Is the main source for energy for your body carbohydrates?

The body's FIRST source of energy will be carbohydrates, if they are readily available - if you have eaten them within the hours prior to physical activity (PA). If they are eaten in absence of PA, they will be broken down, converted to, and stored as glycogen in the blood, liver and muscles until PA takes place. However, there is a capacity to each of these, and if you eat more than capacity, the remainder will be stored as fat. Once PA has begun, the body will first use muscle glycogen, then tap into blood, then last liver.

On the other hand, in the complete absence of carbohydrates over an extended period of time, the body, lacking any glycogen, will be forced to burn fatty acids to produce energy. This process is slow, or better stated more difficult, and cannot be done as quickly (by quickly, to fuel for high intensity exercises). You will simply not have the energy to move fast. But slower movement should be ok.

Last, in the absence of both, the body will resort to it's lean tissue, specifically muscle, for fuel.