UV light exposure versus vitamin D supplementation: A comparison of health benefits and vitamin D metabolism in a pig model

There is limited data on the effect of UV light exposure versus orally ingested vitamin D₃ on vitamin D metabolism and health. A 4-week study with 16 pigs (as a model for human physiology) was conducted. The pigs were either supplemented with 20 µg/d vitamin D₃ or exposed to UV light for 19 min/d to standardize plasma 25-hydroxyvitamin D₃ levels. Important differences were higher levels of stored vitamin D₃ in skin and subcutaneous fat, higher plasma concentrations of 3-epi-25-hydroxyvitamin D₃ and increases of cutaneous lumisterol₃ in UV-exposed pigs compared to supplemented pigs. UV light exposure compared to vitamin D₃ supplementation resulted in lower hepatic cholesterol, higher circulating plasma nitrite, a marker of the blood pressure-lowering nitric oxide, and a reduction in the release of pro- and anti-inflammatory cytokines from stimulated peripheral blood mononuclear cells. However, plasma metabolome and stool microbiome analyses did not reveal any differences between the two groups. To conclude, the current data show important health relevant differences between oral vitamin D₃ supplementation and UV light exposure. The findings may also partly explain the different vitamin D effects on health parameters obtained from association and intervention studies.