Gladden, L. B., Spriet, L. L., Donovan, C. M., Bonen, A., & Brooks, G. A. (1997). The role of skeletal muscle in lactate exchange during exercise. Medicine and Science in Sports and Exercise, 29(5), Supplement abstract 424.

"Lactate is not simply a dead-end metabolite which is produced in skeletal muscle in response to hypoxia. Instead, lactate is produced by many tissues under many different conditions, transported throughout the body, and taken up and metabolized by many tissues. This 'lactate shuttle' necessarily involves lactate metabolism and the membrane transport of lactate, especially in skeletal muscle. Historically, skeletal muscle was viewed as a producer of lactate; however, skeletal muscle (both at rest and during exercise) is now known as not only a major source of lactate, but also as the most important consumer of lactate." (p. S74)

Stephen Seiler (https://www.sportsci.org/news/news9707/seiler.htm) commented on this symposium. Some of his remarks, which summarize the content of the presentations and are appropriate for coaching science, are listed below.

• "In 1970, Lehninger's biochemistry text defined lactic acid as a metabolic end product which escapes from muscle cells as waste during conditions of oxygen deficiency. In 1996, the same text describes lactate as an intermediate produced in fully oxygenated tissue as a means of coordinating energy storage and utilization in different tissues."
• "Skeletal muscle not only takes up lactate, but . . . it does so even at very high metabolic rates."
• "It appears that muscle begins to be a significant consumer of lactate at blood concentrations of about 2mM. By 8mM all fibers become lactate consumers, even at rest. What are the pathways for lactate removal once it is taken up by muscle? In type I fibers [slow-twitch] the answer is oxidation and transanimation. However, in IIb fibers glyconeogenesis is a major removal pathway. Yes folks, skeletal muscle CAN move lactate back up the glycolytic pathway to reform glycogen. We knew the liver could manage this, but skeletal muscle glycolysis was assumed to be a one-way path. . . . but in skeletal muscle [does not use similar mechanisms to the liver] direct pyruvate kinase reversal occurs."
• "It also appears that lactate transport is faster in oxidative fibers, and that transport capacity increases with training (~30% in a recent study from Copenhagen). . . . but it looks like some transporters are designed for exporting lactate out of the muscle, while others specialize in import. The likely possibility . . . is that lactate is pushed out of an active glycolytic fiber via 'release transporters' only to be taken up by an immediately adjacent oxidative fiber with a high density of 'uptake transporters'."
• "George Brooks capped things off by saying that the lactate shuttle hypothesis [reported in this journal] he proposed in the 80s is alive and well. Indeed, it appears that lactate serves the body well as a vehicle for moving carbon around in the body.'