Lactic acid

Lactic acid is a biologically important acid and a by-product of anaerobic metabolism. The conjugate base of lactic acid is called lactate. Most of the time, the term "lactic acid" refers to L-lactic acid, the form of lactic acid produced in animals. Some bacteria produce a different form, called D-lactic acid. There is evidence that lactic acid in animals may be an important fuel for myocardial energy metabolism.

ME/CFS
Elevated blood lactate has been found in both resting ME/CFS patients and following exertion. ME/CFS patients with higher elevated lactate at rest may have more severe post-exertional malaise after activity.

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https://www.khanacademy.org/test-prep/mcat/physical-sciences-practice/physical-sciences-practice-tut/e/enzymes---passage-2