By Juliet M. Getty, Ph.D.
 
Spending 30 to 90 minutes in the sun will give the average person all the required daily vitamin D their body needs. But for a horse, the hair coat alone creates such a significant barrier to absorption that it typically takes five to eight hours of exposure to ultraviolet light for horses to produce enough vitamin D to satisfy the daily requirement. Compound that with additional barriers, like fly spray, coat conditioner, a blanket or sheet, or decreased body oils due to bathing, and it becomes apparent that in some cases, horses may not be getting enough of this vital ingredient.

How does sunshine convert to vitamin D? The key is in the skin’s oils, which contain a derivative of cholesterol called 7-dehydrocholesterol. When exposed to sunshine, this compound is converted to cholecalciferol, also known as 25-hydroxy-cholecalciferol, or vitamin D3 for short.
 
Vitamin D3 is actually a hormone. A hormone, simply put, is a substance that is produced in one place and delivers a message to another place. D3 is produced in the kidney, and its message is to ensure correct blood calcium levels which are critical to the proper function of your horse’s bones, joints, and muscles. D3 looks first to increase absorption by the intestine of ingested calcium, then if necessary it will signal the bones to give up calcium, and finally, it will instruct the kidneys to reduce calcium losses through urine.
 
Vitamin D2, another form of vitamin D, is found in plants. Plants make D2 from sunlight exposure, much in the same manner as D3 is made in animals, except that D2 is associated with ergosterol rather than cholecalciferol. Most vitamin supplements contain the animal form – vitamin D3 – because it tends to be more stable and therefore has a longer shelf life. But when your horse eats fresh grass, he is getting the plant form. Once inside your horse’s body, they both have the same function.
 
Vitamin D deficiency is more common than you might think, and horses that are kept indoors are at the highest risk. The reduced intensity of sunlight during the winter, or at higher latitudes (starting with the upper one-third of the U.S. and extending north into Canada), inhibits vitamin D production. Frequent bathing with soap inhibits the body’s ability to produce vitamin D because the precursor in the body oil (7-dehydrocholesterol) is washed away. Pasture grazing is important, since vitamin D does not survive in hay.

Deficiency causes reduced appetite, slowed growth, physitis in growing horses, bone demineralization (leading to stress fractures and bone deformities), and poor muscle contraction.
 
Horses do best when they receive at least 6.6 IU of vitamin D per kilogram of body weight. For an 1100 pound (500 kilogram) horse, this translates into 3300 IU/day. Sunlight exposure — five to eight hours per day under optimal conditions — will produce this amount of vitamin D.
 
Vitamin D toxicity is unusual but possible, and, somewhat confusingly, the signs of toxicity are similar to deficiency: reduced feed intake, poor growth, and an unthrifty appearance. An upper limit of 44 IU per kilogram of body weight (22,000 IU for an 1100 pound horse) has been established. Improper supplementation can cause excessively high intake; check all your supplements and fortified feeds to make certain you’re feeding a safe amount.

The very good news is that sunlight exposure cannot lead to excessive vitamin D production. So give your horse as much time as possible outdoors, with minimal chemical or physical barriers, and let that vitamin D message work its wonders for the good of your horse.
 
Dr. Juliet Getty has taught and consulted on equine nutrition for more than 20 years. The Getty Equine Nutrition website (www.GettyEquineNutrition.com) offers helpful articles, a library of previous teleseminars and articles, and a nutrition forum. Subscribe on the website to her free and informative monthly e-newsletter, “Forage for Thought.” Dr. Getty serves as a distinguished advisor to the Equine Sciences Academy and her articles on equine nutrition are internationally published. She is available for individual consultations. Contact Dr. Getty directly at gettyequinenutrition@gmail.com or 740-663-2333 (Eastern Time Zone).

Main Article Photo: Giving your horse as much time as possible outdoors in the sunlight will help his body fulfill its Vitamin D requirements.