As I think I posted on another thread, I was also interested in Vitamins D and B12. Even before donation, I was already very low on D, but afterwards, routine blood work at my family doc showed I was also quite low on B12. Oddly, the doc never mentioned that the B12 had any link to donation (though I don't think he's taken care of too many donors). Anyway, I now take a daily B12 pill, and 25 drops of D in a glass of water once a week. I can't say I feel any different (fortunately, I had not had any fatigue or other special symptoms), but if it makes my body happy, fine with me.
Be well, Snoopy