> I wondered how do people by this stuff believing it helps them

Read Amazon reviews. It's pretty amazing what people can come up with.

I take the usual suspects: Magnesium (I run almost every day) and a Omega 3/Vitamin D combo (I don't eat fish and we don't get a lot of sun here in northern Germany). Seems like a reasonable thing to be doing but I can't for the life of me tell any difference with or without them. I do buy the cheap ones though - maybe I'd feel 10 years younger if I'd buy the top shelf ones...