It's not always simple to know what's good for your health. We've seen that the placebo effect
is far more complicated than it seems (or is usually presented) as its effects have grown stronger and stronger over the years across many clinical trials. Doctors themselves aren't always certain what's the "best" health advice, and they're constantly re-evaluating
whether current recommendations are actually valid -- to develop better recommendations. Here are just a few links on vitamins and some of their (mistaken) benefits.
If you'd like to read more awesome and interesting stuff, check out this unrelated (but not entirely random!) Techdirt post