Vitamin D is known as the sunshine vitamin because the only good source of vitamin D is the sun. We absorb it from the sun through our skin unless you wear sunscreen.

15 minutes of high-noon sun exposure 3 times a week is a way to get the amount of vitamin D we need to get all the sunny benefits it offers. For us in the upper Midwest that doesn't work so well come December. Plus, we've been warned by doctors for years to stay out of the sun because of the damaging ultra violet rays.

According to a recent study published in the Archives of Internal Medicine 50 to 75% of Americans aren't getting the optimal levels of vitamin D. The only food where vitamin D is found naturally is in fatty fish like salmon and cod liver oil.
You can get vitamin D in fortified dairy products, like milk, but you'd need to drink 4 glasses of milk a day and most people just don't do that.
The experts say most multi-vitamins have a substantial amount of vitamin D, but to insure you are getting enough, doctors recommend taking a vitamin D supplement every day with 1,000 - 2,000 IU's (international units, that's how they are measured) in it. That's significantly higher than what doctors used to recommend.

They are discovering it promotes weight loss, reduces risk of heart disease and mortality, fewer bone fractures and helps fight cancer. That's pretty good for a cheap little pill with no down sides.