If a friend asks me, I try to give a thoughtful answer, but generally, the people who ask me these types of questions, are some of the most boring people I've met, so I keep the conversation brief.
First, I answer the question objectively and then while they're hesitating, I move the conversation to something else they care about. If they insist, though, I flatly give them an honest answer, which, so far, forces them to move on.
Here's what I mean:
"Wow! Why did you lose so much muscle mass?"
"Because I haven't been lifting heavy weights."
"..."
"So how's business?"
"But why haven't you been?"
"Because I don't want to."
"..."
"Are sales up?"
"Oh, yeah. We recently added..."