Lactic acid can also be used as an important fuel or as a source for glucose and glycogen synthesis. When you exercise intensely, for instance, lactic acid produced in your fast-twitch fibers can actually go to an adjacent slow-twitch fiber, which can then use it as fuel.
Approximately 75% of the lactic acid made during exercise is used as fuel. The remaining 25% is converted to glucose in the kidney and liver. The removal of accumulated lactic acid helps avert excessively high levels, and the conversion of lactate into glucose helps maintain sufficient levels of blood glucose, which is important during prolonged exercise.
However, that is not all lactic acid can do. Did you know that when you work your legs, for example, your inactive muscles (such as your biceps) can release lactic acid from their glycogen stores? This lactic acid travels to the liver via the bloodstream, where it is converted to glucose. This glucose is shuttled back via the blood to the previously active muscles and serves as a substrate for glycogen re-synthesis, so even your inactive muscles play a role in muscle recovery.
Since lactic acid is half the size of glucose, it more easily crosses cell membranes. Unlike glucose, which requires insulin for its transport across the cell membrane, lactic acid needs no hormonal support and crosses the membrane via facilitated transport. In addition, muscles can release large quantities of lactic acid into the general circulation, where it can serve as a potential fuel source and precursor for gluconeogenesis.