It's actually a bit more complicated than that. Any carbohydrates not immediately used by the body will be stored in the form of glycogen (a long string of glucose molecules linked together).
The body has two storage sites for glycogen: the liver and the muscles. The glycogen stored in the muscles is inaccessible to the brain. Only the glycogen stored in the liver can be broken down and sent back to the bloodstream so as to maintain adequate blood sugar levels for proper brain function.
The liver's capacity to store carbohydrates in the form of glycogen is very limited and can be easily depleted within ten to twelve hours. So the liver's glycogen reserves must be maintained on a continual basis. That's why we eat carbohydrates.
The question no one has bothered to ask until now is this: what happens when you eat too much carbohydrate? Here's the answer: whether it's being stored in the liver or the muscles, the total storage capacity of the body for carbohydrate is really quite limited.
If you're an average person, you can store about three hundred to four hundred grams of carbohydrate in your muscles. In the liver, where carbohydrates are accessible for glucose conversion, you can store only about sixty to ninety grams.
This is equivalent to about two cups of cooked pasta or three typical candy bars, and it represents your total reserve capacity to keep the brain working properly.
Once the glycogen levels are filled in both the liver and the muscles, excess carbohydrates have just one fate: to be converted into fat and stored in the adipose, that is, fatty, tissue.
In a nutshell, even though carbohydrates themselves are fat-free, excess carbohydrates ends up as excess fat. That's not the worst of it. Any meal or snack high in carbohydrates will generate a rapid rise in blood glucose. To adjust for this rapid rise, the pancreas secretes the hormone insulin into the bloodstream. Insulin then lowers the levels of blood glucose.
The problem is that insulin is essentially a storage hormone, evolved to put aside excess carbohydrate calories in the form of fat in case of future famine. So the insulin that's stimulated by excess carbohydrates aggressively promotes the accumulation of body fat.
In other words, when we eat too much carbohydrate, we're essentially sending a hormonal message, via insulin, to the body. The message: "Store fat."
Hold on; it gets even worse. Not only do increased insulin levels tell the body to store carbohydrates as fat, they also tell it not to release any stored fat. This makes it impossible for you to use your own stored body fat for energy.
So the excess carbohydrates in your diet not only make you fat, they make sure you stay fat. It's a double whammy, and it can be lethal.