For years exercise scientists and physiologists have preached that lactate/lactic acid build-up in the muscle is the direct cause of muscle fatigue and a decrease in performance. It is proposed that lactate builds up in the muscle cells during intense exercise and literally “poisons” them, essentially shutting down activity biochemically by reducing the pH or increasing the acidity level. Lactate has therefore been dubbed a metabolic by-product or dead end and as the enemy of human exercise and performance. This may be far from the truth as this article explains.
Unfortunately, the information that gave lactate this unearned reputation is outdated and very questionable since the research on which this theory is based was isolated frog muscle experiments from 1910-1914. The experiments were performed and the lactate theory proposed by A.V. Hill and associates. Basically, these investigators took excised frog muscle and continuously stimulated it with electric shocks until failure and then took lactate samples for analysis. Lactate levels were extraordinarily high, and from this finding the scientists came to the conclusion that lactate buildup must have been the culprit for the fatigue and ultimate failure of the muscle. Hill also concluded that since there was no blood and therefore oxygen supply to the muscle, that this condition must have been the cause of the accumulation of lactate.
These investigations however were highly flawed in their design, since the muscle had been removed from both its nervous and its circulatory (arteries and veins) systems. Since the electrical stimulus was applied at a fixed voltage from an external source, one cannot correlate this with the situation of a live muscle in an animal with a nervous system and brain that regulates nervous and motor input to the muscles. One must account for the possibility of central or nervous fatigue limiting activity. With the absence of a circulatory system, how was the generated lactate supposed to be transported away from the working muscle via the veins? Also, the regulation of metabolism, which is highly dependent on hormonal control, was eliminated with the loss of blood circulation. Clearly, there were big problems with these early experiments, but amazingly the theory in all its weakness has been upheld to the present day!
Few people realize that lactate generation is actually necessary to allow moderate to intense exercise to occur. It is the conversion of a product known as pyruvate to lactate that enables the glycolytic (a fast energy generating metabolic process using glucose) system to continue working at a fast rate. Lactate is formed during moderate to intense exercise, when the human body relies heavily on carbohydrates and the glycolytic system to produce energy. All metabolic processes are highly regulated and only a fixed amount of energy-supplying product or substrate may be used at a time before a backup or “metabolic bottleneck” develops. A good analogy to visualize the regulation of a metabolic pathway is to consider energy substrates as an army of soldiers marching to, and through a tunnel. When the first few soldiers enter, they can move quickly and unrestricted, but as the number trying to enter increases, the process slows down dramatically. The conversion of pyruvate to lactate prevents the excess pyruvate from clogging up the pathway, bringing glycolysis to a grinding halt. This slowing of glycolysis obviously does not occur, because if it did running events such as the 400m would be impossible. As we shall see in another article on this site, the accumulation of lactate in the blood is the direct result of this “redirection” of energy in the body not because of a lack of oxygen in the muscles as Hill proposed. (see The Lactate Threshold – Reality or Fallacy? ). Lactate as will be described below, is actually a useful and readily available source of energy for the body to utilize.
Recent research is supplying some very interesting information concerning the role of lactate and muscle performance. George Brooks at the University of California at Berkeley has dedicated much of his career to exploring the role of lactate during exercise. Brook’s investigations indicate the presence of a “lactate shuttle” that allows for the transportation of lactate from one muscle to another. The glycogen (stored carbohydrate) stored in muscle is destined for use in this tissue only, unlike liver tissue, which is able to release glucose into the bloodstream to be used by the rest of the body. The lactate shuttle is proposed as being a means for muscles to be able to “share” and redistribute their glycogen stores to other muscles and tissues in the form of lactate not glucose. For many years it was thought that lactate was a metabolic by-product that to be of any use it had to be transported via the blood to the liver to generate glucose via a process known as the Cori Cycle, but there is evidence to indicate that tissue such as red muscle, heart and brain tissue can directly oxidize the product. Therefore lactate can be utilized by tissue very close to, or even far from the source of generation. The shuttle works via the interaction of the circulatory system and the presence and operation of special transporter proteins located in muscle called mono carboxylic acid transporters (MCTs). These transporters are able to efficiently transport lactate from the blood into adjacent or distant muscles in the body. The inactive muscle can actually store the lactate, thereby further lowering the concentrations in the blood and active muscle. According to Brooks, lactate is far from a metabolic dead-end and may in fact be the most important metabolic fuel used by muscles especially during exercise. Estimates are that approximately 70% or more of the lactate generated during exercise is actually consumed or oxidized while only 19% is converted to glycogen.
In conclusion, the dubbing of lactate as a metabolic dead-end and as an exclusive cause of muscle fatigue was hasty but may have seemed appropriate at the time. Since then however technological advances in research have provided some quite contradictory evidence to the role of lactate. Amazingly, lactate may in fact be a “super fuel” for the body during exercise sessions that produce large quantities of the product. Exercise science will continue to investigate the role and contribution of lactate to exercise but in the meanwhile runners and athletes alike can rest assured that lactate is not the enemy but may in fact be an ally.