Nutrition is the provision of nutrients to the cells in the body necessary to support life


Nutrition is the provision of nutrients to the cells in the body necessary to support life.  In horses, this is typically done through forages, grains, oilseeds, minerals, vitamins and water.  The assimilation of nutrients in the animal is a complex process and is often compartmentalized into individual nutrients and areas of digestion to aid in ease of understanding.  Below is a description of nutrients and their relevance to the functioning of the horse.


While not technically considered a nutrient, it must be given consideration due to its extreme importance in any biological system.  It is free, always available and generally taken for granted, but air is to be considered the first limiting nutrient because without it, death would ensue more quickly than the elimination of any other nutrient.  Given its importance, it is strange that it is given so little thought, until it becomes contaminated to the point of inducing disease, such is the case with COPD (chronic obstructive pulmonary disease).

Ventilation should be the first consideration in any closed facility (barn or arena), yet in many facilities, it would appear aesthetic appeal and anthropomorphic comfort take priority over proper ventilation.  While it may feel more comfortable to keep the barn warm during the winter, it is a disservice to the animal to allow air to stagnate and become polluted with particulate matter and ammonia.

Ammonia causes irritation of the mucosal lining of the airways in humans and horses and can permanently damage the animal’s ability to exchange gases in the lungs.  Considering the horse’s extreme oxidative capacity and the demands to draw on these in events of endurance and speed, it is especially important to ensure the air the horse breathes is of the highest quality.  Failure to accommodate this most basic need is sure to result in suboptimal performance and possibly respiratory disease.


Water makes up approximately 67% of the total body weight of a normal adult horse, making it the most abundant constituent of the horse’s body. Therefore an average sized horse of 450 kg contains approximately 300 L of water. Water provides the medium for the solubilisation and passage of a multitude of nutrients from the blood to the cells and the return of metabolic products to the blood. It also serves as the medium in which the vast number of intracellular metabolic reactions take place. Thus, maintenance of proper water balance in the horse is imperative. In horses, dehydration from disease, prolonged exercise, or transport impairs health, well-being, and physical and cognitive performance.

The routes by which water is lost from the body can vary according to environmental and physiological conditions such as ambient temperature and extent of physical exercise. At ambient temperature of 68 degrees F, about 1,400 mL of the 2,300 mL taken in is normally lost in the urine, 100 mL is lost in sweat, and 200 mL in the feces. The remaining 600 mL leaves the body as insensible water loss – evaporation from the respiratory tract and diffusion through the skin are examples of insensible water loss. During exercise, transportation or high environmental temperatures, the loss of water through the skin in the form of sweat can increase exponentially.

Maintenance of Electrolyte Balance

The term electrolyte refers to the anions and cations that are distributed throughout the fluid compartments of the body. They are distributed in such a way that within a given compartment, the blood plasma for example, electrical neutrality is always maintained, with the anion concentration exactly balanced by the cation concentration.

The cationic electrolytes of the extracellular fluid include sodium, potassium, calcium, and magnesium, and these are electrically balanced by the anions, chloride, bicarbonate, and proteins, along with relatively low concentrations of organic acids, phosphate, and sulphate. Most of the electrolytes are categorized nutritionally as macrominerals and are discussed below.

One of the more important factors determining the distribution of water among the water compartments of the body is osmotic pressure. The term osmosis is used to describe the movement of water from a solution with higher water concentration (lower solute) toward the solution with lower water concentration (higher solute). The amount of pressure required to exactly oppose osmosis into a solution across a semi-permeable membrane separating it from pure water is the osmotic pressure of the solution.

The theoretic osmotic pressure of a solution is proportional to the number of solute particles per unit volume of solution. This concentration is expressed in terms of osmolarity, or osmoles per liter, of solute particles. The importance of this in regards to the horse in particular is due to the composition of its sweat. Whereas, human sweat is considered to be hypotonic – less solute particles per liter of water in the sweat than is contained in the body; horses are the exact opposite and have a hypertonic sweat, which means there is more solute per liter of water relative to the internal fluid in the body.

While this may seem a trivial point, it provides insight into the importance of maintaining hydration and electrolyte balance in exercising horses; and also explains why after extensive sweating horses begin to lose the stimulus to drink water. When humans sweat and are not allowed access to water, the osmolarity of the blood increases, triggering hormone cascades which signal the brain to drink. When horses sweat and are not allowed to replenish their electrolytes, the opposite happens the osmolarity of the blood decreases, decreasing the desire to consume water and thus increasing the risk of further dehydration. In fact, if the horse is only given water during extensive exercise, the problem will be exacerbated, as it will speed up the drop in osmolarity of the blood and hasten the eventual lack of desire to drink water. Ergo the old adage, ‘you can lead a horse to water, but you can’t make it drink’. Many observers of exercising horses have noted after long bouts of exercise a horse’s refusal to drink water. As just explained, this is likely due to a large loss of electrolytes through sweat and lack of replenishment to the point that the thirst stimulus is impaired. Proper supplementation with electrolytes before, during and after strenuous exercise will help avoid this situation and keep your horse properly hydrated, ensuring maximal performance, enhanced recovery time and improved health.


