you may also want to check out: http://www.vitamindcouncil.org
It's an excellent site for all things Vitamin D related. Also, if you're sunning yourself for about 20 minutes a day between the hours of noon & 2PM during the months of May - Sept (in the Northern Hemisphere)you should be getting a good amount. Remember: no sunblock, no sunglasses, no glasses & no contacts when sunning yourself. The unfiltered light stimulates your pituitary gland which is a master gland for thyroid, adrenals etc....