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Vitamin D: Does Your Horse Have Enough?

Vitamin D, aka The Sunshine Vitamin, is often overlooked or understudied when it comes to horse nutrition. For horses that are kept outside each day, all year round, they get sufficient levels to meet their requirements without additional supplementation. However for horses that are outside for less than 5 hours a day, supplementation is definitely needed. In fact it takes 5-8 hours of outside sun exposure for horses to produce enough Vitamin D. The key to producing enough Vitamin D3 lies in the skin's oils. Hence, if you are bathing your horse frequently with soap you are stripping the horse's ability to produce Vitamin D3. (1)

What is Vitamin D3 and why is it so important? The skin's oils contain a cholestral derivative called 7-dehydrocholesterol, that when exposed to sunshine it gets converted to cholecalciferol, which is then converted to Vitamin D3. D3 is simply a hormone, produced in the kidney, and increases calcium blood levels via increased absorption, removes calcium from bones, and reduces urinary losses. And as everyone knows we need calcium for bones, joints, proper muscle function. D2 is the plant form of Vitamin D, while D3 is the animal produced form. If a horse is eating grass, then he is ingesting D2 which acts the same way as D3 in the horse's body. In supplements you will find D3 because it is more stable and lasts longer. (1)

Deficiencies are common to horses stabled most of the time, or horses that are stalled during the day, out at night. Also horses that are bathed frequently, or during the winter months when the sun is low and limited, experience low D3. Deficiencies include reduced appetite, physitis in growing horses, slow growth, bone demineralization, fractures and deformities, and in many cases poor muscle contraction. Recommended daily requirements for most horses include 6.6 IU of vitamin D per kg of body weight, or 3300 IU/day for an 1100 lb (500 kg) horse(1,3) Horses that are outside for 5-8 hours per day (unbathed, no coat sprays or bug sprays, no outerwear) will fulfill their requirements of this vitamin.(1) However in hay, there is virtually none. So consider supplementation through feeds or specified supplements if this is the case. Horses cannot overdose on D3 when exposed to sun. However if over supplemented, signs of toxicosis include depression, anorexia, and limb stiffness.(3)

Does blanketing affect absorption? The verdict is still out on that, although some studies suggest no, that absorption is not affected. However, if the horse is getting enough D3 through diet, then it really doesn't matter if the horse covered up or not and how many hours it spends outside.(2)

But Wait! It doesn't stop here!

Recent studies are indicating that the benefits of vitamin D not only maintain good bone health, but also boost the immune function and squelch inflammatory airway disease (IAD) in horses. Horses with IAD often exhibit symptoms such as inflamed airways, mild, intermittent cough, nasal discharge, mucus accumulation in the trachea and poor athletic performance. General management strategies often include reducing dust in bedding and feeds, maximizing turnout and using anti-inflammatories and bronchodilators. Increasing Vitamin D3 in horses with IAD is now a new consideration! In one study circulating blood levels of Vitamin D were lower in horses with IAD; it is still inconclusive if this is the cause or the effect of IAD (3). Interestingly enough, lower levels of Vitamin D were associated with certain white blood cells being low that fight infection (neutrophils). KER suggests adding Omega-3 fatty acids to the diet as they have anti-inflammatory properties as well.(3)

If you suspect a Vitamin D deficiency, speak to your veterinarian and/or your nutritionist.

1. Vitamin D – Why is it called “The Sunshine Vitamin?” By Juliet M. Getty, Ph.D.

2. Do Blanketed Horses Get Enough Vitamin D? Kentucky Equine Research Staff, June 8, 2012

3. Vitamin D for Respiratory Health in Horses, Kentucky Equine Research Staff, February 29, 2016

Do you have a horse that requires a specialized diet? Do you need help in determining the best combination? Send an email to and share your story!


Jean Klosowicz, Equine Nutrition Consultant & Educator

Superior Equine Health and Nutrition Inc.

Bruce Mines, ON

fb: @SuperiorEquine


Healthy Horses. Happy Owners. Superior Results!

Superior Equine Health and Nutrition Inc. ~

The articles contained in this column are for the purpose of education and are not intended to take the place of proper veterinary care. They may be used in conjunction with such care to facilitate healing and maintain health of the horse.

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