Minerals are typically referred to as macro or micro minerals, the differentiation simply relating to the quantity needed in the diet. The macro minerals are typically required in gram quantities and are expressed as a percent of the total diet, whereas micro minerals are typically given in milligram (one thousandth of a gram) quantities and are typically expressed as a concentration of the total diet given as parts per million (ppm) or mg/kg.

Macro Minerals:

Macro minerals interact with each other and must be supplied in proper quantities and ratios to maintain appropriate animal function. The seven essential macro minerals are: calcium (Ca), phosphorus (P), sodium (Na), magnesium (Mg), potassium (K), sulfur (S), and chlorine (Cl).

Calcium is the most abundant divalent cation of the body, averaging about 1.5% of the total body weight. Bones and teeth contain about 99% of the calcium. The other 1% of the body’s calcium is distributed in both intracellular (inside the cell) and extracellular (outside the cell) fluids.

  • Structural component of bones and teeth, role in intracellular and hormonal secretion regulation, muscle contraction, blood clotting, and activation of some enzyme systems
    • Supplemental Sources
      • Calcium carbonate (limestone) – best choice
      • Calcium sulfate
      • Calcium oxide
      • Calcium amino acid proteinate
    • Deficiency
      • Rickets, osteomalacia, osteoporosis, tetany, parathyroid hyperplasia, stunted growth, laryngospasm
    • Max Tolerable Level
      • 2% of total dietary intake
      • Approximately 200 grams for 500 kg horse

Among the inorganic elements, phosphorus is second only to calcium in abundance in the body. Approximately 85% of the body’s phosphorus is in the skeleton, with the remainder associated with organic substances of soft tissue.

  • Structural component of bone, teeth, cell membranes, phospholipids, nucleic acids, nucleotide coenzymes, ATP-ADP phosphate energy transferring system in cells, participates in regulation of pH and osmotic pressure of intracellular fluids
    • Supplemental Sources
      • Monocalcium phosphate – best choice
      • Tricalcium phosphate
      • Defluorinated Phosphate
      • Because most supplemental forms of phosphorus are associated with calcium, it is important to balance phosphorus requirement before calcium
    • Deficiency
      • Neuromuscular, skeletal, hematologic, and renal manifestations; rickets, osteomalacia, anorexia
    • Max Tolerable Level
      • Ratio Ca:P; 1.2:1 or higher
      • 1 % of total dietary intake, assuming maintain Ca:P ratio
      • Approximately 100 grams for 500 kg horse

Magnesium as a cation in the body ranks fourth in overall abundance, but intracellularly it is second only to potassium. Approximately 60% of magnesium is in bone and remaining 40% in extracellular fluids and soft tissues.

  • Component of bones; role in nerve impulse transmission, protein synthesis, enzyme activation (in glycolysis and many ATP-dependent reactions).
    • Supplemental Sources
      • Magnesium oxide – best choice
      • Magnesium sulfate (Epsom salts)
    • Deficiency
      • Depression, muscle weakness, tetany, abnormal behavior, convulsions, depressed serum levels, growth failure
    • Max Tolerable Level
      • 0.8% of total dietary intake
      • Approximately 80 grams for 500 kg horse
      • Magnesium sulfate may have a laxative effect at lower concentrations.

98% of the body’s potassium is intracellular, making it the major intracellular fluid cation. Potassium influences the contractility of smooth, skeletal and cardiac muscle, and profoundly affects the excitability of nerve tissue. It is also important in maintaining electrolyte and pH balance.

  • Functions as an electrolyte; role in water, electrolyte and pH balances, cell membrane transfer
    • Supplemental Sources
      • In a diet with adequate forage, normally no supplementation required
      • Potassium carbonate
      • Potassium chloride
    • Deficiency
      • Profuse sweating, diarrhea or use of lasix (furosemide) or other diuretics main causes of deficiency, unlikely to encounter under normal dietary conditions
      • Muscular weakness, mental apathy; cardiac arrhythmias, paralysis, bone fragility, adrenal hypertrophy, decrease growth rate, weight loss
    • Max Tolerable Level
      • NRC (2005) states 1%, but this is far too low to be considered a maximum tolerable level
      • Unlikely to encounter excess, difficult to induce excess through dietary intake

Approximately 30% of the sodium in the body is located on the surface of bone crystals. From that site, it can be released into the bloodstream should low serum sodium levels develop (hyponatremia).

