Glycemic index (GI) is the index that can provide a measure of a particular food about how quickly it can raise blood sugar (blood glucose) level after eating the particular food. Glycemic index of glucose is considered as 100 and all other foods are compared with that of glucose. Glycemic index is a good indicator of how quickly they can raise blood sugar level after we eat them and provide diabetic individuals a guide about various foods they should eat liberally and the foods they are better off if they avoid. Glycemic index indicates the ability of raising blood sugar level of particular food item after consumption.
How accurate is glycemic index?
There are various practical limitations of using glycemic index as an indicator for foods to eat and foods to avoid by a diabetic individual. The limitations are:
Glycemic index do not take into consideration the actual amount of carbohydrate consumed. For example, a food item with low glycemic index (55 or less is low glycemic index) can have large amount of carbohydrate in it and actually may not be good for diabetics and a food item with high glycemic index (more than 70 is high glycemic index) can have very small amount of carbohydrate in it and actually may be good for diabetics.
Glycemic index chart of various foods usually provide only single value. However, variation of glycemic index a food item is possible due to various factors such as cooking method/process, processing, ripeness of the food, storage length etc. Glycemic index do not these factors into account and hence, it may not be accurate.
There may be individual variations of glycemic index as well as variation in same individual depending on various factors such as insulin resistance, blood glucose level, gastrointestinal disturbances etc.
Glycemic index of several food items when consumed at a time (as we generally do in our meal with several/various food items), may not reflect the individual glycemic indices of several foods, but reflect a glycemic index of its own. Hence, eating a food with high glycemic index along with several other foods with low glycemic index will not have much effect on rapid rise in blood sugar level.
To overcome the faults (inaccuracy) associated glycemic index (GI), currently Glycemic Load (GL) is used for guiding diabetic individuals about their food selection.
Examples of foods with low glycemic index (55 or less):
Most vegetables, beans (soy, black, kidney, lentils etc.), fruits (e.g. peaches, mango, guava, strawberry etc.), seeds (pumpkin, flux, seasame etc.), whole grains (brown rice, wheat, millet, rye etc.)
Examples of foods with medium glycemic index (56-69):
Potato, banana, grapes, rice, whole wheat bread etc.
Examples of foods with high glycemic index (70 or more):
White rice, white bread, corn flakes, most breakfast cereals, glucose, maltose, bagels, pretzels etc.
Glycemic Index and Weight Loss
Glycemic index of a food indicates how fast the food can raise blood sugar level after consumption. If a food can raise blood sugar very fast its glycemic index is high, which indicates that a food with high glycemic index is digested and absorbed very fast and foods with low glycemic index, get digested and absorbed slowly.
How glycemic index is calculated?
For calculating the glycemic index of food the area under curve (AUC, which is the area of two-hour blood glucose response curve) for the particular food (test food) following fasting (which is usually no calorie intake for 12 hors) is calculate. The AUC for glucose is standard and known.
Glycemic index of a food is calculated by dividing the AUC of test food by AUC of standard food (most commonly glucose is used) multiplied by 100.
The most common reference or standard food used for calculating glycemic index is glucose. Advantage of using glucose as reference food is that its GI value is maximum (100), and it has universal acceptance. However, sometimes white bread is also used. Use of white bread has an advantage for the people who eat white bread as staple food, because it is possible to find if replacement of the dietary staple with a different food would result in better blood glucose control. The disadvantage is that, the reference food is not well defined.
Glycemic index of a food and weight control:
Glycemic index is and indication of how rapidly the food is digested and absorbed. Hence, a food with high glycemic index can be digested and absorbed very fast leading to weight gain. Because, the food may not be metabolized as rapidly as it is absorbed and the portion absorbed but not metabolized will be deposited as fat, leading to weight gain. It is therefore important to eat more low glycemic index foods for reducing body weight. Although it is important to have low glycemic index food, it is more important to have lower total daily calorie intake for weight loss as well as for diabetes control.