The Glycemic Index (GI) is a numerical Index that ranks carbohydrates based on their rate of glycemic response (i.e. how long it takes for a particular food convert in to glucose in our body). Glycemic Index uses a scale of 0 to 100, with higher values given to foods that cause the most rapid rise in blood sugar. Pure glucose serves as a reference point, and is given a Glycemic Index (GI) of 100.
Your body performs best when your blood sugar is kept relatively constant. The less flucatuations occurs in your blood sugar, the better off you are. If your blood sugar drops too low, you become lethargic and/or experience increased hunger. These leads to eating more food. And if your blood sugar goes too high, your brain signals your pancreas to secrete more insulin. Insulin brings your blood sugar back down, but primarily by converting the excess sugar to stored fat. These leads to fat storage. Also, the greater the rate of increase in your blood sugar, the more chance that your body will release an excess amount of insulin, and drive your blood sugar back down too low.
Therefore, when you eat foods that cause a large and rapid glycemic response, you may feel an initial elevation in energy and mood as your blood sugar rises, but this is followed by a cycle of increased fat storage, lethargy, and more hunger!
The theory behind the Glycemic Index is simply to minimize insulin-related problems by identifying and avoiding foods that have the greatest effect on your blood sugar.
If you eat food that have low Glycemic Index (low GI number), there will be less rapid fluctuations in the blood glucose level leading to a stage where you will not feel hungry soon and insulin secretion will also be minimal. These, potentially, result in eating smaller amount and ultimately weight loss.
A word of caution: Just don’t go by GI value. Also look at the calories taken by eating a particular portion of food. e.g. a peanut have lower GI than an apple but it has a lot more calories compared to apple. So eating more peanuts may not be healthy.