The Glycemic Index (GI) is a relative ranking of carbohydrate in foods according to how they affect blood glucose levels. Carbohydrates with a low GI value (55 or less) are more slowly digested, absorbed and metabolised and cause a lower and slower rise in blood glucose and, therefore usually, insulin levels.
-High GI carbs cause glucose levels to spike and then crash, this usually can be explained when you eat something sugary and after you are hungry again and tired, because your glucose levels dropped down, is like a vicious cicle. This are caused by the sugary foods, including candy, cookies, cakes, and sweet drinks, pure fruiy juice etc.
-Low GI carbs are digested and released slowly giving you a sustained energy. these are usually veggies,whole grains and cereals, and fruits with low sugar high fuber content.
So when people say carbs "make you fat" (even though there is no such a thing) they are referring to the insulin release, Insulin plays a role in regulating blood sugar levels and converting food energy into fat.
Eating more calories than the body needs will lead to excess glucose levels. If the cells do not remove glucose from the blood this means if you dont use this energy right away, the body will store it in the tissues as fat, as energy reserves.
Normally this spike of glucose can cause this, so we need to focus in medium or low glycemic index carbohydrates, or you can modify the GI by combining high GI foods with fiber, protein and or fat. An example can be an orange juice with scrambled eggs or a portion of nuts or almonds, this will slow the glucose release and thus lower the GI index avoiding the spike of your blood sugar levels.
And of course there is ways when you without noticing are increasing its GI of a meal, for example when you eat a whithe bread toast with jam.. wich is not one of my recomendations.
Also the Glycemic Index Can Change by for example:
-The Preparation. Fat, fiber, and acid (such as lemon juice or vinegar) lower the glycemic index. The longer you cook starches like pasta, the higher their glycemic index will be.
-The Ripeness. The glycemic index of fruits like bananas goes up as they ripen.
-Or as i said before Other foods eaten at the same time. Bring down the overall glycemic index of a meal by combining a high-glycemic index food with foods that have lower ones.
Your age, how active you are, and how fast you digest food also affect how your body reacts to carbs.