I am not sure the running friend is correct about lactic acid. If running is aerobic I don't think it builds up in the muscles, never mind being dumped in the gut. It acts as a secondary fuel source and I believe it gets metabolized in the muscles. But even if it didn't, how would it wind up in one's gut? It would more likely be swept out of the bloodstream by the liver, as that is the main clensing agent for blood. And the liver would then break it down into reusable components and store them for a rainy day.