Glycemic index
The glycemic index (GI) is a ranking system for carbohydrates based on their effect on blood glucose levels, invented by Dr. David J. Jenkins in 1981.
Carbohydrates that break down quickly during digestion have the highest glycemic indices. The blood glucose response is fast. Carbohydrates that break down slowly, releasing glucose gradually into the blood stream, have low glycemic indices.
Glycemic index values for different foods are calculated by comparing measurements of their effect on blood glucose compared with a reference food.
Scientific publications tend to use glucose as the reference food, so that glucose has a glycemic index value of 100. This has the advantages that it is universal and it results in maximum GI values of about 100, so that GI values can be interpreted intuitively as percentages on an absolute scale.
GI values are commonly interpreted as follows:
- Low GI - GI less than 55
- Intermediate GI - GI between 55 and 70
- High GI - 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.
Other reference foods are used, for example white bread. For people whose staple carbohydrate source is white bread, this has the advantage of conveying directly whether replacement of the dietary staple with a different food will result in faster or slower blood glucose response. Disadvantages are that the reference food is not well defined, and the GI scale is culture dependent. The glycemic effect of white bread depends on factors such as flour variations, and is of little interest to people whose staple is (for example) rice, corn, or millet.
The presence of fat or dietary fiber can inhibit carbohydrate absorption, thus lowering the GI reading. For example, breads with high amounts of fiber have a lower GI reading than white bread, as does bread with butter on it.
A problem with the glycemic index as a measure of carbohydrate absorption is that it measures only the effects of blood glucose. It does not measure well carbohydrates that are processed through different pathways, eg fructose or fructose-containing foods (fruit, vegetables). Glycemic index is also not particularly well correlated with the measures of insulin response.
For extensive data on GI values of foods (based on glucose reference), see the links below.