Body & HealthFact
Lactate isn't waste, it's fuel.
When muscles under heavy load need more energy than oxygen can supply, they break down sugar without oxygen and produce lactate. It is not discarded: the heart and resting muscles burn it directly, and the liver converts it back to glucose in the Cori cycle. The burning you feel comes mainly from hydrogen ions released alongside, not from lactate itself.
Useful: An easy cool-down speeds up lactate clearance because working muscles use it as fuel.
Source: en.wikipedia.org