You'r body naturally makes sufficient amounts if you spend 20 minutes or so a day in they sun.

An amazingly important and versatile vitamin, which our bodies can produce naturally with direct sunlight. You cannot get too much Vitamin D, as our bodies have a safety switch which turns off excess manufacturing. It has an incredible capability, which the FDA doesn't want you to know about...one which will allow you to cruise through the Sun-challenged Winter...without having mercury and anti-freeze injected into your body. Very cool, short video. Watch it. This can be an influenza-free Winter for RIA reviewers...

I got tested for vitamin D.  I get enough from the sunlight living here in AZ, but if not I would be taking supplements.  Deficiency can drastically impact your mood.  It is possible to overdose and to test it costs about $15 to get a blood draw.

There's nothing wrong in taking vitamins. Saying that, I think the best way to get your vitamin D is being under the sunlight and drinking milk.

Not even worthy of supplementation. Exposure to sunshine provides enough of it through the body's own processes. Is added to milk, as well. Essential to the diet, but not scarce at all.