The primary function of carbohydrates is to provide fuel to the body; every gram of carbohydrates has 4 calories. Carbohydrates are essential to keep the body from using protein for energy, and complex carbohydrates such as whole grains help to sustain energy over a longer period of time, as they are harder to break down.
Carbohydrates provide glucose to the body, stabilizing the body's glucose levels. However, when the body does not have enough carbohydrates, it breaks down protein for that purpose, meaning that protein cannot be used to build muscle.
Carbohydrates give energy to muscles and the central nervous system. They also provide fiber to the body, helping to regulate digestion.