  • Functions as an electrolyte; role in water, pH, and electrolyte regulation, nerve transmission, muscle contraction
    • Supplemental Sources
      • Salt (sodium chloride)
      • Sodium bicarbonate (should not to be used in performance horses undergoing testing)
    • Deficiency
      • Anorexia, nausea, muscle atrophy, poor growth, weight loss
    • Max Tolerable Level
      • NRC (2005) states 2.4% maximum tolerable intake.
      • Horse unlikely to voluntarily consume this much under normal conditions

Chloride is the most abundant anion in the extracellular fluid. Approximately 88% of chloride is found in extracellular fluid, and just 12% is intracellular. Its negative charge neutralize the positive charge of sodium ions with which it is usually associated. In this respect, it is of great importance in the maintenance of electrolyte balance. Chloride has important functions in addition to its role as a major electrolyte. It is require for the formation of gastric hydrochloric acid, secreted along with protons from the parietal cells of the stomach. Also, it acts as the exchange anion in the red blood cell.

  • Functions as a major anion; maintains pH balance, enzyme activation, component of gastric hydrochloric acid
    • Supplemental Sources
      • Supplementation beyond normal salt consumption not usually necessary
      • Salt (sodium chloride)
      • Potassium chloride
    • Deficiency
      • Loss of appetite, failure to thrive, muscle weakness, lethargy, severe hypokalemia, metabolic acidosis, depraved appetite
    • Max Tolerable Level
      • Based on NRC (2005) max intake of salt of 6% of total, 3.6% chloride of total dietary intake
      • Unlikely to encounter issues, especially with free access to water

Component of sulfur containing amino acids, thiamin, biotin and lipoic acid

    • Supplemental Sources
      • Not recommended
    • Deficiency
      • Sulfur deficiency in horses has not been described
    • Max Tolerable Level
      • 0.5% of total dietary intake

Micro Minerals:

Micro minerals, or trace minerals, are present in animal body tissues in extremely low concentrations. They are nutrients required in small amounts, generally in milligram (mg) or microgram (ug) amounts per day, but play critically important roles in animal nutrition. There are 10 essential trace minerals recognized in animal nutrition: iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), selenium (Se), cobalt (Co), iodine (I), chromium (Cr), molybdenum (Mo), and nickel (Ni). Of these, it is rare to supplement Cr, Mo, or Ni, but the rest are normally included in horse feeds or mineral and vitamin premixes.

Over 65% of body iron is found in hemoglobin, up to about 10% is found as myoglobin, about 1% to 3% is found as part of enzymes, and the remaining body iron is found in the blood or in storage. Iron, a metal, exists in several oxidations states varying from Fe+6 to Fe-2, depending on its chemical environment. The only states that are stable in the aqueous environment of the body (and in food) are the ferric (Fe+3) and the ferrous (Fe+2) forms.

  • Necessary component of hemoglobin and myoglobin for oxygen transport and cellular use; facilitates transfer of electrons in electron transport chain
    • Supplemental Sources
      • Not recommended
      • Dicalcium phosphate – used for phosphorus supplementation contains iron
      • Iron oxide
      • Ferrous carbonate
    • Deficiency
      • Listlessness, fatigue, palpitations on exertion, dysphagia, anemia, decreased serum iron, hematocrit and hemoglobin, decreased resistance to infection
    • Max Tolerable Level
      • 500 mg/kg total dietary concentration
      • Some forages may contain higher levels than this, but there are no reports of iron toxicity from feeding higher levels from forages
      • Any form of iron supplementation is strongly discouraged and should only be done so under the supervision of a veterinarian

Zinc can exist in several different valence states, but it is almost universally found as the divalent ion (Zn+2). Zinc is found in all organs and tissues (primarily intracellularly) and in body fluids. Most of the zinc is found in bone, liver, kidney, muscle and skin. The primary role of Zn in the body appears to be related to its association with enzymes and proteins both as part of the molecule and as an activator. There are over one thousand known proteins associated with Zn.

  • Role in energy metabolism, protein synthesis, collagen and keratin formation, carbon dioxide elimination, sexual maturation, taste and smell functions.
    • Supplemental Sources
      • Bioplex Zinc – best choice
      • Zinc sulfate
      • Zinc oxide
    • Deficiency
      • Poor wound healing, subnormal growth, anorexia, changes in hair, hoof, skin inflammation, anemia, retarded development of reproductive system, low plasma zinc levels
    • Max Tolerable Level
      • Excessive intakes of Zn may aggravate marginal Cu and Fe deficiency
      • 500 mg/kg total dietary concentration

