~ January 1998 No.33 ~
The stomach and large intestine break food down into nutrients which then pass through the intestinal wall into the blood stream to be transported to the various parts of the body. The digestion and metabolism of sugar (glucose linked to fructose) is a complex process that involves the hormone insulin, a hormone produced by the islets of Langerrhans which are specialized cells found in the pancreas. The release of insulin is triggered by a rise in blood glucose that follows a meal containing sugar. The secreted insulin lowers blood glucose levels by increasing storage and utilization of glucose by tissues in the body. Several hours after a meal,blood glucose levels return to what they were before the meal.
The ability of the body to digest, absorb and metabolise dietary sugar can be measured by means of a standard test called a glucose tolerance test. A patient is given a fixed amount of glucose to eat. Blood samples are taken before and periodically after the glucose is eaten and are tested for their glucose content. The rise and fall of the glucose concentration after the"test meal" can be used to estimate the glucose tolerance of the patient.
The rise and fall of blood glucose following a meal that is usually wellcontrolled, but becomes a problem in diabetics who are unable to produce insulin fast enough or in large enough quantities to metabolize sugar that is eaten. Rapid rises, and high concentrations of blood glucose concentrations are not desirable. Some diabetics need to take insulin to control blood glucose levels. Others can avoid certain foods to help control their glucose levels.
Scientists have found that although many foods contain glucose, the rate at which the foods are broken down and the speed at which the glucose is taken up by the body can vary. Using a test similar to the glucose tolerance test,subjects (non-diabetics) are fed a fixed amount of a particular food and their blood glucose levels are measured. The rate at which the test food is absorbed is compared to the rate at which glucose (or in some cases standard white bread) alone is absorbed. A ratio can be calculated that is an indication of how fast the body responds to that particular food. This ratio is called the glycemic index. People trying to avoid sharp rises in blood glucose levels would therefore be encouraged to eat foods with a low glycemic index.
Athletes have recently become interested in the glycemic index. Depending on the type and duration of the activity, it may be desirable to have slow or fast uptake of glucose. Some athletes are using glycemic index values to plan their diets leading up to and during competitions. At the present time there is no good evidence that this results in any improved performance.
A useful site on the www that contains a long list of foods and their glycemic index can be found at :Revised International Table of Glycemic Index (GI) and Glycemic Load (GL) Values-20021