Vitamins: tiny organic compounds that have a huge impact on the health and well-being of your horse. Sometimes gleaned from the diet, sometimes manufactured within the digestive tract, vitamins have the power to promote and regulate virtually all of the body’s normal functions, and they need be present only in minute amounts.

There is still much we don’t know about vitamins, and much that is misunderstood. One of the most common misconceptions about vitamins is that “if some is good, more is better.” Horses can become vitamin-deficient, and these deficiencies can have devastating effects on a horse’s normal functions, but equally dangerous are toxicities from an “overdose”–a real possibility with some (but not all) of the vitamins. Further, different species have different vitamin requirements, so assumptions extrapolated from human medicine might not necessarily apply to horses. Vitamin requirements don’t really vary with the amount of work a horse does, either; the pleasure horse and high-performance athlete have almost identical needs. And while we frequently succumb to marketing ploys designed to convince us that our horses are in need of supplemental vitamins in their diet, the reality is that horses usually receive an excellent daily dose of the vitamins they require–those they cannot manufacture for themselves–from their forage (pasture or hay).

Researchers have classified vitamins into two categories that affect how the vitamins are absorbed, stored, and excreted by the body: fat-soluble and water-soluble. Vitamins A, D, E, and K are fat-soluble vitamins, which tend to be stored in the body (and thus can build up toxicities if there is an excess), while the B vitamins and vitamin C are water-soluble, meaning that any excess not used quickly by the body tends to be excreted rather than stored.

Vitamins also can be classified according to their source. Under normal conditions, the horse quite efficiently produces his own vitamins C, D, and niacin (one of the B-complex vitamins) from other organic molecules he ingests, and the beneficial microbes living in his cecum and large intestine, as part of their symbiotic bargain, produce all of the other B vitamins, as well as vitamin K. Only vitamins A and E are not produced within the horse’s body and must be sourced from vegetable matter in the diet