Under the Microscope:Vitamin D is an essential vitamin for human health. A deficiency of this vitamin leads to abnormal calcium and phosphorous metabolism and defective bone formation. The deficiency disease is called rickets in children and osteomelacia in adults.
The physical symptoms of rickets are bow legs and pigeon breast. But there is evidence now that vitamin D may play a much broader role in human health, helping to protect us from a variety of disorders including TB, multiple sclerosis (MS) and cancer. The evidence is reviewed by A Mandavilli in Nature Medicine, Vol 13, No. 4 (April 2007) and I incorporate the highlights into this article.
Vitamin D3 or cholecalciferol is normally made in human skin from an inactive precursor, 7 dehydrocholesterol, by reactions that are promoted by exposure to the ultraviolet component of sunlight. It is also abundant in fish liver oil, but normally we do not need supplementary vitamin D as long as the skin receives sufficient exposure to sunlight.
Vitamin D3 is not biologically active itself, but is the precursor of the active 1,25-dihydroxycholecalciferol. Conversion to the active agent is completed in the kidney. Because this active form of vitamin D3 is transmitted from the kidney to other sites in the body, particularly the small intestine and bone, where it regulates calcium and phosphorus metabolism, it is considered to be a hormone (a chemical "messenger" synthesised by one organ for the regulation of biological activity of another tissue).
In 1895, the Danish scientist Neils R Finsen discovered an unusual way to treat TB of the skin, for which he won the 1903 Nobel Prize in physiology or medicine. He found that light from an electric arc lamp cured most cases. This was before the age of antibiotics which transformed TB into a curable disease, and Finsen's discovery was a very significant breakthrough. It began the trend of sending TB patients to sanatoriums in sunny locations, but nobody understood how it worked. Recent work published by Barry Bloom and colleagues from the Harvard School of Public Health (Science, Vol 311, pp1770-1773, 2006) suggests that vitamin D, made under the stimulus of sunlight, promotes the production in the body of a compound called cathelicidin which kills various viruses and bacteria, including the bacterium that causes TB.
African Americans are eight times more susceptible to TB than Caucasians. The dark skin of African Americans is much more efficient at filtering out ultraviolet light than lighter skin and consequently blood levels of vitamin D are significantly lower in African Americans. Studies have shown that, consequently, African Americans seem to produce significantly less of the TB microbe-killing cathelicidin than Caucasians and this might explain the higher rate of TB. Vitamin B supplements are now planned in African TB trials.
Apart from its role in building strong bones, probably the strongest evidence for other preventive roles for vitamin D is the case of MS. Several studies have shown that MS is rarer in a "sunlight belt" near the equator and is more common in northern latitudes where ultraviolet light from the sun is less intense. In the US , for example, incidence of MS is four times higher in the north-west than in the south-east.
Mice suffering from a disease similar to human MS quickly succumb to the disease when deprived of vitamin D, but the symptoms diminish when they are administered vitamin D. And people with the highest blood levels of vitamin D appear to have the lowest risk of MS (Journal of American Medical Association, Vol 296, pp 2832-2838, 2006). Research is now testing whether high doses of vitamin D will ease MS symptoms in people as they do in mice.
There is evidence also that places with less sunshine have higher rates of cancer. Animal studies indicate that vitamin D stops the proliferation of cancer cells, inducing them to commit suicide. The recommended daily intake of vitamin D is 400 international units (IU), and other studies (American Journal Public Health, Vol 96, pp 252-261, 2006), indicate that upping daily intake to 1,000 IUs cuts the risk of colon cancer by 50 per cent and breast and ovarian cancers by 30 per cent. Other studies indicate favourable results for prostate cancer.
As summer takes over from winter, the susceptibility of people to succumb to flu virus drops about one hundred-fold and nobody understands why, although there are many hypotheses - eg the virus is more stable in wintry air. However, the reason may simply be vitamin D and sunlight. This idea is easily testable although the drug companies are not very interested in doing a big trial. Vitamin D is not a new substance and it would not be easy to make lots of money out of it.
Vitamin D seems to be much more important than we had thought. The recommended daily intake of 400 IUs may therefore be somewhat too low. However, daily intake greater than 2,000 IU may be toxic. Fish-oil is high in vitamin D, but probably the best way to boost intake is the natural way - exposure to sunlight. Long winters, longer working hours and fear of skin cancer have conspired together to lower average blood concentrations of vitamin D below optimum levels. The function of sunscreen is to protect against ultraviolet light, thereby minimising risk of skin cancer. This is important, but the realisation of the importance of vitamin D is sufficient reason to justify brief periods of sun-bathing unprotected by sunscreen. Ten minutes of summer sun can produce far more vitamin D than the recommended minimum of 400 IUs. William Reville is associate professor of biochemistry and public awareness of science officer at UCC - understandingscience.ucc.ie