Food provides the building blocks for recovery and adaptations. Adequate nutrition speeds up the recovery process and maximizes the extent your body can adapt to become stronger, faster and more powerful.
What happens when you train?
During training you put stress on your body so that by the end of the session you are in a worse position than when you started. In the next few hours, your body is in overdrive trying to repair damaged cells. Once you are back to normal functioning, your body then goes through physiological changes to adapt to the training stimulus so that next time, you are able to handle things better.
During training your body has also metabolized carbohydrates, fats and proteins to produce energy. In high intensity training you will use more carbohydrates. During lower intensity training you may use a mix of carbs and fats. After or during your session you may feel fatigued if your body has used all the carbohydrates available.
Not enough fluids may also leave you feeling fatigued. Fluid is used to regulate your body temperature. When you over-heat you don't function as efficiently. Fluid is also important for blood flow. Blood carries oxygen and nutrients to your working muscles. When you are dehydrated your blood is thicker and doesn't travel as easily to your working muscles which means your heart has to work harder and you tire faster.