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I found the podcast Science Vs very helpful in understanding this topic - https://gimletmedia.com/episode/vitamins-supplements-worth/ https://gimletmedia.com/episode/vitamins-supplements-worth/ I was recommended Vitamin D by the wife of a Chiropractor. I do 2000IU somewhat daily to every other day in the winter & 5000IU if I feel a cold coming on with some extra zinc. This has made a significant improvement in my ability to fend off colds. That said, I do try to exercise a few times a week & eat as reasonably healthy as one can do in the far north midwest of America.