Dr. Youngquist: I don't know about vitamins. It depends on if they're water soluble or not, but yes most of the time if you take supplemental vitamins most of it ends up in your urine anyway, regardless of how much water you take. And that's why you take some B12 vitamins and you'll notice that your urine becomes distinctly kind of dark yellow-orange in color, and it's not because you're suddenly dehydrated. It's because of the concentration of those vitamins.