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Exposing skin to sunlight presents both health risks and health benefits.[1] On the one hand, long-term sunlight exposure is known to be associated with the development of skin cancer, skin aging, immune suppression and eye diseases such as cataracts.[2] On the other, sunlight is a major source of vitamin D-producing Ultraviolet B radiation, which has a wide range of positive health effects, including possibly inhibiting the growth of some cancers.[3] Sun exposure has also been associated with the timing of melatonin synthesis and reduced risk of seasonal affective disorder.[4] A number of public health organizations state that there needs to be a balance between having the risks of having too much and the risks of having too little sunlight.[5][6] There is a general consensus that sunburn should always be avoided.

However, not all physicians agree with the assertion that there is an optimal level of sun exposure, with some arguing that it is better to minimize sun exposure at all times and to obtain vitamin D from other sources.[7]

Serum levels of 25(OH) D3 are below the recommended levels for a large portion of the general adult population and in most minorities, indicating that Vitamin D deficiency is a common problem in the United States.[8]

Contents

Benefits

According to the U.S. National Institutes of Health Office of Dietary Supplements, most people in the United States can meet their vitamin D needs through exposure to sunlight, even though a large portion have serum 25(OH)D3 levels below recommendations.[1] Ultraviolet (UV) B radiation with a wavelength of 290-315 nanometers penetrates uncovered skin and converts cutaneous 7-dehydrocholesterol to previtamin D3, which in turn becomes vitamin D3.[9][10][11] Season, geographic latitude, time of day, cloud cover, smog, skin melanin content, and sunscreen are among the factors that affect UV radiation exposure and vitamin D synthesis.[11] The UV energy above 42 degrees north latitude (a line approximately between the northern border of California and Boston) is insufficient for cutaneous vitamin D synthesis from November through February;[12] in far northern latitudes, this reduced intensity lasts for up to 6 months. In the United States, latitudes below 34 degrees north (a line between Los Angeles and Columbia, South Carolina) allow for cutaneous production of vitamin D throughout the year.[13]

Complete cloud cover reduces UV energy by 50%; shade (including that produced by severe pollution) reduces it by 60%.[14] UVB radiation does not penetrate glass, so exposure to sunshine indoors through a window does not produce vitamin D.[15] Sunscreens with a sun protection factor of 8 or more appear to block vitamin D-producing UV rays, although in practice people generally do not apply sufficient amounts, cover all sun-exposed skin, or reapply sunscreen regularly.[16] Skin likely synthesizes some vitamin D even when it is protected by sunscreen as typically applied.[1]

The factors that affect UV radiation exposure and research to date on the amount of sun exposure needed to maintain adequate vitamin D levels make it difficult to provide general guidelines. It has been suggested by some vitamin D researchers, for example, that approximately 5-30 minutes of sun exposure between 10 AM and 3 PM at least twice a week to the face, arms, legs, or back without sunscreen usually lead to sufficient vitamin D synthesis and that the moderate use of commercial tanning beds that emit 2%-6% UVB radiation is also effective.[17][11] Individuals with limited sun exposure need to include good sources of vitamin D in their diet or take a supplement.

Risks

Despite the importance of the sun to vitamin D synthesis, it is prudent to limit exposure of skin to sunlight[16] and UV radiation from tanning beds.[18] According to the National Toxicology Program Report on Carcinogens from the Department of Health and Human Services, broad-spectrum UV radiation is a carcinogen thought to contribute to most of the estimated 1.5 million skin cancers and the 8,000 deaths due to metastatic melanoma that occur annually in the United States[16][19] Lifetime cumulative UV damage to skin is also largely responsible for some age-associated dryness and other cosmetic changes.

It is not known whether a desirable level of regular sun exposure exists that imposes no (or minimal) risk of skin cancer over time. The American Academy of Dermatology advises that photoprotective measures be taken, including the use of sunscreen, whenever one is exposed to the sun.[20]

Is there a safe level of sun exposure?

