This is a revised version of a posting from a few weeks ago. I beleive this to be the final version.
The glycemic index or GI is a measure of the effects of carbohydrates (laden foods) on blood sugar levels. Carbohydrates (laden foods) that break down quickly during digestion and release glucose rapidly into the bloodstream have by definition a high GI; carbohydrates (laden foods) that break down more slowly have by definition a low GI.
The concept was developed by Dr. David J. Jenkins and colleagues in 1980–1981 at the University of Toronto. The purpose of their research was to find out which foods were most suitable for subjects with Diabetes Miletus.
A lower glycemic index suggests slower rates of digestion and absorption of the foods' carbohydrates and may also indicate greater extraction of carbohydrate digestion. A lower glycemic response usually equates to a lower insulin demand but not always, and may improve long-term blood glucose control.
You can find additional info relating to the GI at http://www.glycemicindex.com The Official Website of the Glycemic Index and Database.
Glycemic Index Defined
The glycemic index of a food is defined as the area under the two hour blood glucose response curve (AUC) following the ingestion of a fixed portion of carbohydrate (usually 50 g). The AUC of the test food is divided by the AUC of the standard (glucose) and multiplied by 100. The average GI value is calculated from data collected in 10 human subjects. Both the standard and test food must contain an equal amount of available carbohydrate. The result gives a relative ranking for each tested food in comparison to 50g of pure glucose.
Glycemic Indexes Of Foods
Classification GI range Examples
Low GI 55 or less most fruits and vegetables, whole-grain breads, pasta, legumes/pulses, milk, yogurt, fructose
Medium GI 56–69 whole wheat products, basmati rice, sweet potato, table sugar
High GI 70 and above corn flakes, puffed rice, baked potatoes, watermelon, croissants, white bread, white rice, glucose (100)
The glycemic effect of foods depends on a number of factors such as the type of starch (amylose vs. amylopectin), physical matrix of the food, fat and protein content of the food and organic acids or their salts content. Adding vinegar, for example, will lower the GI of a meal. The presence of fat or soluble dietary fiber can slow the gastric emptying rate, thus lowering the GI
The glycemic index can be applied only to only foods with reasonable carbohydrate content, as the test relies on subjects consuming enough of the test food to yield about 50 g of available carbohydrate. Many fruits and vegetables contain very little carbohydrate per serving, and the average person is not likely to eat 50 g of carbohydrate from these foods. Fruits and vegetables tend to have a low glycemic index and a low glycemic load.
Related Glycemic Indexes
The glycemic load (GL) is a ranking system for carbohydrate baring food portions based on their glycemic index (GI) and the portion size. Glycemic load or GL combines both the quality and quantity of carbohydrate into one ‘number’. It is one of the better best ways to predict blood glucose values of different types and amounts of food.
The formula is: GL = (GI x the amount of carbohydrate) divided by 100.The usefulness of glycemic load is based on the idea that a high glycemic index food consumed in small quantities would give the same plasma glucose effect as larger quantities of a low glycemic index food. By way of example white rice has a somewhat high GI, so eating 50g of white rice at one sitting would give a particular glucose curve in the blood, while 25g would give the same curve but half the height. Since the peak height is considered by many experts the most important parameter for diabetes control, multiplying the amount of carbohydrates in a food serving by the glycemic index gives an idea of how much effect an actual portion of food has on blood sugar level.
Many have questioned the value of using glycemic load as a basis for weight loss programs; however, glycemic load is generally a useful concept in dietary designs targeting health maintenance. Studies have shown that elevation in blood sugar and insulin levels may lead to increased diabetes risk. Studies concluded (repeatedly) that women whose diets had the highest glycemic index were 21 percent more likely to develop type 2 diabetes than women whose diets had the lowest glycemic index.
The Insulin Index is a measure used to quantify the typical insulin response to various foods. The index is similar to the Glycemic Index and Glycemic Load, but rather than relying on blood glucose levels, the Insulin Index is based upon blood insulin levels. This measure can be more useful than either the Glycemic Index or the Glycemic Load because certain foods (e.g., lean meats and proteins) cause an insulin response despite there being no carbohydrates present, and some foods cause a disproportionate insulin response relative to their carbohydrate load.
Holt et al. have noted that the glucose and insulin scores of most foods are highly correlated, but high-protein foods and bakery products that are rich in fat and refined carbohydrates "elicit insulin responses that were disproportionately higher than their glycemic responses." They also conclude that insulin indices may be useful for dietary management and avoidance of non-insulin-dependent diabetes mellitus and hyperlipidemia.
Explanation of Index
The insulin index shows how much insulin is present in a subject’s blood as a result of particular food consumption, the glucose index shows how much glucose is present in the blood as a result of a particular food, and the satiety index shows how much a particular food decreases one's propensity to eat more.
Glucose (glycemic) and insulin scores were determined by feeding 1000 kilojoules (239 kilocalories) of the food to the participants and recording the area under the glucose/insulin curve for 120 minutes then dividing by the area under the glucose/insulin curve for white bread. The result being that all scores are relative to white bread. The satiety score was determined by comparing how much food was eaten by participants at a buffet after being fed a fixed number of calories of a particular food while blindfolded (to ensure food appearance was not a factor), then dividing that number by the amount eaten by participants after eating white bread. White bread serves as the baseline of 100. In other words, foods scoring higher than 100 are more satisfying than white bread and those under 100 are less satisfying.
Limitations And Criticisms
• The glycemic index does not take into account other factors other than glycemic response, such as insulin response, which is measured by the insulin index and can be more appropriate in representing the effects from some food contents other than carbohydrates.
• The glycemic index is significantly altered by the type of food, its ripeness, processing, the length of storage, cooking methods, and its variety
• The glycemic response is different from one person to another and even in the same person from day to day, depending on blood glucose levels, insulin resistance, and other factors.
• The number of grams of carbohydrate impacts blood sugar levels more than the glycemic index.
• Lowering glycemic index leads to small (momentary) improvements in blood sugar levels, but consuming fewer total carbohydrates would benefit the blood glucose profile much more.
• Carbohydrate impacts glucose levels most profoundly, and two foods with the same carbohydrate content are generally comparable in their effects on blood sugar. A food with a low glycemic index may have high carbohydrate content or vice versa; this can be accounted for with the glycemic load.
• Most of the values on the glycemic index do not show the impact on glucose levels after two hours.
• The GI of foods is determined under experimental conditions after an overnight fast (with only 10 test subjects), and might not apply to foods consumed later during the day because glycemic response is strongly influenced by the composition of the previous meal.
Relevance And Application
The Glycemic Index holds extraordinary relevance to a diabetic subject (or anyone using exogenous insulin). The ability to comparatively rate the release rates of dietary glucose holds life saving value to the clinical diabetic.
For non diabetic and non insulin using subjects the value of such data is questionable at best. The Glycemic Index holds almost no valuable info for the non insulin using athlete. Food volume and meal frequency has a far greater “GI” slowing effect than the carbohydrate bearing matrix or starch chemistry.
As the above overview illustrates dietary carbohydrate quantity dictates the total daily plasma insulin volume. The simple conclusion is that over consumption of carbohydrates not the comparative GI of said carbohydrate is the larger problem.