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

•Differences in the metabolic routes of dietary-supplied vs. skin-produced vitamin D.•UV exposure was associated with higher levels of vitamin D3 in plasma and tissues.•3-epi-25-hydroxyvitamin D3 increased in response to UV exposure.•exposure to UV light affects nitric oxide concentrations independently of vitamin D.•UV exposure reduces cytokine release in blood cells independently of vitamin D. 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.