Currently, the use of rations containing levels of 3 to 5 times the recommendations in vitamin D should be ensuring sufficiency. However, literature indicates that average serum concentration of vitamin D are insufficient or on the lower end of the reference range. Indeed, bone disease, including rickets, still occur in swine raised in modern operations. Traditional confinement practices limit exposure to sunlight and vitamin D synthesis, and vitamin insufficiency occurs even with dietary supplementation. The aim of this study was to determine the effect of limited sun exposure on serum concentration of vitamin D and the expression of vitamin D synthesizing enzymes in the liver and kidney of pigs on a vitamin D sufficient diet. White-pigmented grower pigs (29.7 ± 2.3 kg) fed 15% CP diet ad libitum providing >1,200 IU vitamin D3/kg of feed were exposed to sunlight for 1 h each day at solar noon for 14 d at the spring equinox (March pigs, n = 10) or summer solstice (June pigs, n = 5) and again before slaughter in June (March pigs) and September (June pigs). Blood for the analysis of 25(OH)D was collected before and after sunlight exposure. Traditionally housed pigs served as controls. After initial sun exposure, blood samples were collected from June pigs daily for 5 d and weekly for 8 wk to determine vitamin D3 and 25(OH)D decay, respectively. Kidney and liver samples were collected from the June pigs at slaughter after sun exposure for analysis of messenger RNA expression of vitamin D binding protein and synthesizing/degrading enzymes.
Average daily gain (ADG) was not influenced by sunlight exposure. June pigs had fewer days on feed, lower ADG and were slaughtered at a lighter weight. Exposure to sunlight increased 25(OH) vitamin D for all pigs. March pigs, had lower concentration of 25(OH)D than June pigs born on-farm. Initial sunlight exposure increased serum concentration of 25(OH)D in March pigs by 200% and June pigs by 67%. Serum concentration of vitamin D3 was decreased by 72 h with 25(OH)D decreased by wk 4 after exposure. Expression of vitamin D binding protein, vitamin D synthesizing CYP2R1, CYP27A1, CYP2D25, or degrading enzyme CYP24A1 were not influenced by sunlight exposure. Expression of CYP27B1 was decreased in the kidney but tended to be increased in the liver after sun exposure.
These results suggest limited sun exposure can efficiently increase serum concentration of vitamin D in growing pigs with varying levels of vitamin sufficiency. The lack of major changes in vitamin synthesizing enzymes suggests the 14-d exposure period did not saturate the capacity of slaughter-weight pigs to synthesize vitamin D.
Alexander, B. M., Ingold, B. C., Young, J. L., Fensterseifer, S. R., Wechsler, P. J., Austin, K. J., and Larson-Meyer, D. E. (2017). Sunlight exposure increases vitamin D sufficiency in growing pigs fed a diet formulated to exceed requirements. Domestic Animal Endocrinology, 59, 37-43. https://doi.org/10.1016/j.domaniend.2016.10.006