I don't think there are many chemicals that work that way, though. Is there any one chemical that is so great for us that only a very biased diet would give it?

Different soil conditions could lead to deficiencies -- if you only "eat local" you might have to pay attention to selenium or magnesium (http://tin.er.usgs.gov/geochem/doc/averages/se/usa.html) due to geography or geology. But setting aside deficiency, what chemical would we want to load up on in isolation?

(ok, caffeine comes to mind. but that's not really for health, and I get it pretty well from ground plant products.)