Ultimate Guide to Horse Nutrition

Nutrition

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.

Carbohydrates

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.

Protein

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 21^st 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.

Fat

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 alpha-linolenic acid (18:3 n-3), an omega-6 and omega-3 fatty acid, respectively.

The essentiality of the fatty acids linoleic acid and alpha-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, alpha-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.

Vitamins

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

These are vitamins that are soluble (form solutions) in lipids. As a result, they tend to be more bioavailable when supplemented orally and they can be stored in fat tissue within the body for extended time periods.

Vitamin A

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
    • 16,000 IU/kg of dry matter intake

Vitamin D

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

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 alpha-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
    • 1,000 IU/kg of dry matter intake

Vitamin K

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 beta-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.

Thiamin (B1)

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 alpha-ketoglutarate in the blood.

• Oxidative decarboxylation of alpha-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

Riboflavin (B2)

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

Niacin (B3)

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

Pantothenic Acid (B5)

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 (B7)

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 (B6)

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

Folic Acid (B9)

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

Cobalamin (B12)

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

Ascorbic Acid (C)

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