by What is Lactic Acid and its Role in the Body
Formation of Lactic Acid
Lactic acid is primarily formed within human muscle cells and tissues during periods of intense exercise and physical exertion. During exercise, muscles contract continuously which requires a rapid supply of energy in the form of adenosine triphosphate (ATP). ATP is primarily generated by breaking down glucose and glycogen within the muscle cells. However, under conditions of high-intensity exercise, the muscle cells shift to an anaerobic metabolic pathway which produces lactic acid as a byproduct instead of carbon dioxide and water. This is because the muscle cells do not have enough oxygen available to sustain aerobic respiration to meet the energy demands.
The anaerobic pathway involves the rapid breakdown of glucose into pyruvic acid through a series of steps called glycolysis. In the absence of sufficient oxygen, the pyruvic acid is then reduced to lactic acid. This builds up lactic acid levels within the muscle cells and tissues, leading to the burning sensation commonly felt during intense exercise. The built-up lactic acid also diffuses out into the bloodstream leading to elevated blood lactic acid levels.
Role of Lactate Dehydrogenase Enzyme
The conversion of pyruvic acid to lactic acid is catalyzed by the enzyme lactate dehydrogenase or LDH. There are 5 isoenzymes of LDH found in human tissues with differing cellular locations and roles. LDH converts pyruvic acid to lactic acid during intense exercise to maintain ATP production through glycolysis when oxygen levels are low. Conversely, it also converts lactic acid back to pyruvic acid when oxygen levels increase after exercise, helping to clear lactic acid built up during exertion. The lactate/pyruvate redox reaction is crucial in maintaining cellular energy production through glycolysis under varying oxygen conditions.
Reasons for Lactic Acid Build Up During Exercise
There are two primary reasons behind the build-up of lactic acid within exercising muscles and tissues:
1. Oxygen Debt: During intense exercise like sprinting or weightlifting, oxygen demand outstrips oxygen delivery and supply to the muscles. The cardiovascular and respiratory systems cannot provide enough oxygen to support full aerobic respiration. This oxygen deficit or debt results in a shift to anaerobic glycolysis and lactic acid production.
2. Fast Twitch Muscle Fibers: Skeletal muscle contains two main types of muscle fibers – slow twitch and fast twitch fibers. Fast twitch fibers contract rapidly but rely more heavily on anaerobic glycolysis during high-intensity activities. They produce more lactic acid as a result. Sports involving speed or power engage more fast twitch fibers leading to greater lactate accumulation.
Physiological Effects of Lactic Acid
The build up of lactic acid in the body during intense exercise produces both positive and negative physiological effects:
Positive Effects:
– Acts as a buffer to maintain pH levels during exercise when H+ ions accumulate
– Fuels gluconeogenesis in the liver to resynthesize glucose from lactate after exercise
– Transports hydrogen ions away from muscles into circulation, delaying fatigue
Negative Effects:
– Causes burning sensation in muscles due to H+ ion accumulation
– Leads to metabolic acidosis if lactate levels become too high
– Linked to central fatigue in the brain if blood lactate rises substantially
– May directly inhibit muscle contraction at very high concentrations
The key is balancing lactic acid production during activity with clearance via aerobic pathways after exercise ends to prevent deleterious impacts. Training helps improve the lactate threshold.
Role in Athletic Performance
Lactic acid is an important metabolic marker closely linked to exercise performance and conditioning level across many sports. Higher lactate threshold and tolerance enables athletes to:
– Engage in higher-intensity exercise for longer periods before fatiguing
– Recover more rapidly after intense bursts of activity
– Maintain power and speed over multiple bouts or rounds of high exertion
Sports like running, cycling, and swimming rely heavily on a high lactate threshold to sustain near-maximal effort for an extended time. Resistance and interval training have both been shown to push the lactate threshold higher increasing work capacity. Lactate testing during training enables coaches to track adaptive changes and periodize workouts accordingly. Understanding one’s lactate profile is invaluable for optimizing performance.
In summary, lactic acid is a normal byproduct of anaerobic glycolysis during intense exercise when oxygen delivery lags behind energy demands. While it causes fatigue directly at very high levels, it also performs important roles in energy production and pH regulation in exercise and recovery. Conditioning adaptations improve lactate clearance enhancing muscular endurance and performance across a wide range of athletic endeavors.
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