Solar ultraviolet-B (UVB) radiation is the primary source of vitamin D for most people. Skin pigmentation has adapted to solar UV doses where people live for a thousand years or more [Jablonski and Chaplin, 2010]. Skin pigmentation is dark in the tropical plains regions in order to reduce the adverse effects of UV such as free radical production, leading to skin cancer, and folate destruction; since UVB doses are high in the tropical plains areas, sufficient UVB reaches the 7-dehydrocholesterol to produce adequate amounts of vitamin D3 if people spend sufficient time in the sun. In the tropical forests, skin pigment is lighter since people are often shaded from full sun. As people migrated out of Africa, skin pigmentation progressively lightened to where it is very pale in Northern Europe. Those in Asia have a different skin pigmentation type, so their skin did not lighten as much at higher latitudes. Those living from 20 deg. to 40 deg have an additional ability to tan in order to better match the solar UV conditions as they change during the year.
Solar UVB doses have been found protective against a number of diseases. One way this is done is through the use of ecological studies. In ecological studies, geographical variations in disease outcome are compared to indices of risk-modifying factors. The first ecological study linking solar UVB doses to reduced risk of cancer was published in 1980 by the brothers Cedric and Frank Garland. They noticed that colon cancer rates were highest in the Northeast, lowest in the Southwest, and that the distribution was inversely correlated with annual solar radiation doses. They hypothesized that since vitamin D production is the primary physiological effect, vitamin D very likely reduced the risk of colon cancer. This hypothesis has been extended in subsequent ecological studies [Grant and Garland, 2006; Grant, 2012]. It has also been supported in observational studies [Gandini, 2011; Grant, 2010], mechanism studies [Fleet, 2012], and randomized controlled trials [Bolland, 2011; Lappe, 2007].
Alternatively, seasonal variations in disease outcome can also be used. Cannell used this approach to hypothesize that influenza was largely seasonal due to lower UVB and vitamin D production in winter [Cannell, 2006]. This hypothesis has been supported in randomized controlled trials [Aloia and Li-Ng, 2007; Urashima, 2010] and observational studies [Sabetta, 2010]. However, a recent paper reports that influenza has a frequency that more closely matches temperature rather than the seasonal variation in 25-hydroxyvitamin D [25(OH)D] concentrations [Shaman, 2011]. The reason for this finding is likely that influenza virus lives longer outside the body when the air is cold and has low humidity. However, one can affect vitamin D levels but not so much temperature and humidity.
So, what is reasonable regarding UV irradiance? The best time to produce vitamin D is when the sun is highest, both by time of day and by season. The solar elevation angle should be at least 45 degrees for efficient vitamin D production. Vitamin D production rates depend on many factors including skin pigmentation, amount of body surface irradiated, the angle the sun hits the body (e.g., whether standing or lying), and age (production efficiency decreases with increasing age). With whole-body exposure, a young person can make 10,000 IU vitamin D3 or more in a few minutes in the summer. Sunscreen reduces the production of vitamin D, so should not be worn when trying to produce vitamin D. If one gets some sunlight each day as the sun rises in spring, the skin adapts with a protection factor of two-to-four, partly through tanning, partly through developing a thicker outer layer (stratum corneum).
What about the risk of skin cancer and melanoma? For squamous cell carcinoma, integrated lifetime UVB irradiance is the most important risk factor. However, smoking is also an important risk factor, and diet plays a role, too. For basal cell carcinoma, both integrated and intermittent UV irradiance are important risk factors. For melanoma, sunburning is a risk factor. Those with pale skin and/or freckles have increased risk. Those who are chronically in the sun, such as through occupation, do not have a higher risk of melanoma than do those who do not. One of the risk reduction factors for melanoma is skin wrinkling or elastosis, either through UV irradiance or from smoking. Smokers have reduced risk of melanoma [Grant, 2008a]. It is not understood why elastosis reduces risk of melanoma. Diet also plays a role, with fatty foods increasing risk, fruits and vegetables reducing risk [Millen, 2004]. Vitamin D likely reduces risk of melanoma, too [Field, 2011].
Squamous cell carcinoma is relatively rare, but is a moderately deadly. Basal cell carcinoma is common, but very seldom fatal. Melanoma is moderately common, and has a moderate risk of death if not caught early. Those who develop basal cell carcinoma or squamous cell carcinoma in a location where it is warm enough are able to produce enough vitamin D to reduce risk of vitamin D-sensitive diseases such as many types of cancer [Grant, 2008b; Tuohimaa, 2007].
As for indoor tanning, lamps generally have a UV spectrum similar to that of midday, mid-latitude sunlight, so produce vitamin D as well [Moan, 2009].
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