Lactic Acid

Lactic acid is the product of anaerobic respiration, the burning of stored sugars without sufficient oxygen by cells. Anaerobic respiration is much less efficient than aerobic respiration, for which there is enough oxygen to fully utilize the stored sugar energy. Essentially, anaerobic respiration causes the halving of glucose molecules (C₆H₁₂O₆) into lactic acid molecules (C₃H₆O₃). The lactic acid builds up in muscles, accounting for the soreness in overworked muscles. This build-up of lactic acid may also lead to cramps. One advantage of anaerobic respiration is that it can take place very quickly and in short bursts, as opposed to aerobic respiration, which is designed for slower and more steady use of muscles. Eventually the build-up of lactic acid is carried away in the bloodstream and the lactic acid is converted to carbon dioxide (CO₂) gas and water vapor, both of which are exhaled. If lactic acid levels in the bloodstream rise faster than the body's natural pH buffers—combinations of acids, salts, and bases that maintain a constant pH level—can neutralize them, a state known as lactic acidosis may exist. Lactic acidosis rarely happens in healthy people. It is more likely the result of the body's inability to obtain sufficient oxygen (as in heart attacks or carbon monoxide or cyanide poisoning) or from other diseases such as diabetes.

The ability of the body to metabolize, or break down, lactic acid is decreased significantly by alcohol, which impairs the liver's ability to carry out normal metabolic reactions. Thus, alcoholics often have sore muscles from lactic acid build up that was not caused by exercise. Lactic acid can also lead to a build-up of uric acid crystals in the joints, since lactic acid reduces the elimination of uric acid and related compounds. This build-up can lead to gout, a very painful disease.