I have been looking for a book for years, but have very little detail about it (if it's even right). I don't know title or author. I do remember that I enjoyed it immensely!
What I can remember is (I think), there was a farmer (boy/man) who looked after sheep or cattle and aliens (I think they were cat like/feline) who removed all the toxins and impurities (caused by earths environment) from the boy. The boy became really strong and fast as all of a sudden he was picking up sheep? etc.