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Glycemic index (also glycaemic 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. Such carbohydrates require less energy to be converted into glucose, which results in faster digestion and a quicker increase of blood glucose. 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.

The glycemic index of a food is defined by the area under the 2 hour blood glucose response curve following the ingestion of a fixed portion of carbohydrate (usually 50 g). The AUC of the test food is divided by the AUC of the standard (either glucose or white bread) and multiplied by 100.

The average GI value is calculated from data collected in 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

Glycemic index values for different foods are calculated by comparing measurements of their effect on blood glucose with an equal carbohydrate portion of a reference food. The current validated methods use glucose as the reference food, giving it a glycemic index value of 100 by definition. This has the advantages in that it is universal and it results in maximum GI values of approximately 100. White bread can also be used as a reference food, giving a different set of GI values (if white bread = 100, then glucose ≈ 140). 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 would result in faster or slower blood glucose response. The disadvantages with this system are that the reference food is not well-defined, and the GI scale is culture dependent.

Glycemic index of foods

GI values can be interpreted intuitively as percentages on an absolute scale and are commonly interpreted as follows:

Classification	GI range	Examples
Low GI	Below 55	sour dough bread (54), apple juice (40), pumpernickel (50), oatmeal (48), pasta (54), Indian Basmati Rice (58)
Intermediate GI	56 to 69	croissant (67), coca-cola (63), raisin bran (61), wholemeal bread (65)
High GI	Above 70	white bread (70), corn flakes (80), doughnut (76), white rice (98)

A low GI food will release energy slowly and steadily and is generally appropriate for everyone, especially diabetics, dieters and endurance athletes. A high GI food will provide a rapid rise in blood sugar levels and is suitable for energy recovery after endurance exercise.

The glycemic effect of foods depends on a number of factors such as the type of starch (amylose vs amylopectin), 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 fiber 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. Many brown breads, however, are treated with enzymes to soften the crust, which makes the starch more accessible. This raises the GI, with some brown breads even having GI values over 100.

The glycemic index can only be applied to foods with a reasonable carbohydrate content, as the test relies on subjects consuming enough of the test food to yield about 50 g of available carbohydrate. High fat or high protein foods such as meat, eggs, nuts and cheese have a negligible GI. Furthermore, many fruits and vegetables (but not potatoes) contain very little carbohydrate per serving, and also have very low GI values. This also applies to carrots, which were originally and incorrectly reported as having a high GI. Alcoholic beverages have been reported to have low GI values, however it should be noted that beer has a moderate GI. Recent studies have shown that the consumption of an alcoholic drink prior to a meal reduces the GI of the meal by approximately 15%

Disease prevention

Several lines of recent scientific evidence have shown that individuals who 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 . In the past, postmeal hyperglycemia has been a risk factor mainly associated with diabetes, however more recent evidence shows that postprandial hyperglycemia presents an increased risk for atherosclerosis in the non-diabetic population.

Weight control

Recent animal research provides compelling evidence that high GI carbohydrate is associated with increased risk of obesity. In human trials, it is typically difficult to separate the effects from GI and from other potentially confounding factors such as fibre content, palatability, and compliance. In the study (Pawlak et al, 2004), male rats were split into high and low GI groups over 18 weeks while mean bodyweight was maintained. Rats fed the high GI diet were 71% fatter and had 8% less lean body mass than the low GI group. Postmeal glycemia and insulin levels were significantly higher and plasma triglycerides were three-fold greater in the high GI fed rats. Furthermore, pancreatic islet cells suffered "severely disorganised architecture and extensive fibrosis". The evidence in this study showed that continued consumption of high glycemic index carbohydrates would likely have led to the development of severe metabolic abnormalities.