According to a recent study submitted by the University of Ottawa to the Department of Health and Human Services in Washington, D.C., there is not enough information to determine a safe level of sun exposure at this time.[12]

Sunlight is the only listed carcinogen that is also known to have health benefits, in the form of helping the human body to make Vitamin D. This makes sunlight unique on the list of known carcinogens.[21]

Homo sapiens evolved near the equator, and human skin colour is a visible indication of direct sun exposure due to the pigmentation level. With new evidence of Vitamin D receptors in all body tissues, experts advise having a balance between Vitamin D from sun exposure and Vitamin D from supplements. The only way to quantify adequate levels of Vitamin D is with a serum 25(OH) D3 test.[22]

References

  1. ^ a b c "Dietary Supplement Fact Sheet: Vitamin D". Office of Dietary Supplements, National Institutes of Health. http://dietary-supplements.info.nih.gov/factsheets/vitamind.asp.  
  2. ^ Lucas RM, Repacholi MH, McMichael AJ (June 2006). "Is the current public health message on UV exposure correct?". Bulletin of the World Health Organization 84 (6): 485–91. doi:/S0042-96862006000600018. PMID 16799733.  
  3. ^ http://news.bbc.co.uk/1/hi/health/4090972.stm
  4. ^ Mead MN (April 2008). "Benefits of sunlight: a bright spot for human health". Environmental Health Perspectives 116 (4): A160–7. PMID 18414615. PMC 2290997. http://www.ehponline.org/members/2008/116-4/focus.html.  
  5. ^ http://www.cancer.org.au//File/PolicyPublications/PSRisksBenefitsSunExposure03May07.pdf
  6. ^ http://www.sunsmartschools.co.nz/be/Editor/Doc/PS_RisksBenefits_SunExposureSept08.pdf
  7. ^ http://www.skincancer.org/scf-journal-2008.html
  8. ^ Zadshir A, Tareen N, Pan D, Norris K, Martins D (2005). "The prevalence of hypovitaminosis D among US adults: data from the NHANES III". Ethnicity & Disease 15 (4 Suppl 5): S5–97–101. PMID 16315387.  
  9. ^ Hayes CE, Nashold FE, Spach KM, Pedersen LB (March 2003). "The immunological functions of the vitamin D endocrine system". Cellular and Molecular Biology 49 (2): 277–300. PMID 12887108.  
  10. ^ Holick MF (October 1994). "McCollum Award Lecture, 1994: vitamin D--new horizons for the 21st century". The American Journal of Clinical Nutrition 60 (4): 619–30. PMID 8092101. http://www.ajcn.org/cgi/pmidlookup?view=long&pmid=8092101.  
  11. ^ a b c Holick, Michael F. (February 2002). "Vitamin D: the underappreciated D-lightful hormone that is important for skeletal and cellular health". Current Opinion in Endocrinology & Diabetes 9 (1): 87-98. doi:10.1097/00060793-200202000-00011.  
  12. ^ a b Cranney A, Horsley T, O'Donnell S, et al. (August 2007). "Effectiveness and safety of vitamin D in relation to bone health". Evidence Report/technology Assessment (158): 1–235. PMID 18088161.  
  13. ^ Holick MF (2006). "Vitamin D". in Shike, Moshe; Shils, Maurice Edward. Modern nutrition in health and disease. Hagerstwon, MD: Lippincott Williams & Wilkins. ISBN 0-7817-4133-5.  
  14. ^ Wharton B, Bishop N (October 2003). "Rickets". Lancet 362 (9393): 1389–400. doi:10.1016/S0140-6736(03)14636-3. PMID 14585642.  
  15. ^ Holick MF (2005). "Photobiology of vitamin D". in Feldman, David Henry; Glorieux, Francis H.. Vitamin D. Amsterdam: Elsevier Academic Press. ISBN 0-12-252687-2.  
  16. ^ a b c Wolpowitz D, Gilchrest BA (February 2006). "The vitamin D questions: how much do you need and how should you get it?". Journal of the American Academy of Dermatology 54 (2): 301–17. doi:10.1016/j.jaad.2005.11.1057. PMID 16443061.  
  17. ^ Holick MF (July 2007). "Vitamin D deficiency". The New England Journal of Medicine 357 (3): 266–81. doi:10.1056/NEJMra070553. PMID 17634462.  
  18. ^ "The association of use of sunbeds with cutaneous malignant melanoma and other skin cancers: A systematic review". International Journal of Cancer 120 (5): 1116–22. March 2007. doi:10.1002/ijc.22453. PMID 17131335.  
  19. ^ http://ntp.niehs.nih.gov/index.cfm?objectid=BD4CD88D-F1F6-975E-792094AC1CE4B062
  20. ^ American Academy of Dermatology. Position statement on vitamin D. November 1, 2008. [1]
  21. ^ http://ntp.niehs.nih.gov/ntp/roc/toc11.html
  22. ^ http://www.nlm.nih.gov/medlineplus/ency/article/003569.htm
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