Vitamin D is mainly derived from radiation, such as sunlight. So if you are a beach bum chances are you don't need extra, but if you live in an area that gets pretty heavy winters, getting a little extra in this period may be wise. Nowadays the FDA recommends milk be enriched with Vitamin D, but it's a highly anabolic hormone that may be of greater benefit than you think. The basic recommendation is 400 International Units, but as with Vitamin E, getting 800 IU or so makes more sense. Calciferol as it's called is known as the sunshine vitamin. I never believed you could get vitamins through sunlight, not even as a gullable kid. How funny it is then that as an educated adult I had to change my mind. One of the best alternate sources of Vitamin D is by letting your body's cells manufacture it themselves from cholesterol under the influence of unfiltered sunlight. Depending on the darkness of your skin and your normal daily exposure to sunlight, this will take 30 to 200 minutes.  

It can regulate the phosphorus balance in the body and helps to maintain the heart muscle and maintains healthy nerves. Vitamin D is also a classified steroid hormone. Something you already learned if you read my article on milk. After all it is manufactured from cholesterol, just like the sex-hormone precursor dehydro-epiandrosterone. As such it increases the absorption and use of calcium and thereby assists in maintaining healthy bones. It also stimulates the thyroid and helps in blood clotting.