I told my nutritionist that I was bipolar, and she highly recommended vitamin D to me. My regular doctor told me the same thing.

If you're curious, my nutritionist told me 2000 IU a day is what she recommends. (That amount is safe and healthy.) She said it helps a lot with depression in particular.

I only started it taking vitamin D a week ago, so I can't attest to this yet... but I'm hoping it works out well.