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
