Abstract
Lactate had been considered to be a waste which is produced due to lack of oxygen supply during exercise. However, it is now recognized that lactate is not a waste but an intermediate of carbohydrate and therefore an oxidizable substrate. Production of lactate does not necessarily mean lack of oxygen but rather increase in glycolysis and glycogenolysis. Therefore, lactate is an intermediate which balances glycolytic flow with mitochondrial oxidation. The transport of lactate into and out of tissues are done via transporter called MCT (Monocarboxylate Transporter). There are several isoforms in MCTs. MCT1 is abundant in slow type muscle fibers and is related to oxidation of lactate. MCT4 is rich in fast type muscle fibers and is related to extrusion of lactate out of tissues. These characteristics of distribution of MCTs in muscle fibers reflect characteristics of lactate metabolism particularly during exercise when a great deal of lactate is produced. Because lactate is an oxidizable substrate, ingestion of exogenous lactate can prevent hypoglycemia during endurance exercise. Fatigue during exercise is not necessarily caused by accumulation of lactate. For example, in marathon running, runners feel fatigue toward the end of race while there is less accumulation of lactate because of depletion of glycogen. There are some reports showing beneficial effect of production of lactate against fatigue. These suggest that oxidation is the major fate of lactate during and after exercise. Further studies are needed to clarify what is the major cause of fatigue other than lactate during exercise. [J Physiol Sci. 2007;57 Suppl:S51]
