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Thompson, Dixie L. Ph.D., FACSM
Dixie L. Thompson, Ph.D., FACSM, is the director of the Center for Physical Activity and Health and a professor in the Department of Exercise, Sport, and Leisure Studies at the University of Tennessee, Knoxville.
No metabolic by-product of exercise has been more intensely studied than lactic acid (LA). Even with all this attention, scientists continue to argue about the role it plays in the body. Some have labeled LA as a critical link to fatigue, whereas others tout its role as a source of energy. Can they all be correct?
WHAT IS LA?
Lactic acid is produced in muscle as a result of the breakdown of carbohydrates (glucose and glycogen). To understand LA, let's review some basic information about energy production in the body. Adenosine triphosphate (ATP), a molecule that serves as the body's source of energy, is required for muscle contraction. Fats, carbohydrates, and proteins are broken down by the body to yield ATP.
Although LA is constantly produced in the body, it also is being metabolized, so levels remain steady and low at rest. During exercise, ATP must be produced at a higher rate than at rest. Because carbohydrates can be broken down faster than either fats or proteins, carbohydrates are essential sources of ATP during exercise, especially high-intensity exercise. As carbohydrates are broken down, LA is formed, particularly during vigorous exercise.
LA AS AN ENERGY SOURCE
Scientists once thought that LA produced during exercise was essentially useless until after exercise stopped, when it could be reconverted back into carbohydrates by the liver. It is now known that LA is an important fuel for heart muscle and slow-twitch muscle fibers. These tissues can use LA as a fuel (much like glucose) to form ATP. This process requires oxygen, and heart and slow-twitch fibers are quite effective in using oxygen in ATP production.
High-intensity exercise requires a rapid supply of ATP. Fast-twitch muscle fibers are activated during high-intensity movements because these fibers are capable of producing a lot of ATP quickly by breaking down carbohydrates. During vigorous exercise, fast-twitch fibers produce a great deal of LA. Fortunately, slow-twitch fibers and the heart use some of it for energy.
LA'S LINK TO FATIGUE
Because it is an acid, LA releases hydrogen ions (H+). When LA levels rise, the H+ concentrations in muscle fibers and blood also increase. The presence of H+ has been linked with several potential sources of fatigue. Hydrogen ions may interfere with the muscles' ability to contract, and H+ can potentially affect enzymes crucial in ATP production. Although specific mechanisms through which LA is linked with fatigue remain controversial, clearly, individuals cannot sustain vigorous exercise for extended periods when LA levels are greatly increased.
Lactic acid is a naturally formed product of the breakdown of carbohydrates. Although high levels of LA accumulation are generally linked with fatigue, this acid also provides energy for highly oxidative cells. Scientists continue to investigate the complex role that LA plays in exercise.
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