Sunlight And Vitamin D Definition and history of vitamin D
The first scientific description of a vitamin D deficiency, rickets, was provided in the seventeenth century (Whistler, 1645; Glisson, 1650).The healing effects of vitamin D, however, were not understood until the beginning of the twentieth century, the period between 1910 and 1930 when researchers were trying to identify the causes of rickets, an abnormal bone formation in children that results from inadequate bone calcium. Rickets is due to the failure to mineralize bone, thus causing osteomalacia, the softening of the bone structure. A dietary agent in cod liver oil was found to alleviate this problem and was named vitamin D. In 1923 an anti-rickets effect similar to that of cod liver oil was observed in ultraviolet (UV) radiation. Goldblatt and Soames (1923) observed that when a precursor of vitamin D in the skin, known as 7-dehydrocholesterol, was irradiated with sunlight or ultraviolet light, a substance
- Figure 4.1 Vitamin D production through skin photosynthesis.
equivalent to vitamin D was produced. Vitamin D comprises a group of fat-soluble seco-sterols, scarce in most foods but manufactured in the skin of vertebrates through photosynthesis of solar ultraviolet type B radiation (UV-B).
Ultraviolet light is divided into three wavelength spectra: UV-A, UV-B, and UV-C. The shortest and most potent of the three, UV-C (<280 nm), can burn the skin even at small doses. It is completely absorbed by the ozone layer. UV-A (320 nm to 400 nm), known as 'black light,' has a longer wavelength than B or C and is responsible for skin darkening and pigmentation; consequently, we refer to it as the tanning light. UV-B (290 nm to 315 nm) is responsible for photosynthesis and stimulates our skin to produce vitamin D (Figure 4.1). It is also responsible for skin burning and aging. The amount of UV-B present in UV light depends on the angle of incidence of the solar rays and is most prevalent during midday hours at higher latitudes (Figure 4.2). The content of UV-B rays in sunshine is greatest at latitudes of up to 30 degrees north and south of the equator. At an altitude of 1000 meters the content of UV-B rays is 15% higher, whereas the content of UV-A rays is almost the same as at sea level. At latitudes greater than 55 degrees, very little UV-B radiation reaches the earth's surface during the winter months.
UV-radiations (arbitrary units)
UV-radiations (arbitrary units)
UV-radiations (arbitrary units)
time an June 21
site: Washington,D.C.
Figure 4.2 Variation of UV radiation according to months and time of day (adapted from Serafino and Frederick, 1987; courtesy of William B. Grant).
time an June 21
site: Washington,D.C.
Figure 4.2 Variation of UV radiation according to months and time of day (adapted from Serafino and Frederick, 1987; courtesy of William B. Grant).
A much higher proportion of UV energy passes through glass from daylight radiation than from incandescent, halogen, or fluorescent light sources. Glass, however, filters out about 95% of the UV-B radiation present in the atmosphere. As a result, the occupants of buildings receive nine to ten times less UV-B radiation than if they were outside.
Continue reading here: Health And Spectral Quality Of Light
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