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The Glycemic Index Concept

THE MOST RECENT TOOL IN THE DIETARY TREATMENT OF DIABETES MELLITUS AND OTHER CONDITIONS

 
The Glycemic Index (GI) is simply a ranking of foods based on their immediate effect on blood glucose levels. It is a physiological measure of how fast, and to what extent, a carbohydrate food affects blood glucose levels.


In the past, it was assumed that complex carbohydrates ( starches) such as potatoes, maize meal (mieliepap) and bread, were digested and absorbed slowly, causing only slight rise in blood glucose levels. Simple sugars, on the other hand, were believed to be digested and absorbed quickly, producing a large and rapid rise in blood glucose. We now know that these assumptions were incorrect, and that the general public, as well as diabetics, no longer need to avoid sugar altogether provided they use it correctly. In fact, we now know 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 when used alone.


As early as the 1930’s scientists challenged the traditionally held view that the metabolic effects of carbohydrates (CHO) can be predicted by classifying them as either simple or complex. In the 1970’s researchers such as Otto and Crapo examined the glycemic impact of a range of foods containing CHO. To standardize the interpretation of glycemic response data, Jenkins and colleagues from the University of Toronto, Canada, proposed the use of a Glycemic Index (GI) in 1981. The Glycaemic Index (GI) rates food according to its actual effect on blood glucose levels. . The GI  disproved the assumption that equal amounts of CHO from different foods causes a similar glycemic response. Furthermore, the researchers concluded that the CHO exchange lists  which have regulated the diets of most diabetics, do not reflect the physiological effect of foods and are therefore no longer sufficient for controlling blood glucose. Research conducted during  the past two decades shows that it is not the amount of carbohydrate but rather its rate of digestion and absorption which determine the physiological response of the body.


The Glycemic Index offers the following:

  •           reflects the physiological effect of foods,
  •           helps to keep blood glucose levels even,
  •           substitutes the old terms of complex and simple CHO.


Research conducted throughout the world has confirmed  that ranking foods according to their actual effect on blood glucose is scientifically more accurate. Consequently, the Glycemic Index (GI) factor was developed and  foods ranked on a scale from 0 – 100, according to their actual effect on blood glucose levels. On the Index, glucose is taken as 100 since it causes the greatest and most rapid rise in blood glucose -  all other foods are rated in comparison to glucose. Since the GI is a ranking of foods based on their actual effect on blood glucose levels instead of on assumptions, it provides an accurate tool for regulating blood glucose levels. By using the GI concept diabetics, people who suffer from low blood sugar (hypoglycemia),  children with Attention Deficit Disorder (ADD), and sportsmen may optimize their blood glucose control. By using the GI concept in combination with low-fat foods, both triglycerides and blood pressure  may be lowered and HDL-cholesterol (good cholesterol)  may be increased. For those wanting to lose weight, the increased satiety levels and reduced insulin levels (a fat storer) resulting from following a low GI diet may enhance weight loss.  Even people who suffer from  gout  may benefit from following  a low fat, low GI diet. Foods with a low GI release glucose slowly and steadily into the bloodstream and so do not over stimulate insulin secretion. High insulin levels are implicated in many lifestyle diseases including  high blood pressure, high blood cholesterol, high triglycerides, diabetes (type II), hypoglycemia, ADD, obesity and coronary heart disease (CHD).

 

HOW THE GLYCEMIC INDEX (GI) IS DETERMINED

The Glycemic Index (GI) is a physiological measure of how fast, and to what extent, a carbohydrate food affects blood glucose levels. The GI uses the blood glucose response (BGR) to GLUCOSE as the point of reference. The GI of a specific food is determined by comparing the BGR of that food with the BGR of GLUCOSE. GLUCOSE is absorbed quickly from the small intestine and generally causes the greatest and most rapid rise in blood glucose. Usually, 50 grams of carbohydrate (the equivalent of 3 tablespoons pure glucose powder)  is used to establish the Glucose Base or Index reference for a specific test person. A value of 100 would be allocated to the BGR of GLUCOSE for that person. In practice, the BGR of GLUCOSE would be tested at least three times in each test person to ensure an accurate reference for the other tests.


The BGR measurement is done by testing the actual blood glucose level of the test person every 15 minutes for a period of 3 hours. The glucose level readings are plotted on a chart or graph. The area under the curve, which  indicates the rise and fall of BGR, is calculated and represents the absolute BGR of the specific food for that specific test person. To calculate the GI of the food the absolute reading is expressed as a percentage of the absolute reading of the BGR of glucose. In order to ensure accuracy the BGR of eight to twelve people is measured to determine the GI of a certain carbohydrate. The average of the group for a specific food is used as the GI value of that food.


To determine the GI of a specific food, e.g. potatoes, an amount containing 50 grams of carbohydrate is given to a test person to eat. Thus, in the case of boiled potatoes,  a volunteer would be given 250 grams of potatoes which supplies 50 grams of carbohydrate.


Often the GI of a given food is not what one would expect. For example, the GI of South African brown bread is 70 whereas  sweetened, low-fat fruit yoghurt is only 33. For this reason, all foods containing carbohydrate need to be tested to determine their GI. The GI of over 600 foods  has been determined worldwide and new  foods are being tested constantly.