Vitamin D(3) synthesis in the entire skin surface of dairy cows despite hair coverage

How hair-coated animals such as dairy cows synthesize endogenous vitamin D(3) during exposure to summer sunlight has been unclear since vitamin D(3) and its relation to sunlight was discovered. The fur of fur-bearing animals is thought to be comparable to clothing in humans, which prevents vitamin D(3) synthesis in the skin during exposure to sunlight. Different scenarios have been suggested but never tested in cows; for example, that vitamin D(3) is synthesized from sebum on the hair and ingested by cows during grooming or that body areas such as the udder and muzzle that have scant hair exclusively produce the vitamin. To test different scenarios, 16 Danish Holstein dairy cows were subjected to 4 degrees of coverage of their bodies with fabric that prevented vitamin D(3) synthesis in the covered skin areas. The treatments were horse blanket (cows fitted with horse blankets), udder cover (cows fitted with udder covers, horse blanket + udder cover (cows fitted with both horse blankets and udder covers), and natural (cows without any coverage fitted). The cows were let out to pasture daily between 1000 and 1500 h for 4 wk in July and August 2009. Blood samples were collected 15 times during the study and analyzed for content of 25-hydroxyvitamin D(3) [25(OH) D(3)] indicative of the animals' vitamin D(3) status. Results showed that uncovered cows had a higher 25(OH)D(3) concentration in plasma after 28 d of access to sunlight compared with covered cows and that the plasma concentration of 25(OH)D(3) was strongly inversely correlated to the body surface area covered. These results are consistent with findings in humans, wherein the vitamin D(3) status of different individuals was inversely proportional to the amount of clothing worn during exposure to artificial sunlight. Hence, it appears that human clothing and cow hair are not comparable with respect to prevention of vitamin D(3) synthesis and that cows, like humans, synthesize vitamin D(3) evenly over their body surface. That vitamin D(3) should be synthesized from sebum on the hair and obtained by cows as a result of grooming is not supported by the findings in the present study either, because large differences were found between the treatment groups. If grooming were the source of vitamin D(3), then a relatively even 25(OH) D(3) concentration between treatments would be expected, because covered cows would obtain vitamin D(3) by grooming uncovered herdmates.