The glycemic index decides the position of carb containing foods based on their ability to affect the blood glucose levels.
Simple carbs, which are metabolized quickly, increase the blood sugar levels equally fast and have a high GI. The low GI foods contain complex carbs, which are slowly metabolized, and the glucose is released slowly in the blood stream. Low GI foods are considered better for health in most cases.
How glycemic index is decided:
Subjects are given enough quantity of test food that contains 50 gms of carb. On a different day, the subjects are given control food (generally glucose or white bread) that contains same amount of carb. Blood samples are tested for blood glucose levels, before eating and at regular intervals after eating the food samples. Blood sugar levels are plotted on graph.
The glycemic index is found by AUC or area under the curve of two-hour blood glucose levels after consuming the carb food to be tested. The AUC of the test food is divided by the AUC of standard carb and multiplied by 100.
Foods, which increase blood sugar levels quickly, have a high glycemic index compared to foods, which have lesser effect on blood glucose. (However, it does not mean that low glycemic index diet foodis always better or healthier than high GI foods)
Many factors can affect the glycemic reaction of the foods: Amount of fat and protein present in the food, whether the starch content is amylose or amylopectin type of starch, presence of organic acids and their salts in the food, presence of trapped starch molecules etc. If fat or soluble dietary fiber is present, it reduces the GI, because these components decrease the gastric emptying rate. When vinegar is added to a product, it reduces the GI. The treatment of carb food can alter its glycemic effect. Brown breads with high fiber have low GI, but if the brown bread is treated with enzymes (to keep the crust soft), then the starch is easily and quickly metabolized changing its GI to a higher one.
GI values can be correctly calculated if food contains some reasonable quantity of carb. , because the subject needs to consume the amount of test food that would have about 50 gms of available carb. Some vegetables like carrot have very little carb per serving and so has very low GI
Sometimes diabetes patients are advised to select carbs based foods using the glycemic index. The glycemic index diet can help in reducing glucose levels in blood. So it is used as meal planning tool. But some times it might pose difficulties because GI of a food can vary with many factors, like method of preparation, the type of starch, fat and proteins contents of that food, organic acids present in that food etc. then again, GI values for some foods are not known. So these factors cause difficulties in using GI as meal planning tool.
Food products with high GI: White rice, white bread, baked potatoes, candy, corn flakes
Food products with low GI: Pasta, legumes, milk, most fruits and vegetables.
Food products with medium GI: Brown rice, whole-wheat products etc.
The risk of diseases like diabetes type 2 and heart diseases are found to be in a low percentage in people following low glycemic index diet. Combination of a high and a low GI products in a diet results in a moderate GI. So although in regions like Asia high GI foods like rice and potatoes are a staple diet, still the level of obesity and diabetes is not very high. This could be because of the inclusion of fresh fruits, vegetables and legumes in the same diet, which reduces total glycemic effect. Studies have shown that consumption of high GI products for long periods of time can result in increased risk of diabetes, obesity and heart problems.
Advantages of using GI diet system:
- Lessens obesity, diabetes and heart disease risk
- Better metabolism and greater energy level
- Weight reduction
- Monitors blood sugar and insulin
- Reduce appetite and over eating.
Disadvantages of GI system:
In GI, only levels of glucose in blood are considered, while presence of other types of sugars and their effects are not taken into account. Glycemic response can change from person to person and it may vary according to variety or the treatment of the carb food.