The challenges of using glycemic index
to choose foods to eat
~ No.239 ~
The digestion and absorption of the sugar glucose is rapid. Absorbed glucose passes into the blood stream which causes a rise in blood glucose that can be measured. This rise in blood glucose can be used to calculate what has been termed the glycemic index (GI) of the food. In many foods, glucose is not found in the simple molecular form, but rather many glucose molecules are connected to each other to form the string and branch structure that makes up starches. Enzymes in our gastrointestinal tract act on starches and liberate the glucose which is then absorbed. This takes some time, and so the digestion of foods high in starch is slower than foods high in the simple glucose. Foods high is starch have a low GI; foods high in glucose have a high GI. Many people now believe that eating a diet with low GI foods is a way to control their rate of absorption of sugar, which in turn may affect factors leading to obesity and diabetes.
The growing interest in glycemic index has prompted some people to plan a diet based on the GI of foods. Their aim is to consume as many low GI foods as possible. The GI of many foods is now available. But is it as simple as comparing one number to another if you want to develop a diet based on GI? Unfortunately not. Many factors affect the GI of a food.
For example, the degree of ripeness of fruit affects the GI. As a banana ripens, the GI increases as the starch in the banana breaks down to glucose. The physical form of the food also can affect the GI. That means that a cube of cooked potato has a different GI than if the cube was mashed. A raw apple, apple puree and apple juice all have different GIs. Even foods within a class are different. So different types of rice have different GIs. Pasta in the form of macaroni, star pastina and spaghetti also have different GIs.
The GI is based on the speed at which food starches are broken down to glucose and then absorbed, and so the way foods are processed can also affect the foods GI. Grinding, rolling, pressing, and the application of heat or moisture can break down starch granules. This allows digestive enzyme in the gastrointestinal tract to more quickly break down the starch to glucose, giving the processed food a higher GI.
At the present time there is not enough scientific evidence to say that eating a diet containing more low glycemic index foods is good for your health, or that it is a way of preventing diabetes or obesity. The digestion and absorption of foods with different glycemic index values is not the same, and this may have long term effects on metabolism and health. However, the glycemic index value of a food is affected by many factors, and so designing a diet made up of low GI foods needs to be done with care.
Glycemic index and disease
Am J Clin Nutr 2002,76:290S-8S