Changes

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"/>.

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"/>.