I think it has no difference as building on other niches but the problem is you need to know which you will write and real experience about health and fitness to give best advice for your advice.
Do you have a very particular tie in to this niche? Anything personal to you, that you can create content from your own experience can be a micro-niche for you to explore and chisel out a piece of this market. Health is gigantic niche with thousands of micro-niches. Like ericplotz1 said, don't do this if you're on a fishing expedition - you'll drown or get eaten by the sharks!
