Carbohydrates are the most efficient nutritional source for the bodies energy requirements, converting to glucose much more freely than proteins and fats. Every single cell in the body utilizes glucose for fuel and most of this glucose (provided through the metabolism of carbs) is stored in skeletal muscle and the liver as glycogen.
The rest is carried around in the bloodstream to supply your brain and other organs. When you exercise and your blood sugars drop the glycogen stores serve as a reserve, replenishing blood sugar in order to meet your bodies energy needs. Carbs are also important for preventing the protein in the muscle from being converted to glucose when blood sugar and glycogen are low. During exercise your body burns considerably more energy than during your daily activity routine and its need for blood glucose increases. These demands are met by the muscles glycogen reserves, so it's important that you consume adequate amounts of carbs to support energy demands; carb demands will vary from athlete to athlete.
Most people think that a low carbs diet is the way forward (low simple carb diet certainly is), with the theory centering around the fact that if carbs are restricted the body wil resort to a process known as Ketosis. This is where the body breaks down triglycerides for use as an energy source. However, the Ketosis process is not as efficient at supplying energy or replenishing glycogen as ingesting carbs is! This will mean that the body will have to use protein in foods and muscle tissue for energy. It does this by converting certain amino acids in the protein into glucose; one of these amino acids is glutamine, which plays a huge role in immunity and not one that you want your body 'cannibalizing' for energy unnecessarily! Put simply, if you want to gain muscle you need carbohydrates...FACT!