1. The response of 25-hydroxyvitamin D [25-(OH)D] to artificial ultraviolet irradiation applied to a known area of dorsal skin was investigated in 18 subjects, small quantities of ultraviolet energy being used. Ultraviolet irradiation was administered on days, 1, 3, 5, 8, 10, 12, 15 and 17, a total of 15 min being delivered over this time. In 15 subjects plasma 25-(OH)D showed a significant increase after a total of 15 min exposure but three subjects failed to demonstrate any increment. Plasma 25-(OH)D did not increase in any subject after 2·5 min of ultraviolet irradiation (irradiation on days 1 and 3).

2. Responses were compared in young and old, in male and female and in normal and osteomalacic subjects. No significant difference in response was found between these groups.

3. When plasma volume was taken into account, it was possible to calculate the increase in nmol of plasma 25-(OH)D/cm2 skin irradiated. This was 0·024 nmol/cm2 with no sex difference, over the 17 days of irradiation.

4. Exposure to ultraviolet irradiation over a small area of dorsal skin led to a rapid rise of plasma 25-(OH)D in most subjects with a subsequent plateau in subjects studied for up to 15 min total exposure. This contrasted with the prolonged increase in plasma 25-(OH)D continuing over several weeks in response to whole-body ultraviolet irradiation. This may indicate that cutaneous synthesis of vitamin D is rapid but limited, and that the considerable rise in plasma 25-(OH)D during whole-body irradiation may originate from vitamin D synthesized during the first few exposures.

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