I heard a long Quirks & Quarks story about Vitamin D a few weeks ago. The US and Canadian RDAs for Vitamin D, 400 IU, are based on amounts meant to aid bone health, not for these other benefits. The amount in the typical multivitamin will be 400 IU, not the 800 IU recommended here. Unless you wash down your multivitamin with fortified milk every day, you're probably not going to ingest the amount Vitamin D they think you should. Balance that with your sunlight exposure in your region. I'm in New England so as long as I don't keep myself cooped up, I'll get enough Vitamin D from sunlight alone maybe half the year.