The Glycemic Index (GI) is a ranking that illustrates the rate at which carbohydrates within foods end up as glucose within the bloodstream. The GI is determined by a specific foods actual blood glucose response compared to Glucose as a reference food. The index is ranked on a scale from 0 to 100 comparing different food types relative to pure glucose. Foods that are digested quickly, rapidly releasing glucose and have a rapid effect on blood sugar levels have a high GI ranking, e.g. white bread. Conversely, those foods that take longer to release their carbohydrates in the form of glucose have a low GI ranking e.g. lentils.
The GI specifically aids in diet regulation providing a resource for establishing optimal intake of nutrients via particular carbohydrate rich foods. It is especially useful in determining what are appropriate foods for specific types of training, ailments, conditions and training ambitions. By establishing the rate and extent of Glycemic response to foodstuffs a judgement can be made to the timing of ingestion, type and quantity of food.
It is often surprising to learn the GI of foods considered to release energy relatively slowly. For example, white rice can have a GI as high as 87, only 13 points different to ingesting pure glucose. Many such foods have elevated GI values due to being highly refined.