I'm afraid that the best medical evidence disagrees with this; systematic meta-analyses of several large randomised controlled trials show that multivitamins have no effect in the prevention of major diseases (cardiovascular disease, cancer) or death; and that indiscriminate multivitamin supplementation may even cause some harms. http://annals.org/article.aspx?articleid=1789253
I'm well aware of this. This is advice for someone who knowingly keeps a nutrient-poor diet. (We're talking about an article on scurvy.) Doctors do advise vitamins in those cases.