It's not necessarily so: we have a slight problem getting rid of retinol, vit A, for example, which can lead to overdose problems and it's common in cod-liver oil supps. It's better to take beta-carotene, although even that has been linked to lung cancer in smokers since, apparently, it can become a pro-oxident rather than an anti-oxident in some situations.
What I'm getting at is that you shouldn't assume that vitamins are safe to take simply because the body will discard whatever it doesn't need. Sometimes it can't or doesn't and high doses can have unexpected and undesirable side-effects.