The ultraviolet radiation in sunlight, though a principal source of vitamin D3 compared to diet, is mutagenic.[1] Supplementing diet with vitamin D3 supplies vitamin D without this mutagenic effect,[2] but bypasses natural mechanisms that would prevent overdoses of vitamin D generated internally from sunlight. Sunlight is the 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] On the other 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.[4] Sun exposure has also been associated with the timing of melatonin synthesis and reduced risk of seasonal affective disorder.[5] A number of public health organizations state that there needs to be a balance between the risks of having too much and the risks of having too little sunlight.[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]
In the United States, 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 country.[8]
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.[2]
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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.[2]
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 also is effective.[11][17] Individuals with limited sun exposure need to include good sources of vitamin D in their diet or take a supplement.
There is some evidence that bright light exposure reduces seasonal affective disorder,[18] and it is a standard treatment for certain circadian rhythm sleep disorders.
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.[19] 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][20] 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.[21]
Prolonged optical exposure to sunlight, especially intense ultraviolet light, may be linked to cataracts and high levels of high energy visible light may be linked to age related macular degeneration. See also snow blindness.
According to a 2007 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]
There is no consensus on which component of ultraviolet radiation (UVA or UVB or UVC) actually is carcinogenic,[22] and the composition of ultraviolet radiation changes throughout the day: At high noon, ultraviolet radiation reaching ground level is 95% UVA and 5% UVB, while before 10am and after 2pm this percentage changes over time to 99% UVA and 1% UVB.[23] This is caused by the reflection of UVB rays back into space due to sun angle as the earth slowly rotates on its axis. The rate of change is faster the farther the position moves away from the equator (i.e. more north or south).[24]
On average over a day, 98.7% of the ultraviolet radiation that reaches the Earth's surface is UVA. UVC is almost completely absorbed by the ozone layer and does not penetrate the atmosphere in any appreciable quantities.[25] As a result, only the combination (UVA, UVB, and UVC) known as "ultraviolet radiation" is listed as a carcinogen, the components are only "likely to become" known carcinogens. Solar radiation, also known as "sunlight" is also listed as a carcinogen because it contains ultraviolet radiation. This means also that the UV Index is a measure of total ultraviolet radiation, and not just Vitamin D-producing UVB.[26]
Sunlight is therefore the only listed carcinogen that is 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.[22]
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.[27]
There are currently no recommendations on the total safe level of lifetime sun exposure.[12] According to epidemiologist Robyn Lucas at Australian National University,[28] analysis of lifespan versus disease shows that far more lives are lost to diseases caused by lack of sunlight than by those caused by too much.[29] Some scientists suggest that it is inappropriate to recommend total avoidance of sunlight.[30]
Most people receive between 50% and 80% of their lifetime sun exposure before the age of 18[31]. If one is fair skinned, 10 minutes of exposure to sunshine at high noon (in summer) will produce 10,000 IU of Vitamin D; darker skin requires longer exposure.[29] Note that summer peak daily UVB radiation can be one thousand times higher than winter peak daily UVB radiation in temperate regions. For example, in Boston, the summer solstice sun peak altitude is 71 degrees and the corresponding UVB radiation is 73% of max (90 degree sun altitude); the winter solstice sun peak altitude is 24 degrees and the corresponding UVB radiation is 0.03% of max (90 degree sun altitude).[32][33] The current recommendations for Vitamin D supplementation (between 200 IU and 400 IU)[34] are not based on sun exposure levels of Vitamin D production, but on fears of toxicity as each person's Vitamin D status depends on dozens of environmental and nutritional factors.[35] Because of this balance between internal production and external supplementation of Vitamin D, it is up to each individual to be aware of how they feel, and to consider sun exposure and Vitamin D status as part of their overall health.[36]
It has recently been discovered that vitamin D receptors are present in most if not all cells in the body. Additionally, experiments using cultured cells have demonstrated that vitamin D has potent effects on the growth and differentiation of many types of cells. These findings suggest that vitamin D has physiologic effects much broader than a role in mineral homeostasis and bone function. This is an active area of research and a much better understanding of this area will likely be available in the near future.[37]
A study in the February 2, 2005 issue of the Journal of the National Cancer Institute looked at markers of sun exposure in more than 500 people who had recently been diagnosed with malignant melanoma. The researchers found that solar elastosis, or sun damage to the skin, was independently associated with a surprising increased survival from melanoma.[38]
Sunscreen use within the last 10 years or during childhood was not associated with worse survival from melanoma. And all measures of sun exposure (i.e., history of severe sunburn, high levels of intermittent sun exposure, solar elastosis) were associated with improved survival from melanoma. Furthermore, participants who reported high skin awareness, but not those who reported skin examinations, had better survival.[38]
If confirmed, the results of this study suggest that whereas excessive sun exposure leads to the development of melanoma, sun exposure may protect against the progression of melanoma into more fatal disease.
There is ample evidence that the death rate in elderly patients increases in winter months. In a recent study of seasonal mortality in terminal cancer patients in the United States, it was found that compared to those patients who died during June, July, and August, the number of deaths of patients increased an average of 20% in January, February, and March. This near-sinusoidal pattern was remarkably consistent over a five year period.[39]
Mortality in the general population declines in the late summer to early fall months. In the Mediterranean countries, the lowest average daily mortality was observed in September. The fewest deaths were in August in Sweden and North America. The fewest deaths in Japan occurred in July. In the southern hemisphere, the lowest mortality in Australia occurred in March and in February for New Zealand.[40]
In Scotland, patients who suffer cardiopulmonary arrest (heart attack) in winter have a significantly lower likelihood of surviving. People who arrested in winter were 19% less likely to survive compared to those who arrested in summer.[41] For Atrial Fibrilation (AF), significantly more hospital admissions occurred in winter compared to summer.[42]
Deaths due to variceal bleeding in France occurred with a clear annual periodicity and peaked in winter (December, January) in the overall population. The distribution of cumulative monthly deaths differed by 24%, with a peak 14% above average in December and a trough 10% below average in July.[43] In the French Three-City study, high blood pressure, defined as a systolic blood pressure of 160 mmHg or higher, or a diastolic blood pressure of 95 mmHg or higher, was detected in 33.4 per cent of participants during winter and 23.8 percent during summer.[44]
The preceding details evidence that does not address direct sun exposure, but only seasonal variations in death rate. Factors involved in the observed increase in mortality events during Winter months, such as lower temperatures in Winter, must be kept in mind, as the corresponding increase in Hypothermia is a natural confounding mortality factor during the Winter.[45] Another confounding factor in Winter mortality events is a difference in atmospheric pressure.[46]
Over the past several years, levels of ultraviolet radiation have been tracked at over 30 sites across North America as part of the UV-B Monitoring and Research Program (UVMRP) at Colorado State University. The below image shows levels of UVB Radiation in June 2008.[47] UVMRP is a data collection and research program of the United States Department of Agriculture (USDA)[48]
UVB Levels in Vitamin D Equivalents:[49]
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