Editing Lactic acid (section)

===Metabolism and exercise===
{{See also|N-Lactoylphenylalanine}}
During power exercises such as [[sprint (running)|sprinting]], when the rate of demand for energy is high, [[glucose]] is broken down and oxidized to [[pyruvate]], and lactate is then produced from the pyruvate faster than the body can process it, causing lactate concentrations to rise. The production of lactate is beneficial for [[Nicotinamide adenine dinucleotide|NAD<sup>+</sup>]] regeneration (pyruvate is reduced to lactate while NADH is oxidized to NAD<sup>+</sup>), which is used up in oxidation of [[glyceraldehyde 3-phosphate]] during production of pyruvate from glucose, and this ensures that energy production is maintained and exercise can continue. During intense exercise, the respiratory chain cannot keep up with the amount of hydrogen ions that join to form NADH, and cannot regenerate NAD<sup>+</sup> quickly enough, so pyruvate is converted to lactate to allow energy production by [[glycolysis]] to continue.<ref name="Ferguson 2018">{{cite journal | last1=Ferguson | first1=Brian S. | last2=Rogatzki | first2=Matthew J. | last3=Goodwin | first3=Matthew L. | last4=Kane | first4=Daniel A. | last5=Rightmire | first5=Zachary | last6=Gladden | first6=L. Bruce | title=Lactate metabolism: historical context, prior misinterpretations, and current understanding | journal=European Journal of Applied Physiology | volume=118 | date=2018 | issue=4 | issn=1439-6319 | doi=10.1007/s00421-017-3795-6 | pages=691–728| pmid=29322250 }}</ref>

The resulting lactate can be used in two ways:
* [[Oxidation]] back to [[pyruvate]] by well-oxygenated [[muscle]] cells, heart cells, and brain cells
** Pyruvate is then directly used to fuel the [[Krebs cycle]]
* Conversion to [[glucose]] via [[gluconeogenesis]] in the liver and release back into circulation by means of the [[Cori cycle]]<ref name=mcardlekatch>{{Cite book|vauthors=McArdle WD, Katch FI, Katch VL|title=Exercise Physiology: Energy, Nutrition, and Human Performance|year=2010|publisher=Wolters Kluwer/Lippincott Williams & Wilkins Health|isbn=978-0-683-05731-7|url-access=registration|url=https://archive.org/details/exercisephysiolo00mcar_0}}</ref>
** If blood glucose concentrations are high, the glucose can be used to build up the liver's [[glycogen]] stores.

Lactate is continually formed at rest and during all exercise intensities. Lactate serves as a metabolic fuel being produced and oxidatively disposed in resting and exercising muscle and other tissues.<ref name="Ferguson 2018" /> Some sources of excess lactate production are metabolism in [[red blood cell]]s, which lack [[mitochondria]] that perform aerobic respiration, and limitations in the rates of enzyme activity in muscle fibers during intense exertion.<ref name=mcardlekatch/> [[Lactic acidosis]] is a [[physiology|physiological condition]] characterized by accumulation of lactate (especially {{sc|L}}-lactate), with formation of an excessively high proton concentration [H<sup>+</sup>] and correspondingly low [[pH]] in the tissues, a form of [[metabolic acidosis]].<ref name="Ferguson 2018" />

The first stage in metabolizing glucose is [[glycolysis]], the conversion of glucose to pyruvate<sup>−</sup> and H<sup>+</sup>:

:{{chem2|C6H12O6 + 2 NAD+ + 2 ADP(3-) + 2 HPO4(2-) -> 2 CH3COCO2- + 2 H+ + 2 NADH + 2 ATP(4-) + 2 H2O}}

When sufficient oxygen is present for aerobic respiration, the pyruvate is oxidized to {{chem2|CO2}} and water by the Krebs cycle, in which [[oxidative phosphorylation]] generates ATP for use in powering the cell.
When insufficient oxygen is present, or when there is insufficient capacity for pyruvate oxidation to keep up with rapid pyruvate production during intense exertion, the pyruvate is converted to lactate<sup>−</sup> by [[lactate dehydrogenase]]), a process that absorbs these protons:<ref name=Robergs>{{cite journal | vauthors = Robergs RA, Ghiasvand F, Parker D | title = Biochemistry of exercise-induced metabolic acidosis | journal = American Journal of Physiology. Regulatory, Integrative and Comparative Physiology | volume = 287 | issue = 3 | pages = R502–R516 | date = September 2004 | pmid = 15308499 | doi = 10.1152/ajpregu.00114.2004 | s2cid = 2745168 }}</ref>

:{{chem2|2 CH3COCO2- + 2 H+ + 2 NADH -> 2 CH3CH(OH)CO2- + 2 NAD+}}

The combined effect is:

:{{chem2|C6H12O6 + 2 ADP(3-) + 2HPO4(2-) -> 2 CH3CH(OH)CO2- + 2 ATP(4-) + 2 H2O}}

The production of lactate from glucose ({{chem2|glucose → 2 lactate- + 2 H+}}), when viewed in isolation, releases two H<sup>+</sup>. The H<sup>+</sup> are absorbed in the production of ATP, but H<sup>+</sup> is subsequently released during hydrolysis of ATP:

:{{chem2|ATP(4−) + H2O → ADP(3-) + HPO4(2-) + H+}}

Once the production and use of ATP is included, the overall reaction is

:{{chem2|C6H12O6 -> 2 CH3CH(OH)CO2- + 2 H+}}

The resulting increase in acidity persists until the excess lactate and protons are converted back to pyruvate, and then to glucose for later use, or to {{chem2|CO2}} and water for the production of ATP.<ref name="Ferguson 2018" />