CW, there's nothing wrong with an app that collects and tracks diet, activity, or other information (until you get into data from instrumentation devices and more complex data inputs). Diet exercise are fine. Symptoms are fine. Even reporting them are fine (analytics). The issue becomes the key word: correlating. The FDA's (currently incompletely addressed) stance is that as long as you don't DO anything with the data, you're golden. "observe and report," so to speak.

Once you actually try to interpret (read: do anything interesting with) the data, you hit a regulatory wall. Well, regulatory hurdles, but not inconsequential (21 CFR part 11 compliance, quality systems, documentation well beyond anything the average developer is used to, etc.). Hurdles, but not complete roadblocks. So excite for some of the stuff in the works. Particularly looking forward to massiveHealth, but haven't seen anything since the funding a while back and details were nonexistent. I had some preliminary work on a great product and even a trial designed and ready to roll, and all the regulatory work (my bailiwick) lined out. But didn't get the grants funded. Sadly, I shelved it.