Why sunshine is vital for a young horse’s health - and it’s pretty key for people too (especially now)

Horses grazing at Mill Ridge Farm in Kentucky. Some studies suggest horses require between two and eight hours in a field to obtain all their vitamin D requirements. Photo: University of Kentucky/Jimmy Henning

It takes up to 100,000 years for a photon to travel from the sun’s core where it is created, to the sun’s outer surface, and then a further eight minutes to travel the 99 million miles to Earth.

Only a small amount (ten percent) of the ultraviolet (UVB) radiation photons will make it through the atmosphere, the ozone layer capturing the rest, heating the atmosphere and stopping us all being fried. 

The light that makes it through coats the planet and every living creature walking on it. When it hits the skin of a horse (or human) it reacts with cholesterol in the skin and produces cholecalciferol (pre-vitamin D3). This is then synthesized by the liver to produce calcifediol, which then travels to the kidneys and is converted to calcitriol, an active form of D3.

Vitamin D3 – the sunshine vitamin - is important because it regulates calcium and phosphorus uptake. And that is important, because it is well known that calcium is utilized for strong bones. 

If vitamin D3 is synthesized from UVB light from the sun, though, where does that leave young racehorses who are stabled 24/7, often exercised in the dark, or early morning when UV light is weak, and even if trained late in the morning, are not out long enough to absorb sufficient sunlight to synthesize necessary levels of D3?

Around 40 percent of Americans have mild vitamin D deficiency. A chronic deficiency causes rickets with weak, poorly developed bowed and deformed limbs. This was treated and virtually eradicated with oral supplementation of D3 rich cod-liver oil in the early 20th century.

The literature on vitamin D3 is limited and sometimes contradictory where equines are concerned. 

Some studies suggest vitamin D deficiency could be common, some say it is rare, but since it is reckoned that a horse would require somewhere between two and eight hours in a field depending on weather and if rugged, and whether the horse is young and growing (an 18-month-old growing young horse could require up to 90 percent more vitamin D than a mature animal) to obtain all its vitamin D requirements, it is obvious that the young racehorse shall not obtain its D3 requirements in the way nature intended. 

In the absence of turnout, oral supplementation seems to be effective, and indeed most, though not all feed companies, list added vitamin D levels in their nutritional analysis. Hay and alfalfa contain highly variable levels of vitamin D2, depending on how it is prepared and its age, and modern farming methods may have a negative impact on levels. Also vitamin D2 is perhaps not processed by the body as efficiently as D3.

Bone strength

While oral supplementation of vitamin D3 is effective, it is thought that it does not provide the same bone strength as vitamin D3 synthesized naturally, and excessive oral over-supplementation can result in toxicosis, which can lead to calcification of organs and even death in very extreme cases. This does not happen when synthesized naturally through the skin as, when the animal has produced enough D3, it begins to synthesize other sterols.

In addition to vitamin D deficiency resulting in bone weakness, leading to possible increased predisposal to fractures, as well as poorer joint health, including physitis and increased risk of stress fractures and sore shins, it is linked to other conditions in the horse, with lesser or more degrees of certainty - ulcers, Inflammatory Airway Disease, exertional rhabdomyolysis, gait stiffness, appetite suppression, depression, loss of condition, immune suppression, poor muscle contraction. 

I have found no research linking vitamin D deficiency, or lack of natural D3 production, playing some role in bone bruising, but it would be logical to consider it may be a potential factor.

Anyone who has ever worked in a Thoroughbred stable knows these conditions are many of the most common problems affecting young Thoroughbreds in training. 

We know for certain that nearly all young Thoroughbreds in training, who do not have daily turnout, must be naturally deficient in vitamin D3. Oral supplementation is effective for most equines, but in the case of a young racehorse undergoing intense training, oral supplementation may prove inadequate. 

Other conditions

Wolff’s law states that bone remodels according to the stresses placed upon it. No horse’s bones are stressed more than a young Thoroughbred’s in training, yet it has been suggested that a lack of adequate daily sunlight - even with oral supplementation - produces an animal with poorer bone integrity.

With the myriad other conditions possibly linked to vitamin D deficiency, and indeed toxicity, this may be an area where further research could be strongly indicated.

Perhaps the use of UV lamps, to improve absorption through the skin, allowing endogenous synthesis of vitamin D3 naturally, could improve bone density in young racehorses in training. I am not aware of any studies that have addressed this, but in humans UV lamps have been shown to raise circulating vitamin D3 levels. It may, however, just be Wolff’s law again, horses outside run about more than stabled horses, stressing their bones in multiple ways, thus strengthening them.

In the winter of 2018-19, I was overseeing a string of horses at Payson Park in Florida. There was a 2-year-old filly, turning three. Twice she had been taken out of training. She would get too tight, too stiff to train. We stopped with her and turned her out daily for several hours. 

In Europe we call it Doctor Grass and prescribe paddock turnout for horses who are sore, off their feed, have ulcers, or for horses who have just trained off, and for many other reasons, including just a holiday.

After a few weeks, when she started bucking and kicking again and running around the paddock, I put her back in training, starting her off with a small amount of phenylbutazone daily to hopefully keep her feeling good.

She went through her exercise regime fine this time around and ended up winning her maiden special weight at Keeneland in the spring. At the time, I put her improvement down to rest, grass, phenylbutazone, and further maturity. Maybe though, Professor Sunshine had just been looking down and smiling on us all along.

There are a couple of points to bear in mind here for humans also. Studies on jockeys have shown, rather surprisingly, that they demonstrated relatively poor bone density. Cholesterol is a fat. Without its presence in the skin, vitamin D3 cannot be produced through exposure to sunlight. My son recently took part in a fitness assessment of jockeys at the University of Kentucky. He had the lowest body fat they had measured at 4.5%, (athletes average 6-14%, average male 18-25%). Jockeys may need to monitor their D3 levels too.

Finally, optimal vitamin D levels have been shown in studies to be protective in humans against many illnesses, including cold and influenza virus. There are also preliminary reports of vitamin D deficiency possibly being related to higher mortality rates in Covid-19 cases. 

So a bit of sunshine may be beneficial for both horses and humans at this time.

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