The response of serum 25-hydroxyvitamin D concentrations to vitamin D intake and insolation in sheep.

1. Vitamin D-depleted, housed sheep were given diets providing fixed intakes of cholecalciferol ranging from 0.0 to 0.8 microgram/kg body-wt per d for 220 d. Thereafter they were shorn, deprived of dietary cholecalciferol and turned out from 30 June to 30 November. The concentration of 25-hydroxyvitamin D (25-OHD) in serum was determined at frequent intervals. 2. The serum concentrations of 25-OHD took approximately 10 weeks to stabilize after which they reflected dietary intake over 0.1--0.4 microgram/kg body-wt per d. For intakes of 0.4 and 0.8 microgram/kg body-wt per d the mean maximum concentrations were similar, but the rates of increase differed. The latter was proportional to the logarithm of intake over the range studied. 3. Changes in serum 25-OHD due to insolation were similar in all sheep regardless of their starting values, and consequently in some animals reached levels considerably greater than from the dietary source. 4. Although the response of serum 25-OHD to the higher dietary intakes appeared to be limited there was no evidence of any such control over the response ot endogenously-synthesized vitamin.