Vitamin D3 synthesis in the entire skin surface of dairy cows despite hair coverage

Abstract

How hair-coated animals such as dairy cows synthesize endogenous vitamin D3 during exposure to summer sunlight has been unclear since vitamin D3 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 D3 synthesis in the skin during exposure to sunlight. Different scenarios have been suggested but never tested in cows; for example, that vitamin D3 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 D3 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 1500h for 4 wk in July and August 2009. Blood samples were collected 15 times during the study and analyzed for content of 25-hydroxyvitamin D3 [25(OH)D3] indicative of the animals’ vitamin D3 status. Results showed that uncovered cows had a higher 25(OH)D3 concentration in plasma after 28 d of access to sunlight compared with covered cows and that the plasma concentration of 25(OH)D3 was strongly inversely correlated to the body surface area covered. These results are consistent with findings in humans, wherein the vitamin D3 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 D3 synthesis and that cows, like humans, synthesize vitamin D3 evenly over their body surface. That vitamin D3 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 D3, then a relatively even 25(OH)D3 concentration between treatments would be expected, because covered cows would obtain vitamin D3 by grooming uncovered herdmates.