In most animal species, Cu is poorly absorbed; the extent of absorption is influenced by its chemical form and by a substantial number of interactions with other dietary factors. Dietary phytates, high levels of Ca, S, Fe, Zn, Cd, or Mo reduced absorption. Generally, not more than 5 to 10% of the Cu in the diet is absorbed by adult animals, while young animals may absorb 15 to 30%. Copper is required for cellular respiration, bone formation, proper cardiac function, connective tissue development, myelination of the spinal cord, keratinization, and tissue pigmentation. Copper serves as an essential catalytic co-factor of several physiologically important metalloenzymes. It is surpassed only by Zn in the number of enzymes which it activates. At least three Cu enzymes appear to have a role in antioxidant defense. These are the widely distributed intracellular and extracellular superoxide dismutases (SODs), extracellular ceruplasmin, and intracellular Cu thioneins. Due to the redox activity of Cu, this metal ion readily participates in the generation of hydroxyl radical, which damages nucleic acids, proteins, and membranes. Consequently, all cells must establish fine-tuned homeostatic mechanisms to allow cells to accumulate sufficient Cu for essential biochemical reactions, yet prevent the accumulation of Cu ions to toxic levels.

  • Required for proper use of iron by the body, amine oxidase, cytochrome oxidase; role in development of connective tissue – lysyl oxidase, ceruloplasmin, tyrosinase, melanin synthesis.
    • Supplemental Sources
      • Bioplex Copper – best choice
      • Copper sulfate
      • Copper oxide
    • Deficiency
      • Fall in serum copper and ceruloplasmin levels, anemia, neutropenia, leukopenia, bone demineralization, failure of erythropoiesis
    • Max Tolerable Levels
      • 250 mg/kg total dietary concentration

Perhaps more than any other essential trace element, selenium varies greatly in its soil concentration throughout the regions of the world. This, in turn, relates directly to its concentration in food plants. This heterogeneous distribution has had scientific benefit in that it has provided a clear correlation between those selenium-poor regions of the world and the incidence of disease associated with selenium deficiency. Selenium compounds are generally absorbed very efficiently in monogastrics. Selenocysteine and selenomethionine (organic selenium compounds) are almost 100% absorbed. In comparison, selenite (common form used in most horse feeds) has a lower apparent absorption ranging from 30% to about 60%. Selenium performs its functions mainly through selenoproteins. Approximately 30 to 35 selenoproteins can be detected in mammalian tissues. The biochemical reactions catalyzed by mammalian selenoproteins fall into three broad categories: 1) antioxidant defense systems, 2) thyroid hormone metabolism and 3) redox control of cell reactions

  • Protect cells against destruction by hydrogen peroxide and free radicals
    • Supplemental Sources
      • Sel-Plex – best choice
      • Sodium selenite
      • Sodium selenate
    • Deficiency
      • Myalgia, cardiac myophathy, increased red blood cell fragility, pancreatic degeneration, white muscle disease, suppressed immune function
    • Max Tolerable Levels
      • 5 mg/kg total dietary concentration
      • Regulated in many countries so that dietary concentration may not exceed 0.3 mg/kg total dietary concentration. Approximately 3 mg/day for a 500 kg horse.

No Cr-dependent enzyme has been identified. However, Cr is an essential trace element due to its function as a cofactor involved in activation of insulin. The biological action of chromium is believed due to its complexing with nicotinic acid and amino acids to form the organic compound glucose tolerance factor (GTF). GTF is thought to initiate the disulfide bridging between insulin and the insulin receptor. The effectiveness of insulin is greater in the presence of chromium than in its absence. Thus the primary function of GTF is to potentiate insulin action, thereby affecting cellular glucose uptake, and intracellular carbohydrate and lipid metabolism.

  • Normal use of blood glucose and function of insulin
    • Supplemental Sources
      • Biochrome – best choice
      • Chromium propionate
      • Chromium picolinate
    • Deficiency
      • Glucose intolerance, abnormalities in glucose and lipid metabolism, elevated circulating insulin, glycosuria
    • Max Tolerable Levels
      • NRC (2005) does not deal with Cr requirements or toxicity
      • Hexavalent Cr is much more toxic than the trivalent form and is absorbed more efficiently.
      • Acute systemic Cr intoxication is rare but was produced with a single oral dose of 700 mg/kg of body weight Cr (VI) in mature cattle and 30-40 mg/kg of body weight Cr (VI) in young calves.
      • This would equate to a dose of 350,000 mg for a 500 kg horse or 35,000 mg/kg total dietary concentration.
      • An advisable maximum limit to be followed would be 5 mg/kg total dietary concentration

