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Question answered:15/06/08
In 2008 the Vitamin D Council published ‘Diagnosis and treatment of vitamin D deficiency’ [1], this reported:
“Factors that affect cutaneous production of vitamin D include latitude, season, time of day, air pollution, cloud cover, melanin content of the skin, use of sunblock, age and the extent of clothing covering the body. When the sun is low on the horizon, atmospheric ozone, clouds and particulate air pollution deflect UVB radiation away from the surface of the Earth. Therefore, cutaneous vitamin D production is effectively absent early and late in the day and for the entire day during several wintertime months at latitudes > 35°.
For that reason, vitamin D deficiency is more common the further poleward the population. For example, Boston, Massachusetts (latitude 42°) has a 4-month ‘vitamin D winter’ centred around the winter solstice when no UVB penetrates the atmosphere and an even longer period in the fall and late winter when UVB only penetrates around solar noon. In northern Europe or Canada, the ‘vitamin D winter’ can extend for 6 months. Furthermore, properly applied sunblock, common window glass in homes or cars, and clothing, all effectively block UVB radiation – even in the summer. Those who avoid sunlight – at any latitude – are at risk any time of the year. For example, a surprisingly high incidence of vitamin D deficiency exists in Miami, Florida despite its sunny weather and subtropical latitude.
African-Americans, the elderly and the obese face added risk. As melanin in the skin acts as an effective and ever-present sunscreen, dark-skinned patients need much longer UVB exposure times to generate the same 25(OH)D stores compared with fair-skinned patients. The elderly make much less vitamin D than 20-year-olds after exposure to the same amount of sunlight. Obesity is also major risk factor for vitamin D deficiency with obese African-Americans at an even higher risk. Therefore, those who work indoors, live at higher latitudes, wear extensive clothing, regularly use sunblock, are dark-skinned, obese, aged or consciously avoid the sun, are at high-risk for vitamin D deficiency.”
Reference
1) http://www.vitamindcouncil.org/PDFs/diagnosis-vitdd.pdf
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