This can happen in just a few short weeks of higher heart rate training. The lactic acid produced during anaerobic training may inhibit the aerobic muscle enzymes necessary for building an aerobic base. Anaerobic training raises your respiratory quotient. This means the percentage of energy derived from sugar increases and fat burning decreases. In time, this may force more anaerobic metabolism and less aerobic function.
Stress can also inhibit the aerobic system.
Stress is nearly synonymous with anaerobic training. Excess stress raises cortisol levels, which ultimately increases insulin levels, inhibiting fat burning and increasing sugar usage. This promotes anaerobic metabolism and inhibits aerobic activity. Aerobic base training is often the period of training where discipline, dedication, and hard work are most important. Most athletes think these three mental attributes are related to toughing it out, grunting, and training in pain.