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The Glycemic Index (GI) is a ranking system for carbohydrates based on their immediate effect on blood glucose levels. It compares carbohydrates gram for gram in individual foods, providing a numerical, evidence-based index of postprandial (post-meal) glycemia. The concept was invented by Dr. David J. Jenkins and colleagues in 1981 at the University of Toronto.
Carbohydrates that break down rapidly during digestion have the highest glycemic indices. An increased blood glucose response occurs very quickly. Carbohydrates that break down slowly, releasing glucose gradually into the blood stream, have a low glycemic index. A lower glycemic index suggests slower rates of digestion and absorption of the sugars and starches in the foods and may also indicate greater extraction from the liver and periphery of the products of carbohydrate digestion. Additionally, a lower glycemic response equates to a lower insulin demand, better long-term blood glucose control and a reduction in blood lipids.
Glycemic index values for different foods are calculated by comparing measurements of their effect on blood glucose with an equal portion of a reference food. The current scientific validated methods use glucose as the reference food. Glucose has a glycemic index value of 100. This has the advantages in that it is universal and it results in maximum GI values of approximately 100. GI values can be interpreted intuitively as percentages on an absolute scale.
GI values are commonly interpreted as follows:
- Low GI - less than 55
- Intermediate GI - between 56 and 69
- High GI - higher than 70
A low GI food will release energy slowly and steadily and is appropriate for diabetics, dieters and endurance athletes. A high GI food will provide a rapid rise in blood sugar levels and is suitable for post-endurance exercise energy recovery.
Previously, white bread was sometimes used as a reference food (if white bread = 100, then glucose = 140). For people whose staple carbohydrate source is white bread, this had the advantage of conveying directly whether replacement of the dietary staple with a different food would result in faster or slower blood glucose response. The disadvantages with this system were that the reference food was not well-defined, and the GI scale culture dependent.
The glycemic effect of foods depends on a number of factors such as the type of starch (amylose vs amylo-pectin), physical entrapment of the starch molecules within the food, fat content of the food and increased acidity of the meal - adding vinegar for example, will lower the GI. The presence of fat or dietary fibre can inhibit carbohydrate absorption, thus lowering the GI. Unrefined breads with higher amounts of fibre generally have a lower GI value than white breads but, while adding butter or oil will lower the GI of bread, the GI ranking does not change. That is, with or without additions, there is still a higher blood glucose curve after white bread than after a low GI bread such as pumpernickel.
Several lines of recent scientific evidence have shown that individuals that followed a low GI diet over many years were at a significantly lower risk for developing both type 2 diabetes and coronary heart disease. High blood glucose levels or repeated glycemic "spikes" following a meal may promote these diseases by increasing oxidative damage to the vasculature and also by the direct increase in insulin levels.
Criticism
The glycemic index has been criticised for the following reasons: a limited range of data, some variation in GI measurements, GI values affected by cooking or preparation method, difficulty in predicting GI values within mixed meals and daily fluctuations in an individuals glycemic response.
For extensive data on GI values of foods (based on glucose reference), see the links below.
See also
External links
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