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Lactate, also known as lactic acid, has a bad reputation. It's commonly viewed as a waste product that causes fatigue, burning muscles, and [[Delayed Onset Muscle Soreness]] (DOMS). In reality, Lactate is an intermediary in the metabolism of carbohydrates, and is a fuel source that is preferred by working muscles over glucose. A crude analogy would be a steam train, where the coal (carbohydrate) is burnt to form steam (Lactate), and the steam is used to turn the wheels (muscles). This is a flawed analogy, as the creation of lactate from glucose provides direct energy for the muscles, and of course muscles have other fuel sources such as fat, but hopefully you get to the gist.
=Lactate Production=
At one time it was believed that Lactate was a waste product that indicated insufficient oxygen being supplied to the muscles, but this view has changed. Because Lactate is produced by metabolism in the absence of oxygen<ref name="HillLupton1923"/>, the natural conclusion was that the presence of Lactate indicates insufficient oxygen and therefore anaerobic metabolism<ref name="Wasserman-1984"/><ref name="Mizock-1992"/>. However, more recent studies have that Lactate production is related to exercise intensity, not insufficient oxygen supply<ref name="Connett-1986"/><ref name="Richardson-1998"/><ref name="Brooks-1985"/>. Lactate levels appear to depend on many factors, including the metabolism of glucose (to pyruvate), Lactate removal, fast twitch fiber recruitment, and energy demand (ADP/ATP ratio, which is in turn dependent on oxygen levels)<ref name="Gladden-2004"/>.
=Lactate and Fatigue=
Initial studies have indicated that that acidity may reduce force production in muscles<ref name="Hermansen-1981"/> or reduce the rate of glucose metabolism<ref name="Sahlin-1992"/>, leading to fatigue, and of course of Lactate is Lactic Acid in the blood. However, more recent studies have shown that the effects of acidity are not seen in more realistic situations<ref name="Westerblad-2002"/><ref name="Bangsbo-1996"/>. A study of isolated rat muscles showed Lactate can protect against fatigue<ref name="Nielsen-2001"/>. However, more research into the role of Lactate and fatigue are required<ref name="Gladden-2004"/>.
=Lactate as a Fuel=
The view of Lactate as a waste product has changed over time, and Lactate has been shown to be a mechanism for distributing carbohydrate (AKA the "Lactate Shuttle"<ref name="Brooks-2000"/>. During prolonged low-moderate intensity exercise, muscles that initially released Lactate into the blood can become net importers<ref name="Stainsby-1966"/><ref name="Gladden-1991"/>. At higher intensities, the working muscles extract and metabolize Lactate, even while being a net Lactate producer<ref name="Stanley-1986"/>. There is some evidence that muscles at rest will absorb and store Lactate<ref name="Kelley-2002"/>, while at exercise the majority of absorbed Lactate is metabolized by the working muscles<ref name="Kelley-2002"/><ref name="Mazzeo-1986"/>. Lactate is a preferred fuel source for working muscles, as extra Lactate injected will be metabolized in place of glucose<ref name="Miller-2002"/>. A low intensities, Lactate will be converted back to Glucose (AKA Gluconeogenesis)<ref name="Roef-2003"/>. When performing [[Interval Training]], lactate clearance during the recovery period depends on the exercise intensity, so complete rest will not metabolize the lactate as effectively as active recovery<ref name="Menzies-2010"/>.
=Lactate Threshold=
''Main article: [[Lactate Threshold]]''
In the spite of all of the myths around Lactate and lactic acid, the concept of [[Lactate Threshold]] is both valid and important. [[Lactate Threshold]] is a good predictor of athletic performance, especially in runners.
=Lactate and Wound Healing=
<ref name="Mizock-1992">BA. Mizock, JL. Falk, Lactic acidosis in critical illness., Crit Care Med, volume 20, issue 1, pages 80-93, Jan 1992, PMID [http://www.ncbi.nlm.nih.gov/pubmed/1309494 1309494]</ref>
<ref name="Connett-1986">RJ. Connett, TE. Gayeski, CR. Honig, Lactate efflux is unrelated to intracellular PO2 in a working red muscle in situ., J Appl Physiol (1985), volume 61, issue 2, pages 402-8, Aug 1986, PMID [http://www.ncbi.nlm.nih.gov/pubmed/3745033 3745033]</ref>
<ref name="Menzies-2010">P. Menzies, C. Menzies, L. McIntyre, P. Paterson, J. Wilson, OJ. Kemi, Blood lactate clearance during active recovery after an intense running bout depends on the intensity of the active recovery., J Sports Sci, volume 28, issue 9, pages 975-82, Jul 2010, doi [http://dx.doi.org/10.1080/02640414.2010.481721 10.1080/02640414.2010.481721], PMID [http://www.ncbi.nlm.nih.gov/pubmed/20544484 20544484]</ref>
<ref name="Brooks-1985">GA. Brooks, Anaerobic threshold: review of the concept and directions for future research., Med Sci Sports Exerc, volume 17, issue 1, pages 22-34, Feb 1985, PMID [http://www.ncbi.nlm.nih.gov/pubmed/3884959 3884959]</ref>
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