You have two clear markets here, and I can understand the difficulty of your decision.
On the one hand you have a mass-market with the possibility of a manual of advice directed at health devotees... on the other hand, the higher value market of health store proprietors.
Your next step will be to decide which is better for your experience levels. Remember that anyone can put together information on any subject, but without practical experience in a field, your credibility factor is low.
Have you been a health shop proprietor? If not, then you'll need some "big gun" information (scarce, rare, little-known), to sell adequately to them.
So maybe your best bet is to the mass market.
And remember, my view is as an outsider... you've probably done more research and will have a better idea of your priorities.