An indication that fats are being metabolized is low lactate levels during a workout. Higher lactate levels indicate energy is probably being derived more from carbohydrates. The body must use a third source of energy, carbohydrates or more precisely glycogen, to fuel the extended high intensity exercise that is crucial for good most athletic events. Carbohydrates are used both by the aerobic system to produce energy and by the third system, glycolysis or the anaerobic lactate system Glycolysis . Glycolysis may be an unfamiliar term for many coaches, but it is extremely important for most athletic competitions.
Glycolysis means the breakdown of sugars.
When a coach, athlete or sports physiologist use the term anaerobic, they are most likely referring to this energy system. It is very fast, producing quick energy for exercise. One of the end products of this process is a metabolite called pyruvate. When conditions aren't right and pyruvate doesn't break down, it turns into lactate. If the lactate is not removed from the muscle in some way, there will eventually be a problem with muscle contraction because of acidosis and the athlete will have to slow down.