Glycemic index is a measure of the extent to which blood sugar (glucose) levels are raised by a specific amount of carbohydrate-containing food compared to the the same amount of glucose and white bread.
In the not-so-distant past, it was thought that all carbohydrates were the same and they all had the same effect on blood glucose levels. It is now known that some types of simple and complex carbohydrates elevate blood sugars more than others. Carbohydrates that are digested and absorbed quickly have a high glycemic index and raise blood glucose to a higher level. Diets providing low glycemic index carbs have been found to improve blood glucose control in people with type II diabetes, reduce elevated levels of blood cholesterol, triglycerides, and increases levels of beneficial HDL cholesterol, as well as decreasing the risk of developing type II diabetes, heart disease, and some types of cancer. Foods with a high glycemic index may also cause a rapid spike in blood sugar which will cause an enhanced secretion of insulin from the pancreas. This in turn will cause a rapid reduction of blood glucose to the point of hypoglycemia (abnormally low blood glucose). This insulin response and chronic low blood sugar is refered to as reactive hypoglycemia. Reactive hypoglycemia has been shown to be a precursor to type II diabetes.
Foods with low glycemic index lead to a slower insulin response and a more stable blood glucose level. Protien and fats in combination with high glycemic foods help to slow the absorption of the carbohydrate, and can help ward off a rapid spike in glucose and the following sudden drop.
A low-glycemic diet is not recommended for weight loss (as in the South Beach diet), and has not been shown to lower blood glucose in type I diabetics over the long term. It is helpful, however, in avoiding the roller coaster "spike & drop" in glucose levels common for those with a sugar sensitivity such as reactive hypoglycemia, or those with type II diabetes. Low glycemic foods are typically complex carbohydrates, whereas high glycemic index foods are processed grains and simple sugars.
For a glycemic index of common foods and more information, see Free Glycemic Index Chart
Until scientists know more about the glycemic index, people with type I diabetes should eat healthy foods, whether or not they have a high glycemic index.