Vitamin D is an essential nutrient for humans. We are able to synthesize it in our skin, via the ultraviolet-B in sunlight. This is probably the reason why human populations in the higher latitudes have developed lighter-colored skins, since there is less sunlight available to synthesize vitamin D. The darker skin of those living closer to the equator, while protecting them more from the sun's more harmful rays, is not dark enough to inhibit sufficient vitamin D production in their homelands.
Much as iodine is added to salt to ensure a reasonable supply to prevent thyroid problems, vitamin D is often added to milk to address the fact that most people in the higher latitudes don't spend enough time in the sun, especially in their local winter (when the sun isn't strong enough to generate vitamin D in humans).
This is all very simple and easy.
 Endosynthesis woo
Some medical professionals who should know better claim that the majority of vitamin D synthesis takes place not in the skin, but in the eyes. Thus, they recommend looking at a sunlit sky, or even directly into the sun itself, as the sungazers do.
To avoid eye damage, they recommend you do this in the early morning or late afternoon, when the sun is low on the horizon. This is doubly questionable. If it's ultraviolet-B that stimulates production of Vitamin D, then early morning and late afternoon are lousy times of day for it, because it's at those times that the slant angle of the sun through the atmosphere is the steepest. Compared with high noon, almost no ultraviolet-B gets through the Earth's atmosphere when the sun is low.
 Megadose woo
What is strange are the woomeisters who sell vitamin D supplements claiming it will help cure or avoid virtually every condition imaginable. This can be dangerous when taken at high doses (over 5000 I.U. per day), causing vomiting, kidney stones and even heart problems.
 Avoidance woo
The Marshall Protocol claims that vitamin D can be usurped by invading bacteria to suppress your immune system. This flies in the face of other research which supports a link between low vitamin D levels and an increased risk of infection.
 Deficiency diseases
Vitamin D is needed for proper bone growth. Low vitamin D levels in childhood can result in rickets. Among adults, vitamin D deficiency can result in osteomalacia, or contribute to osteoporosis.
Note that vitamin D deficiency is a more severe category than vitamin D insufficiency. While vitamin D deficiency is rare in the developed world, a surprising percentage of otherwise healthy adults may in fact be experiencing vitamin D insufficiency. Insufficiency has been indirectly linked with a variety of problems including sleep disorders and depression. A simple blood test is all that is needed to detect insufficiency. Obesity, spending less than 10 minutes in the sun, wearing sun screen (for example, in your makeup), and hypothyroidism are all conditions that effect vitamin D absorption and creation.
 Deficiency, or inflammation?
A study has found what it claims is strong evidence that 25-hydroxy vitamin D -- also known as calcifediol or calcidiol, the vitamin D looked for in blood tests -- is a negative acute-phase reactant[wp]. That means its concentration goes down in response to inflammation.
As mentioned above, there's a growing body of medical literature showing that low serum levels of Vitamin D are associated with a variety of infections. A common assumption has been that this means Vitamin D deficiency might cause these infections; but if these infections come with an accompanying inflammatory response, the negative acute-phase reactant study opens up the possibility that Vitamin D deficiency isn't the cause of these illnesses but rather a consequence of them. If that's the case -- note the "if" -- then taking Vitamin D supplements might not do anything to improve ones resistance to these infections. These are preliminary results, though, and it would be a mistake to turn them into clinical practice at this stage without controlled trials.
 Mainstream dosage recommendations
The traditional U.S. Recommended Daily Allowance of Vitamin D is 400 I.U. per day for adults. However, the criteria have begun shifting. Current medical wisdom is to aim for a serum concentration of 25-hydroxy-vitamin D that lies within the healthy range. This concentration level can only be established by taking a blood test. If the patient's concentration is below the healthy range, supplements of 1000, 2000, or even 4000 I.U. of vitamin D may be prescribed.
Problems arise because the definition of the healthy range changes from one source to another.
As of 2011, the National Institute of Health sets the healty concentration range as being from 20 to 50 ng/mL. Some articles in peer-reviewed journals, however, put the low end of the healthy range at 30-32 ng/mL; a review in the December 2008 Australian Family Physician even puts the low end at 40 ng/mL.
- ↑ In birds and fur-bearing mammals, such as goats, vitamin D is generated from the oily secretions of the skin deposited onto the feathers or fur and is obtained orally during grooming.
- ↑ NIH: Can I get vitamin D from the sun?
- ↑ NIH: Can vitamin D be harmful
- ↑ WebMD
- ↑ A group that contains over 100 pages about the purported benfits of vitamin D.
- ↑ Science-Based Medicine article on the Marshall Protocol: "a pubmed search will reveal an a growing and interesting literature [sic] that vitamin D deficiency increases, note increases, the risk of a variety of infections."
- ↑ BMJ 2012;345:e4695, "Vitamin D: some perspective please", rapid response tab