In people with a well – balanced diet, protein breakdown accounts for 2 – 5% of total energy required by the body at rest. As exercise continues, glycogen stores are depleted. During endurance exercise, prolonged training session, or training for 7 days a week, when glycogen reserves are diminished, the body breaks down protein to sustain glucose output of the liver.
In endurance sports, such as marathon or ultra-marathon, when the aerobic system is exhausted to provide energy, the body would use protein breakdown to generate energy. Under these circumstances, the body would consume the protein in the muscles. The protein in the neural and connective tissues remains relatively “untouched” and will not be recruited for energy production.
In regular exercise and sports competitions, protein does not play a role in generating energy (see the “energy map”).