Vitamin C, D, zinc, blah, blah, blah...
Here's the real gist:
Eat a balanced diet with adequate portions which supplies all your required daily nutrients.
Take supplements if you wish...in general, they won't hurt you and they'll ensure you get your daily needs.
BUT...your body can only process so much in a given period of time. Know what happens to the excess?
It goes right down the sewers when you use the bathroom.
Take a lot of Vitamin C, for example, and look at what color your urine turns. Kind of a bright, almost flourescent-looking yellow. A sure sign you're taking too much. Of you take too much Vitamin C a day, it will affect your digestive system. (In general, this is caused by supplements, not from foods.)
Short term, this may cause nausea and diarrhea.
Too much, over a long period, can cause problems, too. Like excess iron absorption, which may lead to hemochromotosis and serious damage to your liver, heart, thyroid, pancreas, and central nervous system.
Yeah...these are extreme. But they are real potential problems.
So...eat properly and take supplements if you wish. But don't over do it with high doses for long terms.