A measure that describes how quickly a food releases sugar into the bloodstream. High GI foods release sugar quickly and Low GI foods release sugar slowly. For a number of important reasons relating to energy levels, metabolism, preventing diabetes, and weight control, Low GI foods are healthier for almost all people (the notable exception is that athletes use medium or High GI foods while exercising). Understanding food-labels and choosing Low GI foods is an important part of healthy nutrition.
The glycemic index or glycaemic index (GI) is a number associated with a particular type of food that indicates the food's effect on a person's blood glucose (also called blood sugar) level. A value of 100 represents the standard, an equivalent amount of pure glucose.
The GI represents the rise in a person's blood sugar level two hours after consumption of the food. The glycemic effect of foods depends on a number of factors, such as the type of starch, physical entrapment of the starch molecules within the food, fat and protein content of the food and organic acids or their salts in the meal. The GI is useful for understanding how the body breaks down carbohydrates and only takes into account the available carbohydrate (total carbohydrate minus fiber) in a food.
The glycemic index is usually applied in the context of the quantity of the food and the amount of carbohydrate in the food that is actually consumed. A related measure, the glycemic load (GL), factors this in by multiplying the glycemic index of the food in question by the carbohydrate content of the actual serving. Watermelon has a high glycemic index, but a low glycemic load for the quantity typically consumed.Fructose, by contrast, has a low glycemic index, but can have a high glycemic load if a large quantity is consumed.
GI tables are available that list many types of foods and their GIs. Some tables also include the serving size and the glycemic load of the food per serving.
A practical limitation of the glycemic index is that it does not measure insulin production due to rises in blood sugar. As a result, two foods could have the same glycemic index, but produce different amounts of insulin. Likewise, two foods could have the same glycemic load, but cause different insulin responses. Furthermore, both the glycemic index and glycemic load measurements are defined by the carbohydrate content of food. For example, when eating steak, which has no carbohydrate content but provides a high protein intake, up to 50% of that protein can be converted to glucose when there is little to no carbohydrate consumed with it. But because it contains no carbohydrate itself, steak cannot have a glycemic index. For some food comparisons, the "insulin index" may be more useful.