evidence: Providing vitamin D to confined sheep by oral supplementation vs ultraviolet irradiation - PMID: 2542212 - DOI: 10.2527/jas1989.673794x Providing vitamin D to confined sheep by oral supplementation vs ultraviolet irradiation Serial vitamin D3 (D3) and 25-hydroxyvitamin D3 (25 OH D3) concentrations of plasma were measured in confined, shorn sheep that had either been supplemented with vitamin D3 (50 micrograms/d) or exposed daily to ultraviolet irradiation (UVI). In the sheep administered D3 orally, plasma D3 increased continuously until d 35. This was followed by small fluctuations of the plasma D3 concentrations until a plateau was reached after 56 d of supplementation (.94 ng/ml plasma). Plasma 25 OH D3 concentrations increased continuously and plateaued between d 65 to 75 at about 21 ng/ml plasma. In the UVI sheep, plasma D3 and 25 OH D3 concentrations increased continuously for the first 49 d, then plateaued at 2.03 ng D3 and 29.6 ng 25 OH D3/ml. When a plateau was reached in plasma 25 OH D3 concentrations in both treatment groups, a 3H-labeled tracer dose of 25 OH D3 was given i.v., and disappearance of the 3H-labeled 25 OH D3 was followed. The UVI group had a faster decline in specific activities during the first exponential phase but a slower decline during the prolonged terminal elimination phase. These differences are reflected in the intercompartmental transfer rates.