I came across this article in the LA Times which explain in a clear way how carbohydrates intake work and how carbs are converted to fat storage.

A growing number of top nutritional scientists blame excessive carbohydrates — not fat — for America’s ills. They say cutting carbohydrates is the key to reversing obesity, heart disease, Type 2 diabetes and hypertension.

All carbohydrates (a category including sugars) convert to sugar in the blood, and the more refined the carbs are, the quicker the conversion goes. When you eat a glazed doughnut or a serving of mashed potatoes, it turns into blood sugar very quickly. To manage the blood sugar, the pancreas produces insulin, which moves sugar into cells, where it’s stored as fuel in the form of glycogen.

(note: This is what usually happens in the body. You eat a carb-ful meal and your glycogen stores are empty. So first, your body fills up your glycogen to the max, then it eventually tips over into your fat stores. Any glucose in excess of the needs for energy and storage as glycogen is converted to fat. Fat is stored predominantly as adipose tissue throughout the body and is an energy reservoir. )

If you have a perfectly healthy metabolism, the system works beautifully. However, over time, as our bodies get tired of processing high loads of carbs, which evolution didn’t prepare us for … how the body responds to insulin can change. When cells become more resistant to those insulin instructions, the pancreas needs to make more insulin to push the same amount of glucose into cells. As people become insulin resistant, carbs become a bigger challenge for the body. When the pancreas gets exhausted and can’t produce enough insulin to keep up with the glucose in the blood, diabetes develops.

# source LA Times