If you try with food, try to get it from a natural food store, or a local supplier (farmers market). Produce you get from big grocery stores was picked early and ripened in a truck, so a lot of the nutrients don't have a chance to develop.

I've been taking good vitamins for a couple of years steady now, and I don't get sick often. (that's saying a lot, since I work in a clinic. Seems like every other patient of ours has come in this year with the flu.)