08/26/2025
Vitamins and Minerals
Brian S. Burks, DVM
Diplomate, ABVP
Board Certified in Equine Practice
Vitamins and minerals are part of six essential ingredients required by horses. The others are water, carbohydrates, protein and fat. Horses that are not in heavy work are able to obtain sufficient amounts of these nutrients on good quality pasture and hay (water excepted). Horses only need small amounts of vitamins and minerals, but their lack can have a major impact on your horse’s health.
Vitamins
Vitamins are organic nutrients obtained by the diet or supplementation. They cannot be synthesized by the body. They have a diverse range of function as enzyme co-factors, hormones, and anti-oxidants. Vitamins can be either fat or water soluble.
Fat soluble vitamins are able only to dissolve in fat, not water. The fat soluble vitamins are A, D, E, and K, and are absorbed from the intestinal tract in small fat droplets. They are stored in the body’s fat tissue and liver. Hypervitaminosis is possible with these vitamins, so feeding only the recommended amounts is important, as too much can be harmful. The liver can hold a store of fat soluble vitamins to last as much as six months.
Vitamin A is a group of unsaturated nutritional organic compounds that includes retinol, retinal, retinoic acid, and several provitamin A carotenoids. Vitamin A has multiple functions: it is important for reproduction, growth and development, for the maintenance of the immune system, healthy skin, and good vision. The precursor to vitamin A is present in fresh grass, but rapidly lost from hay. A horse’s vitamin A requirement is 30-60 IU/kg of body weight per day.
Vitamin D is a group of fat-soluble secosteroids responsible for increasing intestinal absorption of calcium, magnesium, and phosphate, and multiple other biological effects. The most important compounds in this group are vitamin D₃ and vitamin D₂. Vitamin D is present in grass, alfalfa, and grass hay. Vitamin D becomes a hormone when activated and is responsible bone formation. Horses require at least 6.6 IU/kg/day or 3300 IU/day. This is produced with about 6-8 hours of sunlight per day.
Vitamin K is a group of structurally similar, fat-soluble vitamins found in foods and in dietary supplements. The body requires vitamin K for complete synthesis of certain proteins that are needed for blood coagulation and for controlling binding of calcium in bones and other tissues.
Vitamin E is a group of eight fat soluble compounds that include four tocopherols and four tocotrienols. It is found in grass and hay, but it decreases in hay over time. Vitamin E deficiency, which is uncommon and usually due to an underlying problem with digesting dietary fat rather than from a diet low in vitamin E, can cause nerve problems. Vitamin E is an anti-oxidant important for nerve function and protection. Its deficiency has been linked to equine degenerative myelopathy and equine motor neuron disease, especially in horses not on pasture, which has plenty of vitamin E, as levels will decrease over time in hay. Healthy horses require about 1-2 IU/kg/day of vitamin E.
Water soluble vitamins include Vitamin C and B, and are not typically stored in the body, so that daily intake is important.
B vitamins are a class of water-soluble vitamins that play important roles in cell metabolism. Though these vitamins share similar names, they are chemically distinct compounds that often coexist in the same foods. In general, dietary supplements containing all eight are referred to as a vitamin B complex. B-vitamins have a variety of functions, helping to maintain healthy skin and muscle, promoting cell growth, and enhancing the immune and nervous systems. There are eight B vitamins. Vitamin B7 is biotin, important in maintaining foot health; horses need at least 20mg per day. These vitamins are generally found in good quality forages and are made by the bacteria in the equine hindgut. Vitamin B12 synthesis requires adequate amounts of cobalt in the diet.
Supplementation of B vitamins might be useful during disturbances of the microbial population in the colon which can occur due to high grain rations, stress, or other illness.
