Originally Posted by GloucesterChief
It is actually a storage of excess sugar. Your body burns fat (slowly) and sugar. It will store excess carbs since it is designed to survive in a feast or famine environment. Your body can turn fat into sugar through glucongensis but it is not very efficient as only about 10% of the fat you consume is turned to sugar. That is why low carb diets make you lose weight. It is also why most low carb Type 2 diabetics are unconcerned with the amount of fat they eat since they need it to maintain healthy blood sugar levels.
Not eating fat or carbs leads to rabbit starvation which is really bad.
It's an excess storage of calories, however they come. Fat is the usual culprit here, as it extremely easy for your body to store fat; it doesn't have to do any converting.
De novo lipogenesis (Carbohydrates to fat) occurs when your glycogen stores are full. It's not a very common occurrence, in reality. It can happen when you over-eat on carbs for days on end and your glycogen stores are full, or when you have a large consumption of glucose (which is gross, and therefore, not likely). The way that excess carbohydrates will cause you to gain weight is by blunting out fatty acid oxidation, much in the same way that calories from alcohol won't make you fat (the body has no way to store these calories and they are prioritized for energy -- however, they, like excess carbs blunt the effects of oxidation of other energy sources).
Low carb diets make you lose weight due to the fact that carbs are hydrophilic. The first few days of a low carb diet will cause you to shed a lot of water weight, and therefore the scale number goes down, but studies show that after this effect takes place, low carbohydrate diets show no statistical advantage over other diets with the same calorie restrictions.