So far we have learned that the difference in lactate production and its removal from the blood results in reduced levels (that is, return toward resting values) during recovery from exercise, and that its removal is faster during controlled exercise-recovery than during rest-recovery. Our next task is to learn what happens to the lactic acid and why its removal is faster during exercise-recovery.
There are four possible fates of lactic acid.
1. Excretion in Urine and Sweat. Lactic acid is known to be excreted in urine and sweat. However, the amount of lactic acid removed in this manner during recovery from exercise is negligible.
2. Conversion to Glucose and/or Glycogen. Because lactic acid is a product of carbohydrate metabolism (glucose and glycogen)during anaerobic work, it can be reconverted to either of these compounds in the liver (glycogen and glucose) and in muscle (glycogen) given the required ATP energy. However, as previously mentioned, glycogen resynthesis in muscle and liver is extremely slow compared with lactic acid removal. In addition, the magnitude of the changes in the blood glucose levels during recovery are also minimal. Therefore, conversion of lactic acid to glucose and glycogen accounts for only a small portion of the total lactic acid removal.
3. Conversion to Protein, Carbohydrates, including lactic acid, can be chemically converted into protein within the body. However, once again only a relatively small amount of lactic acid has been shown to be converted to protein during the immediate recovery period following exercise.
4. Oxidation / Conversion to Co2 and H2o. Lactic acid can be used as a metabolic fuel, mostly by skeletal muscle, but heart muscle, brain, liver and kidney tissues are also capable of this function. IN the presence of oxygen, lactic acid is first converted to pyretic acid and then to Co2 and H2o in the Krebs Cycle and the electron transport system, respectively. Of course , ATP is resynthesized in coupled reactions in the electron transport system.
The use of lactic acid as a metabolic fuel for the aerobic system accounts for the majority of the lactic acid removed during recovery from exercise. Although this holds true for both rest- and exercise-recoveries oxidation accounts for more lactic acid removal in the latter than in the former. AS just mentioned, several organs are known to be capable of oxidizing lactic acid. However, it is fairly well agreed that skeletal muscle is the major organ involved in this process. In fact, most of all lactic acid oxidized by muscle is thought to occur within slow-twitch rather than fast twitch fibers. These are major reasons why lactic acid removal is faster during exercise-recovery than during rest-recovery. For example, the former, both the blood flow carrying lactic acid to the muscles and the metabolic rate of the active muscles are greatly increased. In addition, the type of exercise selected during most exercise-recoveries prudentially recruits slow-twitch fibers to perform the work.