Iodine is unique among the required trace elements in that it is a constituent of the thyroid hormones thyroxine and triiodothryonine. Iodine deficiency is accepted as the most common cause of preventable mental defects in the world today. In humans and farm animals, iodine deficiency is one of the most prevalent deficiency diseases, and it occurs in almost every country in the world. The only known role of iodine is in the synthesis of the thyroid hormones. Thyroxine contains about 65% iodine. Thyroid hormones have multiple functions as regulators of cell activity and growth. They have an active role in thermoregulation, intermediary metabolism, reproduction, growth and development, circulation and muscle function; they control the oxidation rate of all cells. An increase in thyroid hormone levels results in an increase in the basal metabolic rate. Selenium deficiency will have a role in the control of thyroid hormone metabolism. The deiodinating enzyme, which produces most of the circulating T3 is a selenoenzyme with most of the activity occurring the liver, kidney and thyroid. Selenium also plays an indirect role in the control of thyroid hormone synthesis because it is required by another selenoenzyme GSH-Px. In the thyroid, GSH-Px is thought to be the main antioxidant system for neutralizing cytotoxic hydrogen peroxide and its oxidative by-products. Hydrogen peroxide is produced by the thyroid as a cofactor in thyroid hormone synthesis. High iodine intake when Se is deficient may initiate thyroid tissue damage as a result of low thyroidal GSH-Px activity during thyroid stimulation.

  • Thyroid hormone synthesis – metabolic regulator
    • Supplemental Sources
      • Calcium Iodide
      • EDDI
    • Deficiency
      • Enlargement of thyroid gland, myxedema, cretinism, increase in blood lipids, liver gluconeogenesis, and extracellular retention of salt and water.
    • Max Tolerable Intake
      • 5 mg/kg total dietary concentration

At the molecular level, manganese, like other trace elements, can function both as an enzyme activator and as a constituent of metalloenzymes, but the relationship of these functions to the gross physiologic changes observed in manganese deficiency is not well correlated. In the activation of enzyme-catalyzed reactions, the metal can act by binding to the substrate (such as ATP) or the enzyme directly, with induction of conformational changes. Enzymes that can be activated by manganese in this manner are numerous and diverse in function. They include hydrolases, kinases, decarboxylases, and transferases. The activity of most of these enzymes is not, however, affected by a manganese deficiency, largely because the activation is not manganese specific.

  • Essential for normal brain function; role in enzyme systems, collagen formation, bone growth, urea formation, fatty acid and cholesterol synthesis, and protein digestion
    • Supplemental Sources
      • Bioplex manganese – best choice
      • Manganese sulfate
      • Manganese oxide
    • Deficiency
      • Impaired growth, skeletal abnormalities, impaired function of central nervous system, defects in lipid and carbohydrate metabolism.
    • Max Tolerable Intake
      • 400 mg/kg total dietary concentration

Cobalt must be supplied in the diet of monogastric animal species and humans in its active form, vitamin B12. When these species receive adequate dietary vitamin B12 there is no convincing evidence of a requirement for or benefit from dietary Co. Cobalt is, however, a dietary essential for ruminants; ruminal, microogranisms incorporate Co into vitamin B12.

  • The only known function of Co is its participation in metabolism as a component of vitamin B12.
    • Supplemental Sources
      • Vitamin B12
      • Cobalt carbonate
    • Deficiency
      • Technically would result in a B12 deficiency. There are no known cases of either cobalt or vitamin B12 deficiency in horses.
      • Symptoms in other species include megaloblastic anemia, degeneration of peripheral nerves, skin hypersensitivity, glossitis
    • Max Tolerable Level
      • 25 mg/kg total dietary concentration

Other Trace Elements

In more recent years, the trace minerals boron (B), lithium (Li), nickel (Ni), vanadium (V), silicon (Si), tin (Sn), fluorine (F), bromine (Br), germanium (Ge), and rubidium (Rb) have been shown to be essential for various species or the data to date is highly suggestive of essentiality. A further 20 to 30 trace elements occur regularly in feeds and animal tissue, and it is unknown whether they serve some useful purpose or are merely contaminants.

Vitamins are organic compounds that typically function as parts of enzyme systems essential for many metabolic functions. They are classified into fat soluble and water soluble vitamins.

Fat Soluble Vitamins

The term vitamin A is used to refer to retinol and retinal. Retinoic acid is a metabolite of retinal. The term provitmain A refers to beta-carotene and other carotenoids that can be converted into retinol. Vitamin A is recognized as being essential for vision, and for systemic functions including cellular differentiation, growth, reproduction, bone development, and the immune system.

