If you main objective is to know if there's a market for your app, then I would suggest to log on to your iphone and search for "diabetes". Then log on to appannie.com and try to see if some of these apps are successful (check their ranking, you might need to create a free account for that). You won't be able to know how many downloads they are getting... but if I see it I might give you a rough estimation. If no apps are successful, then there is no demand