Carbs are readily available fuel for the body and fat is concentrated, but not so readily available, fuel for the body. Your body has no problem manufacturing fat from the extra carbs.

The other factor with carbs is the glycemic index. See here:

http://www.health.ha...for_100_foods.htm

Eating low glycemic index food does not stress you body's ability to deal a surge of sugars in the blood.

Of course, any diet always requires protein and minerals so that the body can function and repair itself.