Lack of effect of exogenous calcitriol on the cutaneous production of vitamin D3.

Synthesis of vitamin D3 is determined primarily by cutaneous exposure to UV light irradiation (UVR). We studied the effect of exogenous 1,25-dihydroxyvitamin D on the cutaneous synthesis of vitamin D. Eight normal subjects, all caucasian, were randomly assigned to receive 1,25-dihydroxyvitamin D (0.5 micrograms daily) or no treatment for 20 days, followed 30 days later by the other treatment for 20 days. After 5 days, each experimental group (untreated and calcitriol treated) was exposed to one minimal erythema dose of UVR (UVR amplification test). Serum vitamin D3 concentrations reached peak levels 24 h after UVR exposure, and the mean values in the untreated and calcitriol-treated subjects were similar: 13.5 +/- 0.6 (+/- SD) ng/mL (35.1 +/- 1.5 nmol/L) and 13.0 +/- 0.6 ng/mL (33.8 +/- 1.5 nmol/L), respectively (P greater than 0.1). An identical 24-h post-UVR serum vitamin D3 value [13.5 ng/mL (35.0 nmol/L)] was found in a hypoparathyroid patient taking calcitriol (1 microgram daily). Thus, the cutaneous synthesis of vitamin D3 in response to acute UVR exposure was not inhibited by the administration of 1,25-dihydroxyvitamin D orally.