OsteoEd

What factors influence skin synthesis of vitamin D?

The skin synthesizes vitamin D when exposed to sunlight (UVB radiation), which is usually sufficient to meet the daily requirements. However, skin production of vitamin D3 decreases with age beginning in the third decade. One study showed that the 25-OH vitamin D3 levels were three times lower in elderly people age 62 to 80 than in people age 22 to 30 given the same sunlight exposure (1). In the elderly, diminished skin production is often compounded by lifestyle changes that decrease sun exposure. One study found that 54 percent of homebound elderly and 38 percent of nursing home patients were vitamin D deficient (2). Another study of a general medical ward revealed that 57 percent of the 290 patients had low levels of vitamin D (3).

Further variation in vitamin D synthesis occurs because sunlight exposure and UVB radiation absorption vary with degree of skin pigmentation, season, latitude, time of day, atmospheric conditions, and duration of exposure. It has been shown that cutaneous vitamin D synthesis does not occur from November through February in Boston, Massachusetts, and Edmonton, Alberta, Canada (4). However, vitamin D synthesis is fairly constant throughout the year at lower latitudes, as in Los Angeles, California, and San Juan, Puerto Rico (see Figure 1).

Figure 1. Photosynthesis of previtamin D3 after exposure of 7-dehydrocholesterol to sunlight. Measurements were: Boston, Massachusetts, (42 N) after 1 hour (symbol) and 3 hours (symbol); Edmonton, Alberta, Canada (52 N) after 1 hour (symbol); Los Angeles, California, (34 N) (symbol) and Puerto Rico (18 N) in January (symbol). Modified from Webb et al., 1988 (ref. 4)

Also, vitamin D synthesis is significantly decreased and sometimes completely stopped by the application of sunscreen. Studies have shown that chronic use of sunscreens can reduce serum vitamin D levels (5). Clothing, glass, plastic, and Plexiglas also prevent UVB radiation absorption (6, 7). Dark skin pigmentation makes some individuals more vulnerable to vitamin D deficiency at northern latitudes. In developed countries, fortification of dairy products with vitamin D has reduced the incidence of osteomalacia. However, persons who do not ingest adequate amounts of these foods are still at risk unless sunlight exposure is adequate.

Therefore, vitamin D deficiency is more common in people with minimal sun exposure (elderly, institutionalized/homebound, veiled or heavily clothed individuals), those who live at higher latitudes, and/or those who are without vitamin D-fortified foods.

1. Holick MF, Matsuoka LY, Wortsman J. Age, vitamin D and solar ultraviolet. Lancet 1989; 1: 1104-1105.
2. Gloth FM, Gundberg CM, Hollis BW, Haddad JG, Tobin JD. Vitamin D deficiency in homebound elderly persons. J Am Med Assoc 1995; 274: 1683-1686.
3. Thomas MK, Lloyd-Jones DM, Thadhani RI, Shaw AC, Deraska DJ, Kitch BT, Vamvakas EC, Dick IM, Prince RL, Finkelstein JS. Hypovitaminosis D in medical inpatients. N Eng J Med 1998; 338: 777-783.
4. Webb AR, Kline L, Holick MF. Influence of season and latitude on the cutaneous synthesis of vitamin D: Exposure to winter sunlight in Boston and Edmonton will not promote vitamin D3 synthesis in human skin. J Clin Endocrinol and Metab 1988; 67: 373-378.
5. Matsuoka LY, Wortsman J, Hanifan N, Holick MF. Chronic sunscreen use decreased circulating concentrations of 25-hydroxyvitamin D: A preliminary study. Arch Dermatol 1988; 12: 1802-1804.
6. Matsuoka LY, Wortsman J, Dannenberg MJ, Hollis BW, Lu Z, Holick MF. Clothing prevents ultraviolet-B radiation dependent photosynthesis of vitamin D3. J Clin Endocrinol Metab 1992; 75: 1099-1103.
7. Holick MF. McCollum Award Lecture, Vitamin D-New horizons for the 21st century. Am J Clin Nutr 1994; 60: 619-630.

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