The number of dairy cows without access to pasture or sunlight is increasing; therefore, the content of vitamin D in dairy products is decreasing. Ultimately, declining vitamin D levels in dairy products will mean that dairy products are a negligible source of natural vitamin D for humans. We tested the ability of a specially designed UVB lamp to enhance the vitamin D3 content in milk from dairy cows housed indoors. This study included 16 cows divided into 4 groups. Each group was exposed daily to artificial UVB light simulating 1, 2, 3, or 4 h of summer sun at 56°N for 24 d, and the group with simulated exposure to 2 h of summer sun daily continued to be monitored for 73 d. We found a significant increase in 25-hydroxyvitamin D3 (25OHD3) levels in plasma as well as vitamin D3 and 25OHD3 levels in milk after daily exposure for 24 d in all treatment groups. Extending daily exposure to artificial UVB light to 73 d did not lead to an increase of vitamin D3 or 25OHD3 level in the milk. In conclusion, the change in production facilities for dairy cows providing cows with no access to pasture and sunlight causes a decrease of vitamin D levels in dairy products. This decrease may be prevented by exposing cows to artificial UVB light in the stable.

The link between UV light (sunlight) and endogenous cholecalciferol (vitamin D ) synthesis in the skin of humans has been known for more than a 100 years, since doctors for the first time successfully used UV light to cure rickets in children. Years later, it was shown that UV light also had a significant effect on the cholecalciferol status in the body of cattle. The cholecalciferol status in the body is measured as the plasma concentration of 25-hydroxycholecalciferol, which in cattle and humans is the major circulating metabolite of cholecalciferol. Very little is, however, known about the quantitative efficiency of UV light as a source of cholecalciferol in cattle nutrition and physiology. Hence, the aim of this study was to determine the efficiency of using UV light for increasing the plasma 25-hydroxycholecalciferol concentration in cholecalciferol-deprived cattle. Twelve cows deprived of cholecalciferol for 6 months were divided into three treatment groups and exposed to UV light for 30, 90 or 120 min/day during 28 days. UV-light wavelengths ranged from 280 to 415 nm and 30-min exposure to the UV light was equivalent to 60-min average summer-sunlight exposure at 56 °N. Blood samples were collected every 3-4 days and analysed for 25-hydroxycholecalciferol and cholecalciferol. Results showed that increasing the exposure time from 90-120 min/day did not change the slope of the daily increase in plasma 25-hydroxycholecalciferol. Hence, it appears that cholecalciferol-deprived dairy cattle are able to increase their plasma 25-hydroxycholecalciferol concentration by a maximum of 1 ng/ml/day from UV-light exposure.