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 D 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 D or exposed to UV light for 19 min/d to standardize plasma 25-hydroxyvitamin D levels. Important differences were higher levels of stored vitamin D in skin and subcutaneous fat, higher plasma concentrations of 3-epi-25-hydroxyvitamin D and increases of cutaneous lumisterol in UV-exposed pigs compared to supplemented pigs. UV light exposure compared to vitamin D 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 D 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.