Glycaemic Index/Glycaemic Load

Carbohydrates are the source of most of the energy (kilocalories) we eat (nearly 50% in most UK diets), and supply the body with glucose for growth, muscle and brain function. Dietitians used to advise that starchy foods were digested and absorbed slowly, and that sweet sugary foods were digested and absorbed quickly. Many studies feeding humans identical amounts of carbohydrate from different foods have shown different effects on blood glucose levels. The glycaemic index (GI Index) is a way to describe, on a scale of 1 to 100, the blood glucose response to eating a carbohydrate-containing food in comparison to a reference food (white bread or glucose). The carbohydrate from some foods (such as oats, beans and pulses, pasta, whole-grain breads) are absorbed slowly; after eating these, blood glucose levels rise gradually, do not reach high levels, and stimulate a small insulin response. The carbohydrates from other foods (such as potatoes, white bread and cornflakes) are absorbed quickly; after eating these, blood glucose levels rise quickly, reach high levels, and stimulate a large insulin response.

The glycaemic (blood sugar) response to carbohydrate foods is less pronounced when carbohydrate foods are eaten with proteins and/or with fats; this is usually the case with most snacks and meals. The glycaemic response from a cheese sandwich is slower and lower than the glycaemic response to eating the same amount of bread only; the glycaemic response from eating a baked potato is slower when it is eaten together with baked beans. The other factor that needs to be considered is the amount of carbohydrate in a typical portion of food consumed. The Glycaemic Load (GL) is a measure of the total glycaemic response to a food or meal calculated with the GI figure and the amount of carbohydrate in a serving. In practice, the GL is a marker for the blood glucose effects of foods/meals consumed. The small amount of carbohydrate in a typical portion in some high GI foods, such a carrots or watermelon, gives them a low GL figure.

The GI and GL concept are useful for people with diabetes, who need to balance their blood glucose levels with medication. The concept has also been used to plan the diets of sportspeople, to match the supply and demands for glucose in relation to training and performance. Some studies have suggested that diets with overall lower GI levels may reduce the risk of metabolic syndrome and coronary heart disease, and may support the prevention and treatment of obesity. Many dietitians are involved in further research to clarify the best application of GI concepts in dietary advice, to prevent disease and support good health in the population.

March 2006