  • Synthesis of rhodopsin and other light receptor pigments; unknown metabolites involved in growth and differentiation of epithelia, nervous, bone tissue and immune function
    • Supplemental Sources
      • Vitamin A 1,000
    • Deficiency
      • Poor dark adaptation, xerosis, keratomalacia, growth failure, night blindness
    • Max Tolerable Level
      • 16,ooo IU/kg of dry matter intake

Calcitriol, 1,25-(OH)2D3, is considered the active form of vitamin D and functions like a steroid hormone. Initially the target tissues of the vitamin were believed to be limited to the intestine, bone and kidney. The presence of specific receptors for the hormone in many other tissues, however, supports that calcitriol acts in a wide variety of tissues, including the heart, brain, and stomach. Calcitriol plays a role in the parathyroid hormone (PTH)-directed homeostasis of blood calcium concentrations, which impacts several tissues including the intestine, bone and kidney. Hypocalcemia stimulates secretion of PTF from the parathyroid gland. The PTH, in turn stimulates 1-hydroxylase in the kidney such that 25-OH D3 is converted to calcitriol. Calcitriol then acts alone or with PTH on its target tissues, causing serum calcium and phosphorus concentrations to rise.

  • Regulator of bone mineral metabolism, primarily calcium. Cell growth and differentiation
    • Supplemental Sources
      • Vitamin D 500
    • Deficiency
      • Rickets, osteomalacia
    • Max Tolerable Level
      • 44 IU/kg BW/d
      • Equivalent to 22,000 IU/day for 500 kg horse
      • 2200 IU/kg of dry matter intake

Vitamin E includes eight compounds synthesized by plants. These compounds fall into two classes: the tocols, which have saturated side chains, and the tocotrienols (also called trienols), which have unsaturated side chains. All compounds are designated as alpha, beta, gamma, or delta, and possess characteristic biological activity. Another compound, all-rac α-tocopheryl acetate, with vitamin E activity is used in fortification of feed. The principal function of vitamin E is the maintenance of membrane integrity in body cells. The mechanism by which vitamin E functions to protect the membranes from destruction is through its ability to prevent the oxidation (peroxidation) of unsaturated fatty acids contained in the phospholipids of the cellular membranes

  • Antioxidant
    • Supplemental Sources
      • Vitamin E 50%
    • Deficiency
      • White muscle disease, neuropathy and myopathy
    • Max Tolerable Level
      • 1,ooo IU/kg of dry matter intake

Several compounds possess vitamin K activity; these compounds all have a 2-methyl 1,4-naphthoquinone ring. The naturally occurring forms of vitamin K are phylloquinone (K1), isolated from plants, and menaquinones (K2) synthesized by bacteria. Menadione (K3) is not found naturally but is a common synthetic form of vitamin K that must be alkylated for activity. Vitamin K is necessary for the posttranslational carboxylation of specific glutamic acid residues to form γ-carboxyglutamate on 4 of 13 factors required for the normal coagulation of blood. The 4 vitamin K-dependent factors include factors II (pro-thrombin), VII, IX, and X.

  • Activates some blood clotting factors; carboxylates bone and kidney proteins
    • Supplemental Sources
      • Menadione (K3)
    • Deficiency
      • Defective blood clotting
    • Max Tolerable Level
      • 2 mg/kg of dry matter intake – although other research indicates much higher levels can be safely fed.

Water Soluble Vitamins

No maximum tolerable level is given for the water soluble vitamins as they are generally very safe and little evidence is available of toxicity.

Other than thiamin and riboflavin, there are no requirements for the B-vitamins in horses. Recommended concentrations of the B-vitamins, besides thiamin and riboflavin, are based on literature in other species and personal experience.

At the cellular level, thiamin plays essential roles in: 1) energy transformation; 2) synthesis of pentoses and NADPH (a coenzyme form of niacin, nicotinamide adenine dinucleotide phosphate in a reduced form); and 3) membrane and nerve conduction. Thiamin diphosphate (TDP) functions as a coenzyme necessary for the oxidative decarboxylation of both pyruvate and alpha-ketoglutarate. These reactions are instrumental in generating ATP. Inhibition of these decarboxylation reactions prevents synthesis of ATP and acetyl CoA needed for the synthesis of, for example, fatty acids, cholesterol and other important compounds and results in the accumulation of pyruvate, lactate, and α-ketoglutarate in the blood.

  • Oxidative decarboxylation of α-keto acids and 2-keto sugars
    • Deficiency
      • Beriberi, muscle weakness, anorexia, tachycardia, enlarged heart, edema
    • Recommended Dietary Concentration
      • 5 mg/kg of dry matter intake
      • NRC is 5 mg/kg for working and 3 mg/kg for all other classes

Most of the riboflavin in tissues is first converted to one of its coenzyme forms. Synthesis of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) appear to under hormonal regulation. Hormones shown to be particularly important in this regulation are ACTH, aldosterone, and the thyroid hormones, all of which accelerate the conversion of riboflavin into its coenzyme forms apparently by increasing the activity of flavokinase. FMN and FAD function as cofactors for a wide variety of oxidative enzyme systems and remain bound to the enzymes during the oxidation-reduction reactions. Flavins can act as oxidizing agents because of their ability to accept a pair of hydrogen atoms

