The glycemic index (glycaemic index in British English) is a scale used to quantify the effect on blood sugar values by carbohydrates. Foods that rate higher on the index break down quickly and cause blood sugar levels to rise rapidly, while foods that sit lower take longer to break down and release glucose into the blood stream over a longer period of time.

The glycemic index was developed during 1980 and 1981 at the University of Toronto. Dr David J. Jenkins and his fellow researchers were investigating diets for diabetics, when they decided to quantify the effect of carbohydrate intake on blood sugar levels.

There are two scales commonly used for the glycemic index. In one, white bread is taken as the reference point and assigned a GI value of 100. Other values are calculated relative to this. However, the more commonly used scale is to take pure glucose to have a value of 100. This is the scale referenced in the rest of this article.

High GI, low GI and everything in between
The glycemic index of a food is calculated, and then is classified as either high GI, medium GI or low GI. The ranges are as follows:
High GI: 70+
Medium GI: 56-69
Low GI: 55 or below

Dangers of high GI foods
The rapid influx of glucose caused by eating foods high on the GI scale can have negative health effects, as documented by researchers. Diabetics in particular are encouraged to avoid high GI foods, but recent studies have shown that such foods can cause health problems for anyone - in one such study, scientists found that Age-related Adult Macular Degeneration was higher amongst people following a GI diet than the normal population.

Weight loss
Many weight loss plans, such as the South Beach Diet, encourage the consumption of foods low on the glycemic index for weight loss. Diet plans tend to operate on the theory that the rapid rise in blood sugar brought upon by high GI foods cause an insulin spike, which then encourages the body to store fat. In studies conducted on rats, animals which consumed a high GI diet were 71% fatter than their counterparts on a low GI diet.

Examples of high and low GI foods
The glycemic index value of a food is determined by a large number of factors. Processing of the food normally increases the GI, though there are exceptions to this. Common foods that fall into the high, medium and low GI categories are:

High GI
White bread, most types of white rice, many processed breakfast cereals, some fruits such as watermelon

Medium GI
Table sugar, some fruits such as raisins, some whole wheat products

Low GI
Most fruits, some wholegrain products, pasta, dairy products

It is difficult to give accurate guidelines for a lot of foods, because they vary depending on factors like type and preparation. An example of this is porridge - wholegrain porridge oats tend to be low GI, while oats that have been processed tend to be medium GI. Instant oats normally fall into the high GI category.

Balancing out a meal
If a meal is high GI, dieticians often recommend adding a low GI complement to the meal to balance out the GI value. An example of this can be found in a breakfast cereal such as corn flakes - these tend to be high GI, but by adding milk, the GI value of the meal can be reduced.

Glycemic Load
An extension of the glycemic index is the glycemic load (GL). This is simply a measure of how much a serving of one food will impact blood sugar levels, and is calculated by:

Glycemic Load = (Glycemic Index * Carbohydrate Content (grams)) / 100

Insulin Index
A similar index to the glycemic index is the insulin index. This index measures the production of insulin in response to a given food, as opposed to the rise in blood sugar.