Vitamin D inadequacy is a major health concern worldwide, the deficiency is common to all continents, countries, races, ethnics groups, genders, and across all ages. Some surveys suggest that perhaps half of the world’s population has inadequate blood levels of vitamin D. Indeed, in industrialized countries, doctors are even seeing the resurgence of rickets, the bone-weakening disease that had been largely eradicated through vitamin D fortification.
Factors That Affecting vitamin D Levels in Your World
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’ centered 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 (Reference 1).
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 (Reference 2). The elderly make much less vitamin D than 20year olds after exposure to the same amount of sunlight (Reference 3). Obesity is also a major risk factor for vitamin D deficiency with obese African-Americans at an even higher risk (Reference 4). 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.
Roles of Vitamin D
Vitamin D promotes calcium absorption in the gut and maintains adequate serum calcium and phosphate concentrations to enable normal mineralization of bone and to prevent hypocalcemic tetany. It is also needed for bone growth and bone remodeling by osteoblasts and osteoclasts. Without sufficient vitamin D, bones can become thin, brittle, or misshapen. Vitamin D sufficiency prevents rickets in children and osteomalacia in adults. Together with calcium, vitamin D also helps protect older adults from osteoporosis.
Other Roles of Vitamin D
- Modulation of cell growth,
- Neuromuscular and immune function,
- And the reduction of inflammation.
Many genes encoding proteins that regulate cell proliferation, differentiation, and apoptosis are modulated in part by vitamin D. Many cells have vitamin D receptors, and some convert 25(OH)D to 1,25(OH)2D.
1. Levis S, Gomez A, Jimenez C, et al. Vitamin D deficiency and seasonal variation in an adult South Florida population. J Clin Endocrinol Metab 2005;90(3):1557-62
2. Willis CM, Laing EM, Hall DB, et al. A prospective analysis of plasma 25-hydroxyvitamin D concentrations in white and black prepubertal females in the southeastern United States. Am J Clin Nutr 2007;85(1):124-30.
3. Holick MF. McCollum award lecture, 1994: vitamin D – new horizons for the 21st century. Am J Clin Nutr 1994;60:619-30.
4. Yanoff LB, Parikh SJ, Spitalnik A, et al. The prevalence of hypovitaminosis D and secondary hyperparathyroidism in obese Black Americans. Clin Endocrinol (Oxf) 2006;64(5):523-9