  • Electron (hydrogen) transfer reactions
    • Deficiency
      • Cheilosis, glossitis, hyperemia and edema of pharyngeal and oral mucous membranes, angular stomatitis. Rough hair coat; atrophy of the epidermis, hair follicles, and sebaceous glands; dermatitis; vascularization of the cornea; catarrhal conjunctivitis; photophobia
    • Recommended Dietary Concentration
      • 6 mg/kg of dry matter intake
      • NRC is 2 mg/kg of dry matter intake, but expressed as body weight 0.04 mg/kg BW

The term niacin is considered a generic term for nicotinic acid and nicotinamide (also called niacinamide). Approximately 200 enzymes, primarily dehydrogenases, require nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP). Most of these enzymes function reversibly. Although NAD and NADP are very similar and undergo reversible reduction in the same way, their functions are quite different in the cell. The major role of NADH, for from NAD, is to transfer electrons from metabolic intermediates through the electron transport chain, thereby producing adenosine triphosphate (ATP). NAPH, in contrast, acts as a reducing agent in many biosynthetic pathways such as fatty acid synthesis.

  • Electron (hydrogen) transfer reactions
    • Deficiency
      • Pellagra, diarrhea, dermatitis, mental confusion, or dementia
    • Recommended Dietary Concentration
      • 22 mg/kg of dry matter intake

One of the primary functions of pantothenic acid relates to its role as a component of CoA. The synthesis of CoA requires pantothenic acid, cysteine, and ATP. As a component of CoA, pantothenic acid becomes essential for production energy from carbohydrate, fat and protein.

  • Acyl transfer reactions
    • Deficiency
      • Deficiency very rare: numbness and tingling of extremities, fatigue
    • Recommended Dietary Concentration
      • 13 mg/kg of dry matter intake

Biotin functions in cells covalently bound to enzymes. These enzymes replenish oxaloacetate for Krebs cycle, necessary for gluconeogenesis; commit acetate units to fatty acid synthesis; provide mechanism for metabolism of some amino acids and odd-numbered chain fatty acids; succinate formed enters Krebs cycle; allows catabolism of leucine and certain isoprenoid compounds.

  • CO2 transfer reactions; carboxylation reactions
    • Deficiency
      • Severe dermatitis, inflammation
    • Recommended Dietary Concentration
      • 3 mg/kg of dry matter intake

Pyridoxine may be converted into pyridoxine phosphate (PNP) within the intestinal cells, likewise pyridoxal is typically converted pyridoxal phosphate (PLP). PN & PNP may be converted to PLP in the liver. PLP is the main form of the vitamin found in the blood. Other forms of the vitamin, especially PL also may be present in the blood. The coenzyme form of vitamin B6 is associated with a vast number of enzymes, the majority of which are involved in amino acid metabolism.

  • Transamination and decarboxylation reactions
    • Deficiency
      • Dermatitis, glossitis, convulsions
    • Recommended Dietary Concentration
      • 2 mg/kg of dry matter intake

Folate and folacin are generic terms for compounds that have similar chemical structures and nutritional properties similar to those of folic acid. Folic Acid and subsequently dihydrofolate are both reduced by dihyrofolate reductase, a cytosolic enzyme, to generate tetrahydrofolate (THF). THF accepts one-carbon groups from various degradative reactions in amino acid metabolism. These THF derivatives then serve as donors of the one-carbon units in a variety of synthetic reactions.

  • One carbon transfer reactions
    • Deficiency
      • Megaloblastic anemia, diarrhea, fatigue, depression, confusion
    • Recommended Dietary Concentration
      • 0.3 mg/kg of dry matter intake

Vitamin B12 is considered a generic term for a group of compounds called corrinoids because of their corrin nucleus. The corrin is a marocyclic ring made of four reduced pyrrole rings linked together. The corrin of vitamin B12 has an atom of cobalt in the center of it.

  • Methylation of homocysteine to methionine; conversion of methylmalonyl CoA to succinyl CoA
    • Deficiency
      • Megaloblastic anemia, degeneration of peripheral nerves, skin hypersensitivity, glossitis
    • Recommended Dietary Concentration
      • 0.022 mg/kg of dry matter intake
      • 22 µg/kg of dry matter intake

The only functional role of vitamin C categorically established is its ability to prevent and/or cure scurvy. In this role, however, it affects to some extent every body function. For example, normal development of cartilage, bone, and dentine depends on an adequate supply of vitamin C. In addition, the basement membrane lining the capillaries, the “intracellular cement” holding together the endothelial cells, and the scar tissue responsible for wound healing all require the presence of vitamin C for their formation and maintenance.

