Well, of course it is up to you to optionally answer "how are you?" with something like "fine", "good" etc. But it is also quite important to reply with "how are you?", since it would be very impolite if you did not. And, honestly, no one really wants to know about your mood and health; unless it is a friend or relative who asks this, the question is almost always rhetorical.

"How do you do?" is basically the same question as "How are you?". It might be even more superficial, though.