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Notes on Selenium

by Kath Mertens, DVM

     The other day as I was walking through a barn, a client stopped me to ask a question about a salt block she had just purchased for her horse.  “It says it contains selenium,” she said, “and that I should not feed it with any other selenium containing supplement.  Is that true?”

     The short answer is, “Yes, that is true.”  Explaining why takes a little longer, and broaches the topic of nutritional supplementation in general.

     Selenium is a micronutrient, necessary for survival but required only in very small quantities.  Together with Vitamin E it provides a crucial role as an antioxidant.

     Here I digress for a little explanation of veterinary school prerequisites.  For those of you interested in applying to veterinary school and wondering why you need to take all those Chemistry courses, it is so that you can understand terms like “antioxidant” when the time comes to explain them to your clients.  Like right now.

     One way to think of an antioxidant is as a recycling service.  Many, many chemical reactions in your body—or your horse’s body—release energy in a process called “oxidation”, which basically means that electrons are passed from one molecule to another.

     (NB: Physics is also a prerequisite, so that you have some understanding of what an electron is.  I never really liked physics, while my brother has a PhD in the subject.  Interestingly enough he doesn’t really like horses.  But I digress too far.)

     Oxidation is great when it releases energy into an organized chain reaction.  But sometimes oxidation will result in an energized molecule that is just bouncing around waiting to react with something—the wrong thing.  Like a cell membrane, or a protein.  Reacting with the wrong thing causes damage to said membrane or protein.  Enough uncontrolled damage results in disease and possibly death.

     Antioxidants come in and save the day before such chaos ensues.  When it meets a wayward energized molecule, the antioxidant not only takes the extra electrons and renders the molecule harmless, but it returns the electrons to a starting point where they can be recycled to create energy in appropriate reactions.

     Naturally I did not say all this in the barn aisle the other day.  I just explained that while selenium is a required supplement in these parts, it can be toxic in higher quantities, and it is important to read your supplement labels carefully.

     In these parts? you ask.  Yes, parts like the Pacific Northwest.  Selenium is found in soils and incorporated into growing plants, except where it is leached away by significant rainfall.  Here, in the land of rain-drenched soils, local forage (pasture and hay) tends to be low in selenium.  Some horses, especially late gestating and lactating mares, growing foals, or highly competitive athletes, need to have extra selenium provided in their diet to make up for what the local pastures are lacking.

     But you can have too much of a good thing.  In fact, selenium as a nutrient has what doctors call a “narrow margin of safety,” meaning that there is a relatively fine line between just enough selenium and too much.  Long-term, low-grade over-supplementation of selenium causes problems with hair and hoof growth.  Acute massive overdose of selenium causes blood vessel damage and sudden death.

     The take home message is, be careful with your supplements.  Horse owners love to supplement; but you should choose your supplements to meet a specific nutritional need.  There’s no sense in spending money on a supplement that your horse doesn’t need, and there is a legitimate danger of overdose if you add too many supplements with overlapping nutrients.  As my client did with her salt block, read the labels, and speak up if you have any questions.

   --Happy Trails!