The Glycemic Index Concept
THE MOST RECENT TOOL IN THE DIETARY TREATMENT OF DIABETES MELLITUS AND
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.