GLYCAEMIC INDEX (GI) is a measure of the effects of dietary carbohydrates on blood sugar levels. The digestion and absorption of these carbohydrates in the small intestine leads to a rise in their levels in the blood. For glucose, this also provokes the secretion of insulin. The glycemic index of a food is defined as the area under the two hour blood glucose response curve (AUC) following the ingestion of a fixed portion of available carbohydrate (usually 50 g). The AUC of the test food is divided by the AUC of the standard (either glucose or white bread, giving two different definitions) and multiplied by 100. The average GI value is calculated from data collected from a minimum of 10 human subjects. Both the standard and test food must contain an equal amount of available carbohydrate. The result gives a relative ranking for each tested food.
Carbohydrates that break down quickly during digestion and release glucose rapidly into the bloodstream have a high GI. Carbohydrates that break down more slowly, releasing glucose more gradually into the bloodstream, have a low GI.
A lower glycemic index suggests slower rates of digestion and absorption of the foods’ carbohydrates and may also indicate greater extraction by the liver and peripheral tissues of the products of carbohydrate digestion.
A lower glycemic response usually equates to a lower insulin demand but not always, and may improve long-term blood glucose control and blood lipids.
| Classification | GI range | Examples |
|---|---|---|
| Low GI | 55 or less | most fruits and vegetables (except potatoes and watermelon), grainy breads, pasta, legumes/pulses, milk, yogurt, products extremely low in carbohydrates (some cheeses, nuts), fructose |
| Medium GI | 56–69 | whole wheat products, basmati rice, sweet potato, table sugar |
| High GI | 70 and above | corn flakes, rice krispies, baked potatoes, watermelon, croissants, white bread, extruded breakfast cereals, most white rices (e.g. jasmine), glucose (100) |
Some carbohydrates, such as sugars and most starches, are rapidly digested and absorbed as glucose into the body through the small intestine and subsequently used for short-term energy needs or stored for future use. Resistant starch, on the other hand, resists digestion and passes through to the large intestine where it is fermented releasing short chain fatty acids. This process is slow so that the body is supplied with energy over many hours after a meal.
