Time in the sun…we all enjoy it! We love basking in the rays of the sun, soaking up the warmth and the nutrition it provides. An average person can soak up their daily Vitamin D portion by sitting in the sun between 30 – 90 minutes. But, how long does our furry friend need to be in the sun to soak up the sun’s nutrition? It may be longer than you think!
Did you know that your horse’s coat alone creates a perfect barrier to Vitamin D absorption and that it typically takes between 5 – 8 hours of ultra violet light exposure for horses to produce enough Vitamin D. It may take even longer than that as we try to protect our horses against flies by applying fly spray and covering them up with fly sheets; bathing our horses to keep their coats clean and shinny and conditioning the coat to maintain moisture; and, by putting on sheets and blankets to protect them from the harmful ultra violet rays of the sun. All this kindness results in less Vitamin D being produced in our horses.
The simplicity of sunshine to Vitamin D conversion rests in the horse’s skin oils which contain 7-dehydrocholesterol. When exposed to sunshine, this compound is converted into Cholecalciferol which is then again converted into the actual form of Vitamin D (25-hydroxy-cholecalciferol or D3). D3 is produced in the horse’s kidney and the message sent is to ensure that there is the correct levels of calcium critical to the proper fuction of the horse’s joints, muscles and bones. The process is that ingested Calcium is first increased by the intestine and then, bones will be signalled to give up the calcium with the final message sent to the kidneys to reduce the calcium loss through urine.
Vitamin D deficiency is far more common than you may think. Horses that are kept indoors are of high risk of this deficiency. Bathing your horse frequently inhibits your horse’s ability to produce Vitamin D because of body oil being washed away so. The upper third of the USA and Canada, as we are at higher latitudes, have reduced sunlight during the winter months which in turn reduces Vitamin D production. In boarding facilities, turn out is limited and in some cases non-existent. Also, please keep in mind that Vitamin D does not survive in hay, so don’t think that your horse is getting any Vitamin D in eating hay.
Vitamin D deficiency shows as reduced appetite, slowed growth, bone demineralization and poor muscle contraction. Horses do best when they receive at least 6.6 IU of Vitamin D per kg of body weight, as an example, a 1100 pound horse (500 kg) will need at least 5 – 8 hours of sunlight exposure or 3300 IU of Vitamin D per day.
Can we over supplement with Vitamin D. The answer is yes with the toxicity signs being very similar to the deficiency signs, just to confuse things. The established upper Vitamin D limit is 44 IU/kg of body weight, or 22,000 IU for an 1100 pound horse. If you are concerned about your supplemental program, check all your supplements and fees to make sure that you are feeding a safe amount of Vitmain D. However, also keep in mind that excessive sunlight exposure cannot lead to an excessive Vitamin D production. So, you can feel comfortable keeping your horse outside as much as possible without any physical or chemical barriers to soak in the sun!