Your body will create enough Vitamin D for itself, if you will give it 15 minutes of sunshine, three times a week. Without sufficient Vitamin D, you risk osteoporosis if you're an adult, and rickets if you're a child. In America, many people can't (read that won't) get themselves outdoors to even meet this minimal need - so practitioners like Dr. Andrew Weil are recommending supplements at 1000 mg or more of Vitamin D to meet the insufficiency. Sunshine is free.
Source: MedlinePlus, Dr.Weil.Com.