A study following 29,518 Swedish women shows that those who avoided sun exposure were twice as likely to die over 20 years compared to those who spent significant time in sunlight (J Intern Med, published online April 4, 2014). This does not mean that lack of sunlight causes death; it means only that lack of sunlight is associated with increased rate of death in this study. It could mean only that people who are sick do not spend as much time in the sunlight.
You Need Sunlight or Pills to Meet Your Needs of Vitamin D To meet your needs for vitamin D, you must expose your skin to sunlight or take vitamin D pills. Even though vitamin D-fortified dairy products, eggs and liver contain vitamin D, most foods are severely deficient in that vitamin and you are highly unlikely to meet your needs from your diet alone. Excessive exposure to sunlight can cause skin cancers, so it would be helpful to know the least amount of time that you have to expose your skin to sunlight to meet your daily needs for vitamin D.
Researchers have estimated the minimal amount of sun exposure necessary to have your skin make enough vitamin D to equal the same blood level of vitamin D3 that you would get from taking 400 or 1000 IU vitamin D for people living in Miami FL, and Boston MA, during the months of January, April, July, and October (J Am Acad Dermatol, June, 2010;62(6):929). You get the highest concentration of ultraviolet B rays, which are necessary for the skin to synthesize vitamin D, at high noon: * Boston: From April to October at 12 Noon EST, an individual with light skin, with 25 percent of the body surface area exposed, needs to spend 3 to 8 minutes in the sun daily to synthesize 400 IU of vitamin D. It is difficult to get enough sunlight to synthesize vitamin D during the winter in Boston. * Miami: An individual with light skin, with 25 percent of the body exposed, would need to spend 3 to 6 minutes at 12 Noon EST to synthesize 400 IU. Vitamin D synthesis occurs faster in individuals with lighter skin.
Sun Causes the Most Skin Damage at High Noon You do not want to expose your skin to sunlight at high noon because that is the time at which the sun causes the most skin damage per the amount of vitamin D the skin makes. In a study from Australia, researchers calculated the amount of sun exposure needed to have your skin make enough vitamin D to equal the same blood level of vitamin D that you would get from taking 200-600 IU of vitamin D pills per day for people aged 19-50 years with fair skin exposing 15 percent of the body (Med J Aust, 2006 Apr 3;184(7):338-41). Australia and Florida are located roughly the same distance from the equator. In January (mid-summer in Australia), 2-14 minutes of sun three to four times per week at 12:00 Noon was sufficient to ensure recommended vitamin D production. However, the skin can turn red, a sign of sun damage, in as little as eight minutes.
At 10:00 AM and 3:00 PM, your skin can make far more vitamin D and receive far less damage and redness than from noon sun exposure. This is the recommended time to expose your skin to reduce the risk for sun burn and skin cancer. October to March, about 10-15 minutes of sun exposure at around 10:00 AM or 3:00 PM three to four times per week should be enough for fair-skinned people across Australia to produce recommended vitamin D levels. Longer exposure times are needed from April to September, particularly in southern regions of Australia.
My Recommendations If you have very dark skin, it will be very difficult for you to meet your needs for vitamin D through sunlight exposure in most cities in North America. On the other hand, if your skin is fairly light, you probably can meet your needs for vitamin D all year round in Florida and in the summertime in most North American cities. Get a blood test for hydroxy vitamin D in January or February. If it is above 50 nmol/L (20 ng/mL), you are normal and do not need supplementation. If it is below that, you may need to take vitamin D pills during the winter.
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