  • Antioxidant, cofactor of hydroxylating enzymes involved in synthesis of collagen, carnitine, norepinephrine. Horses can produce endogenous vitamin C, no evidence supplementation is needed.
    • Deficiency
      • Scurvy, loss of appetite, fatigue, retarded wound healing, bleeding gums, spontaneous rupture of capillaries
    • Recommended Dietary Concentration
      • 100 mg/kg of dry matter intake

Choline is an essential material for building and maintaining cell structure. It is a constituent of lecithins which are fatty substances (lipids) with one of the three fatty acid molecules replaced by choline which is joined to the glycerol part of the molecule through a phosphoric acid linkage. The free choline pools from which the neurotransmitter acetylcholine is synthesized is maintained through several mechanisms, one of which is the enzymatic hydrolysis of lecithin and sphingomyelin. Choline can be synthesized in liver providing there is a sufficient supply of methionine. Choline plays an essential role in fat metabolism in the liver. It functions by preventing abnormal accumulations of fat by converting excess fat into lecithin or by increasing the utilization of fatty acids in the liver.

  • Neurotransmission, fatty acid metabolism in liver, cell structure
    • Deficiency
      • Unlikely
    • Recommended Dietary Concentration
      • 50 mg/kg of dry matter intake


Carbohydrates are biochemical compounds composed only of the elements carbon, hydrogen and oxygen.  They represent the single largest source of energy for horses.  They come in many different forms, which will impact the overall response of the horse to feeding.  Carbohydrates are polymers made of basic sugar units, such as glucose, fructose, galactose, etc.  The two major classes of carbohydrates in plants are
known as non-structural and structural.  Those that serve as storage and energy reserves and that are available for more rapid metabolism to supply energy (e.g., sugars, starch and pectin) are referred to as non-structural.  Those carbohydrate fractions that are not used for energy storage and provide fiber and anatomical features for rigidity and even water transport are known as structural carbohydrates (e.g., cellulose, lignin etc).  Non-structural carbohydrates are more available for energy metabolism than the structural carbohydrates.


It is often referred to as crude protein because it is a measure of total nitrogen, not the actual protein content of a feed.  The total nitrogen content of a feed is multiplied by 6.25 based on the assumption that true protein contains 16% nitrogen.  Proteins are made up of amino acids.

Amino acids are the building blocks from which proteins are made.  There are 20 standard amino acids required to form proteins (actually 21 – selenocysteine is considered the 21st amino acid as it is required by all mammals).  Amino acids are used to synthesize proteins and other biomolecules.  They can also be broken down and used to produce glucose through gluconeogenesis.  This results in the nitrogen being removed from the amino acid.  The body needs to detoxify this nitrogen, it does so in the liver by turning ammonia (free form of nitrogen) into urea, which is then excreted.  It is important that protein requirements are met, but not exceeded by too wide a margin as it requires the removal of excess nitrogen.


Chemically, fats are triglycerides of fatty acids.  Fat is rich in energy, containing 2.25-2.8 times the energy found in carbohydrates and protein and it is highly digestible.  It is used primarily to increase the energy density of rations.  Specific fatty acids are essential to normal health and maintenance.

Fatty acids are comprised of a straight hydrocarbon chain terminating with a carboxylic acid group.  Fatty acids are components of more complex lipids (commonly called fat).  They are of vital importance as an energy nutrient, but also in the production of bioactive compounds.  Two fatty acids are considered essential, meaning they must be consumed through the diet.  The essential fatty acids are linoleic (18:2 n-6) and α-linolenic acid (18:3 n-3), an omega-6 and omega-3 fatty acid, respectively.

The essentiality of the fatty acids linoleic acid and α-linolenic acid is due to the fact that some of the longer, more highly unsaturated fatty acids into which they can be converted are necessary 1) for the formation of cell membranes and 2) as precursors of compounds called eicosanoids.

There are a few different nomenclatures for fatty acids, but the most common designation is given as: x:y n-z where (x=number of carbon atoms):(y=number of double bonds) (n- z=the first carbon where double bond exists counted from the methyl, or omega end of the chain).  For example, α-linolenic acid would be expressed as 18:3 n-3, meaning it is 18 carbons long, with 3 double bonds, starting at the third carbon when counting from the omega end.

The length of the chains of fatty acids found in foods and body tissues ranges from 4 to about 24 carbon atoms.  They may be saturated (SFA), monosaturated (MUFA, containing one carbon-carbon double bond), or polyunsaturated (PUFA, having two or more carbon-carbon double bonds).

Nutritional interest in the n-3 (omega-3) fatty acids has escalated enormously because of their reported hypolipidemic and antithrombotic effects, which is of particular interest for horses with insulin resistance.  Furthermore, immune system function is impacted by fatty acid composition of the diet.  In very general terms, the omega-6 fatty acids are considered to be pro-inflammatory and the omega-3 anti-inflammatory based on their respective roles in prostaglandin synthesis.