Lactate and Lactic Acid
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.
1 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, the natural conclusion was that the presence of Lactate indicates insufficient oxygen and therefore anaerobic metabolism. However, more recent studies have that Lactate production is related to exercise intensity, not insufficient oxygen supply. 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).
2 Lactate and Fatigue
Initial studies have indicated that that acidity may reduce force production in muscles or reduce the rate of glucose metabolism, 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. A study of isolated rat muscles showed Lactate can protect against fatigue. However, more research into the role of Lactate and fatigue are required.
3 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". During prolonged low-moderate intensity exercise, muscles that initially released Lactate into the blood can become net importers. At higher intensities, the working muscles extract and metabolize Lactate, even while being a net Lactate producer. There is some evidence that muscles at rest will absorb and store Lactate, while at exercise the majority of absorbed Lactate is metabolized by the working muscles. Lactate is a preferred fuel source for working muscles, as extra Lactate injected will be metabolized in place of glucose. A low intensities, Lactate will be converted back to Glucose (AKA Gluconeogenesis).
4 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.
5 Lactate and Wound Healing
It's been known since the 1960 that lactate is an important part of wound healing, with collagen synthesis almost doubled when Lactate is increased to 15 mmol/l in cultures. When you're used to looking at Lactate levels during exercise this seems ridiculously high, but values of 10-15 mmol/l are commonly seen in healing wounds, and this does not appear to be due to low oxygen levels. In fact, oxygen levels seem to have little impact on lactate levels. It's possible that the beneficial effects of Lactate may be in stimulating vascular growth (angiogenesis) and/or increased oxygen supply to the wound through vasodilation.
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