Lactic acid is a by-product produced by the glycolytic pathway and has been viewed as the main cause of fatigue during strenuous exercise. However, there are certain mechanisms which suggest that lactate can actually be used as a fuel source. The mechanisms by which lactate can be moved around the body and used as a fuel source include the Cori cycle and the lactate shuttle hypothesis.
The Cori cycle involves pyruvate and lactate within the muscle entering the bloodstream. From there it can be circulated to the liver and kidneys. Glucose is formed in the liver from the lactate and pyruvate through a process called gluconeogenesis (Garrett and Grisham, 2002). The glucose formed can now re-enter the bloodstream, thus providing energy for working muscles and delaying the onset of fatigue. When in an oxygen deficit during intense exercise, the body must release energy anaerobically. Pyruvate is converted to lactate by lactate dehydrogenase. During the lactic acid fermentation process NAD+ is generated. The NAD+ concentration levels are maintained and this allows for glycolytic metabolism to continue (Granchi et al., 2010).
The lactate shuffle hypothesis suggests that when glucose enters the cell it is converted into pyruvate. From here it can enter the mitochondria and this allows for respiration to occur in the Krebs cycle. The lactate dehydrogenase reaction allows for lactate to be formed and is distributed out of the cell via the monocarboxylate transporter (MCT). This allows for it to be circulated to functional sites where it can be used as a fuel source (Brooks et al., 1999).
It seems that with newer research of lactic acid the view has been shifted from it being a hypoxic waste product as a result of anaerobic metabolism to a fuel source that can be used by resting and contracting muscle (Ferguson et al., 2018). This information would suggest that programming high intensity training which makes anaerobic metabolism the dominant system advantageous for most athletes. This is because the lactic acid generated from this pathway can be used as a fuel source therefore prolonging the onset of fatigue and improving athletic performance.
References:
Brooks, G., Dubouchaud, H., Brown, M., Sicurello, J., & Butz, C. (1999). Role of mitochondrial lactate dehydrogenase and lactate oxidation in the intracellular lactate shuttle. Proceedings of the National Academy of Sciences of the United States of America, 96(3), 1129-1134. https://doi.org/10.1073/pnas.96.3.1129
Ferguson, B.S., Rogatzki, M.J., Goodwin, M.L., Gladden, M.L., Kane, D.A., Rightmire Z. (2018) Lactate metabolism: historical context, prior misinterpretations, and current understanding. European Journal of Applied Physiology, 188(4), 691-728. https://doi.org/10.1007/s00421-017-3795-6
Garrett, RH., Grisham, CM. (2002). Principles of Biochemistry with a Human Focus. Brooks/Cole Cengage Learning.
Granchi, C., Bertini, S., Macchia, M., Minutolo, F., (2010) Inhibitors of lactate dehydrogenase isoforms and their therapeutic potentials. Current Medicinal Chemistry, 17(7), 672-97.
DOI: 10.2174/092986710790416263
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