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
Low GI
55 or less
most fruits and vegetables, whole-grain bread, 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 many factors such as the type of starch (amylose vs. amylopectin), the 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 reducing 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.
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 type of food significantly alters the glycemic index. It’s 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 volume of carbohydrate ingested 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. Foods with a low glycemic index may have high carbohydrate content or vice versa.
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 the composition of the previous meal strongly influences the glycemic response.
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 have a far higher “GI” slowing effect than the carbohydrate bearing matrix or starch chemistry.
The above overview illustrates dietary carbohydrate quantity dictates (or at least is the larger influencer on) the total daily plasma insulin volume. The simple conclusion is that over-consumption of carbohydrates not comparative GI of said carbohydrate is the more significant problem.