Vitamin C, also known as ascorbic acid and l-ascorbic acid, is a vitamin found in various foods and sold as a dietary supplement. Vitamin C is an essential nutrient involved in the repair of tissue and the enzymatic production of certain neurotransmitters. It is a potent antioxidant and is involved in a variety of enzyme systems and is required to synthesize collagen found in tendon and ligaments. Deficiencies of this vitamin are rare as it is synthesized in the hind gut from glucose. Supplementation may be necessary with disturbances of the microbial population in the horse’s colon.
Minerals are defined as elements or chemical compounds that are normally crystalline and formed by geologic processes. Minerals support biochemical processes of the body and serve both structural and functional roles.
Calcium and phosphorus are important minerals of bone. The Ca:P ratio for adult horses is 1:1 to 2.5:1; phosphorus should not exceed calcium intake. Idle mature horses require about 20grams of calcium per day; this can double for a horse in regular heavy exercise.
About 99% of calcium is found in bones and teeth. Bone is 35% calcium. Calcium is important for muscle contraction, cell membrane function, blood coagulation, and the regulation of many enzymes. Alterations in Ca:P can result in the condition known as “big head”, “bran disease”, or “Miller’s disease” where the bones do not form properly and are soft; pathologic fractures can result.
Hypocalcemia, or low blood calcium, can result in hypersensitivity of the phrenic nerve, which passes over the cardiac base to innervate the diaphragm. Each time the heart contracts, the diaphragm will contract, resulting in a condition known as synchronous diaphragmatic flutter or “thumps”.
Iron binds oxygen within red blood cells and transports the oxygen throughout the body for other cells to use. Sulfur is important for structural proteins like those of articular cartilage and connective tissues. Zinc is an important co-enzyme in many systems. Copper is a co-enzyme and is important in elastic tissues such as tendons. Selenium is an anti-oxidant that works in tandem with vitamin E, and is important for the immune and muscular systems.
Minerals also function as electrolytes, which are minerals with charged particles known as ions when dissolved in water. These include sodium, potassium, and chloride. They are important for muscle contraction and conducting nerve impulses. Electrolytes also control the flow of fluids in the body, holding fluids in the vascular system and cells in a controlled fashion.
Sodium is an essential electrolyte that helps maintain the balance of water in and around cells. It is important for proper muscle and nerve function. It also helps maintain stable blood pressure levels. Insufficient sodium in blood is also known as hyponatremia, while too much is hypernatremia. The NRC states that about 50grams (100mg/kg) of sodium is needed per day, though horses only voluntarily eat about 60mg/kg of salt per day.
Potassium is an electrolyte necessary for proper muscle function. Along with other electrolytes such as sodium and calcium, potassium plays an important role in normal muscle contraction and relaxation. Elevated concentrations of potassium, a condition known as hyperkalemia, can be life-threatening.
Forages contain large amounts of potassium, so supplementation is only necessary in horses that sweat large volumes. Horses are obligate potassium secretors and will require supplementation if not eating.
In healthy horses, the kidneys filter out excess potassium from circulation, which is then excreted in urine. When the kidneys fail to clear potassium, or when too much potassium moves from inside to outside of cells, hyperkalemia can result. Abnormally high concentration of potassium in the blood causes muscles to become weak and an irregular heartbeat. At a greater risk of hyperkalemia are horses with a history of hyperkalemic periodic paralysis (HYPP), compromised kidney function, or massive cellular destruction, such as after trauma, burns, or a severe episode of rhabdomyolysis (tying-up).
Classic signs of hyperkalemia include muscle weakness, trembling, depression, and lethargy. While these symptoms are general, when accompanied by changes in cardiac rhythm, hyperkalemia should be suspected. Cardiac changes are not always present in cases of hyperkalemia, so the absence of cardiac abnormalities should not rule out such a diagnosis.
A little bit is good, but too much can be harmful. Vitamins can be toxic. Too much mineral can interfere with absorption and metabolism of other minerals. When offering multiple supplements it is important to total the amounts to be sure not to exceed requirements. Too much supplementation is dangerous for the body and is not economical.
Fox Run Equine Center
www.foxrunequine.com
(724) 727-3481