I apologize in advance for the technicality of this post.
Contrary to popular belief, lactic acid does not cause muscle pain. Ingestion of lactic acid in reasonable amounts causes no problems (it's found in fermented dairy and malt products) and experiments perfusing it into muscle caused no discomfort. Recently, it's been common to say that it's not the lactate but the hydrogen ions - the acidity - that causes pain; this is fallacious in two ways: 1) in cells, the compound found is not lactic acid but lactate ion and 2) the reaction producing lactate is NADH + H + pyruvate = NAD + lactate [NAD is nicotinamide adenine dinucleotide], which actually reduces the hydrogen ion concentration. Muscle pain caused by exertion not due to cellular damage is caused by membrane polarization by ATP-dependent translocases electrically signalling adjacent neurons.
When a contracting muscle does not have adequate oxygen, rather than converting sugars to carbon dioxide and water, the cells convert them to lactate, a much less efficient procedure. This lactate is either reconverted back to pyruvate when oxygen is available, or it is exported to the liver, where it gets reconverted. There are reports (see Noakes' book) that lactate may be shuttled to less active muscles, but this, if it happens, is a minor method of dealing with lactate, as the muscle isozyme has a low affinity for lactate; the exception is heart muscle, which prefers using ketone bodies to sugars for energy.
One of the major ways for muscles to deal with lactic acid is to avoid its overproduction by instead transaminating pyruvate. Pyruvate plus the amino acid glutamate form alpha-ketoglutarate and alanine. Alanine is then sent to the liver, which converts it back to pyruvate. The muscle turns alpha-ketoglutarate back to glutamate by using other transaminases and amino acids (particularly, aspartate). Ingestion of amino acids during exercise does not affect this situation, as they are not taken up by active muscle cells (branched chain amino acids, a common supplement, are taken up only in the presence of insulin, which is absent during vigorous exercise). Transaminases use vitamin B6, but taking B6 in excess is not useful.
Does the use of amino acids to offset lactate production lead to muscle tissue damage? Apparently not. The protein turn-over rate, especially for enzymes, is fast enought to supply the glutamate needed without degrading structural proteins (interestingly, lactate dehydrogenase has an unusually slow turn-over). Yet, muscle damage does occur during exercise, apparently as a result of stress to muscle fiber proteins like myosin and actin and this is the source of soreness felt in the following days. Contraction of muscle fibers requires calcium; a study of runners in a 1000 mile race showed increased serum calcium, regardless of ingestion, presumably from muscle damage.
Take home messge
Anaerobic metabolism is inefficient, but essential when energy requirements outstrip oxygen availability. Some of the burden of energy production is shifted to the liver and heart. There is nothing one can ingest to influence the way lactate is produced or removed.
Next time: Practical lactate tolerance training
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