Vitamin D - Why is it Called the "Sunshine Vitamin?"

By Juliet M. Getty, Ph.D.

Thirty to 90 minutes in the sun will give the average person all the daily vitamin D he/she needs. But it typically takes 5 to 8 hours of exposure to ultra violet light (which is still present, though to a lesser degree, on cloudy days) for horses to produce enough vitamin D. This increased time is due to many barriers including hair, blankets, fly sprays, coat conditioners, and reduced body oils (if recently bathed).

Why the “sunshine vitamin?”

The key is in the skin’s oils -- a derivative of cholesterol called 7-dehydrocholesterol. When exposed to sunshine, this compound is converted to cholecalciferol – which is then converted to the actual vitamin D, known as: 25-hydroxy-cholecalciferol, or D3, for short.

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 it travels to the small intestine to request, “Please increase calcium absorption from the diet.” If there isn’t enough calcium in the diet, D3 travels to the bones and asks, “Please release calcium into the bloodstream.”  Finally, D3 has one more area that it can influence – the kidney. Here, it delivers the message, “Don’t let calcium spill into the urine!” 

The goal of all these three messages is the same – increase blood calcium levels. Vitamin D accomplishes this three ways: (1) increases absorption, (2) removes calcium from bones, and (3) reduces urinary losses. Normal blood calcium levels help your horses’ bones, joints, and muscle function.

Isn’t there also a D2?

Yes. Vitamin D2 (known as 25-hydroxy-ergocalciferol) is found in plants. Vitamin D3 is found in animals. D2 is made from sunlight exposure, much in the same manner as D3 is in animals, except the original starting point is ergosterol (instead of 7-deydrocholesterol). Most vitamin supplements, however, contain the animal source – vitamin D3 – because it tends to be more stable and therefore has a longer shelf life. But when you 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

A deficiency is more common than you might think.

  • Horses that are kept indoors have the highest risk.
  • Frequent bathing with soap inhibits the body’s ability to produce vitamin D simply because the precursor (7-dehydrocholesterol) is washed away.
  • The reduced intensity of sunlight during the winter or at higher latitudes (starting with the upper one third of the U.S., into Canada) inhibits vitamin D production.

Reduced appetite, slowed growth, physitis in growing horses, bone demineralization (leading to stress fractures and bone deformities), and poor muscle contraction, are deficiency outcomes. 

Can a horse get too much?

Yes, and the signs are similar to a deficiency – reduced feed intake, poor growth, and an unthrifty appearance. Fortunately, toxic dosages of vitamin D are unusual. Even so, improper supplementation can cause excessively high intake. An upper limit of 44 IU/kg of body weight (22,000 IU for an 1100 lb horse) has been established. Check all your supplements and fortified feeds to make certain you’re feeding a safe amount. But don’t worry about turnout time — sunlight exposure cannot lead to excessive vitamin D production.


Horses do best when they receive at least 6.6 IU of vitamin D per kg of body weight. For an 1100 lb (500 kg) horse, this translates into 3300 IU/day. Sunlight exposure – 5 to 8 hours/day – under optimal conditions, will produce this amount of vitamin D.

Remember to:

  • Give your horse as much outdoor time as possible.
  • Allow pasture grazing, since hay has virtually no vitamin D remaining.
  • Avoid removing oils through excessive bathing with soap. Instead, rinse with plain water.
  • Allow “naked” outdoor time with no coat or fly sprays.


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