The glycemic index or GI of a food is a measure of the effect that food has on glycemia – the concentration of glucose in the blood.

Foods that can be broken down quickly after ingestion to release glucose into the bloodstream are considered to have a high GI, while foods that are broken down slowly and release glucose gradually into the bloodstream tend to have a low GI. For most people, especially those with diabetes, maintaining a diet made up of low GI foods has several important health benefits.

The concept of glycemic index was first developed by Dr. David J. Jenkins and colleagues in 1980–1981 at the University of Toronto. They were working towards finding the optimal and most beneficial diet for people with diabetes.

Compared with a high GI food, a low GI food takes longer to be digested and for glucose to be absorbed into the bloodstream meaning that the blood glucose level does not peak or “spike” immediately after that food is eaten. These spikes in blood glucose are seen when a high GI food is eaten and glucose is released into the blood immediately after a meal. The slow release of glucose that is seen with low GI foods enables the body to deal with a glucose load more easily as the immediate insulin demand is maintained relatively low which protects against the development of insulin resistance.

The GI is calculated as the incremental area under the two hour blood glucose response curve (AUC) that is achieved after fasting for 12 hours and then eating a fixed amount of carbohydrate (usually 50 g). This area value is divided by the AUC of the standard defining food, which is pure glucose, and multiplied by 100. The average GI value for a particular food is based on the values obtained for 10 humans. Both the standard and test foods have to contain equal amounts of available carbohydrate.