If pharmaceutical companies could patent sunshine, would we even be discussing the best source of vitamin D? Of course not.
In fact, sunshine is the only natural and intended source of vitamin D on the planet. Dietary sources of vitamin D -- with the exception of fatty fish like salmon and tuna -- are unnaturally supplemented with "The Sunshine Hormone" at small levels well below what nature gives you naturally when you expose your skin to the sun.
Why is that important? Because in the past few years we've learned:
(1) The amount of vitamin D you need to regulate proper cell growth in every system in your body is only naturally consistent with the levels of vitamin D you make when you are exposed to regular sunshine.
(2) Dietary supplements are an unreliable way to make vitamin D. In a recent study, most vitamin D pills do not have the vitamin D claimed on the label. http://www.medpagetoday.com/MeetingCoverage/CMSC-ACTRIMS/20522
(3) Cosmetic Dermatologists, who for years have told us to get our vitamin D unnaturally from pills and to avoid the sun, are dangerously low in vitamin D themselves. A 2006 study of dermatologists in sunny Melbourne, Australia, found that 83 percent of dermatologists were vitamin D deficient and that for the skin doctors "the average (vitamin D) level was lower than that of elderly impatients in a Melbourne hospital."
(4) Those who get the most sun exposure have the highest vitamin D levels -- levels naturally consistent with what we now know humans need to regulate cell growth, which greatly reduces your risk of most cancers, heart disease, multiple sclerosis and many other conditions.
This chart shows you clearly that those who get the most UVB from sunlight have "natural and intended" vitamin D levels. Those who don't get regular sunlight fall short:
(5) What's more, your body knows how to regulate vitamin D made from sun exposure --- it naturally uses what it needs and gets rid of any extra. That can't be said of dietary vitamin D or vitamin D supplements. Furthermore, studies show that vitamin D made from sun exposure to the skin lasts at least twice as long in the blood as unnatural vitamin D from dietary supplements.
(6) And finally, think beyond vitamin D. Vitamin D pioneer Dr. Michael Holick, the Boston University researcher who discovered the active form of vitamin D more than 40 years ago, says in his latest book, The Vitamin D Solution, "When you are exposed to sunlight, you make not only vitamin D but also at least five and up to ten additional photoproducts that you would never get from dietary sources or from a supplement. So the obvious question is, why would Mother Nature be making all of these vitamin D photoproducts if they weren't having a biologic effect?"