Actually, there was an article in the NYTimes four years ago about lactic acid, and it turns out that "lactic acid is actually a fuel, not a caustic waste produ

Actually, there was an article in the NYTimes four years ago about lactic acid, and it turns out that "lactic acid is actually a fuel, not a caustic waste product. Muscles make it deliberately, producing it from glucose, and they burn it to obtain energy. The reason trained athletes can perform so hard and so long is because their intense training causes their muscles to adapt so they more readily and efficiently absorb lactic acid." (http://www.nytimes.com/2006/05/16/health/nutrition/16run.html http://www.nytimes.com/2006/05/16/health/nutrition/16run.htm...) So maybe that explains why Phelps can train as he does: if he's producing fuel faster than anyone else, presumably his body is also better adapted to absorbing and using it.