. 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 differ from country to country, extensive work on the GI values of local foods has been done. The latest update appeared in 2015 and is entitled "The South African Glycaemic Index and Load Guide" by Gabi Steenkamp and Liesbet Delport. This publication lists the GI of nearly 800 commonly eaten South African food & food products. This publication is updated every 2 years.
What Is The FACS Message?
The glycemic index concept of knowing whether a food is digested and absorbed quickly (high GI); or more slowly (low GI) 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.
FACS wishes to thank the Association for Dietetics in South Africa for their assistance in compiling this information leaflet.
(A list of references is available from SANCU - (0)12 341 9746.)
Updated for FACS by GSt (2016)