Final answer:
The protein-sparing mechanism during exercise shuts off primarily due to the depletion of glycogen stores. This forces the body to shift towards fat metabolism to meet the energy demands, conserving proteins and amino acids for other functions.
Step-by-step explanation:
The protein-sparing mechanism is designed to conserve muscle tissue by using other fuel sources, such as fats, during times of energy demand. During exercise, this mechanism can shut off when the body’s glycogen stores are depleted. Glycogen serves as a primary energy source at the start of exercise, undergoing physiological processes like glycolysis and the Krebs cycle to produce ATP. As exercise continues and glycogen levels fall, the body will increase fat metabolism. Eventually, muscles switch from glycolysis, which relies on glucose, to the use of fatty acids as fuel, thus conserving amino acids and proteins. Depletion of glycogen stores creates a shift that favors fat utilization to produce energy in order to spare proteins.
However, during high-intensity exercise, when oxygen delivery to the muscles is insufficient, anaerobic glycolysis takes place, leading to the production of pyruvic acid, which is converted to lactic acid. This can contribute to muscle fatigue and is unrelated to the protein-sparing mechanism. When glycogen is depleted, and energy reserves are exhausted, the body lacks the necessary substrate for continued glycolysis driving the shift to fat metabolism.