Lactic Acid - Exercise and Lactate

Exercise and Lactate

During power exercises such as sprinting, when the rate of demand for energy is high, glucose is broken down and oxidized to pyruvate, and lactate is produced from the pyruvate faster than the tissues can remove it, so lactate concentration begins to rise. The production of lactate is a beneficial process because it regenerates NAD+ which is used up in the creation of pyruvate from glucose, and this ensures that energy production is maintained and exercise can continue. The increased lactate produced can be removed in two ways:

• Oxidation back to pyruvate by well-oxygenated muscle cells
  • Pyruvate is then directly used to fuel the Krebs cycle.
• Conversion to glucose via gluconeogenesis in the liver and release back into circulation; see Cori cycle.
  • If not released, the glucose can be used to build up the liver's glycogen stores if they are empty.

Strenuous anaerobic exercise causes a lowering of pH and pain, called acidosis.

The effect of lactate production on acidosis has been the topic of many recent conferences in the field of exercise physiology. Robergs et al. have discussed the creation of H+ ions that occurs during glycolysis. and claim that the idea that acidosis is caused by the production of lactic acid is a myth (a "construct"), pointing out that part of the lowering of pH is due to the reaction ATP-4+H₂O=ADP-3+HPO₄-2+H+, and that reducing pyruvate to lactate (pyruvate+NADH+H+=lactate+NAD+) actually consumes H+. However, a response by Lindinger et al. has been written claiming that Robergs et alii ignored the causative factors of the increase in concentration of hydrogen ions (denoted ). Specifically, lactate is an anion, and its production causes a reduction in the amount of cations such as Na+ minus anions, and thus causes an increase in to maintain electroneutrality. Increasing partial pressure of CO₂, PCO₂, also causes an increase in . During exercise, the intramuscular lactate concentration and PCO₂ increase, causing an increase in, and, thus, a decrease in pH. (See Le Chatelier's principle)

During intense exercise, the respiratory chain cannot keep up with the amount of hydrogen atoms that join to form NADH. NAD+ is required to oxidize 3-phosphoglyceraldehyde in order to maintain the production of anaerobic energy during glycolysis. During anaerobic glycolysis, NAD+ is “freed up” when NADH combines with pyruvate to form lactate (as mentioned above). If this did not occur, glycolysis would come to a stop. However, lactate is continually formed even at rest and during moderate exercise. This occurs due to the metabolism of red blood cells that do not have mitochondria and limitations resulting from the enzyme activity that occurs in muscle fibers having a high glycolytic capacity.