Vitamin D for Horse and Rider

by Kip Tulin, M.D.

With the days getting short and sunshine a bit more scarce, it's a good time to think about Vitamin D, our friend in good health. In preparing a talk on the subject for the monthly Working on Wellness Forum in Sequim, I have been reviewing current research and meeting with a few surprises along the way. It seems that most of us, because of lifestyle, latitude and diet  are not getting enough vitamin D, even if we pay attention to that sort of thing. On a recent afternoon at Freedom Farm I also started to wonder about horses and dogs and vitamin D, so I did a bit more research and thought I'd share.

Almost every creature with a back bone needs vitamin D. It is well known that vitamin D is essential for proper bone health. It is now coming to light (so to speak) that vitamin D is also important in heart, blood, and immune health as well. In humans, vitamin D has been shown to interact with over 3000 enzyme systems. Biology-wise we are not that far removed from our equine cousins so I'm sure this information pertains to them, too.

There are two main forms of vitamin D: D2 (ergocalciferol, derived mainly from plant sources) and D3 (cholecalciferol, derived mainly from animal sources). D3 is many times more powerful than D2. Amphibians, reptiles, birds and mammals can all make vitamin D3 in the skin by the action of ultraviolet light on precursor molecules that we all manufacture.  Humans can absorb the D3 directly from the skin into the lymph system and blood stream. Or we can ingest it in the form of vitamin D rich foods or supplements. Our furry, feathered, and scaled friends may use a combination of that mechanism (in exposed skin areas like the nose) along with something else that's clever: The shaded skin secretes the precursor in the form of an oil that works its way out to the top of the fur where it can be exposed to UV light and converted to D3. As the animal does its grooming and licking, the D3 is taken in orally. Pretty slick, huh?

Modern building-dwelling, clothes- and sunblock-wearing humans have very little UV exposure. Neither do horses that are kept in stables and/or blanketed. The herd-based, open pasture motif at Freedom Farm ensures that the horses get the 5-8 hours of sun exposure (even on cloudy days, by the way) they need to make enough vitamin D. Sun-cured hay provides an additional amount of D2, but this is relatively insignificant.  

OK....for us hairless apes (a.k.a. 'riders'), here's the bottom line: as outdoorsy as we are, even we don't get nearly enough D3 through UV exposure and diet. And there is a difference between the minimum daily requirement (the amount that will prevent obvious disease) and the optimal daily requirement (which will optimize bone, heart, and immune health). Research from centers like the University of California at San Diego suggest that to get an optimum level of 25-hydroxy vitamin D we should take supplemental vitamin D3 at a dose of about 35 i.u. per pound per day (or 75 i.u. per kilogram--you do the math).  So a 150-lb. person would take 4,750 i.u/day. This level is safe and well below the toxic range; yes, like everything else in life, you can overdo it if you go overboard.  

Two thumbs up for the horses who are doing a pretty good job on their own, thanks to being out in the pasture while hangin' with the herd.  Just for the record, veterinary research suggests that horses need a total of about 6.6 i.u. per kilogram of D3 per day.  Since horses have a hard time using calculators with their hooves, that would translate to a 1100 pound horse needing about 3,300 i.u. a day.  

And despite all of my reading, I still haven't figured out how the blind hairless mole rat, who NEVER gets exposed to sunlight and eats a diet devoid of D can function.  Isn't Nature wonderful!

How to get enough Vitamin D in the cold, rainy months.....?