Frownland

Idk about the vitamin d results but if you're getting it, make sure that you're getting something legit. It's a pretty unregulated industry in the US.

Vitamin c helps your immune system in conjunction with other nutrients from your diet but it's not an immune system booster alone unless it needs help battling scurvy from what I've read. The idea that it helps with cold prevention comes from Linus Pauling overstating the results of an experiment with a small homogeneous sample size. Further trials by other scientists showed varying results. The small number of cases that improved or seemed to work were generally attributed the to regression to the mean (i.e. people already suffering colds would be improving anyways and people taking it preventatively just didn't catch it that season).

The vitamins aren't useless, but stuff like Airborne is a scam. If anyone's got more info let me know, my take could be a little dated.

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Studies show that when a given norm is changed in the face of the unchanging, the remaining contradictions will parallel the truth.