Vitamins for Horses

Published in Horses and People Magazine

Vitamins for optimal health!

By Mariette van den Berg BAppSc.(Hons), MSc. (Equine Nutrition)

Supplements for horses are a big business! It is very important that anybody that buys mineral and/or vitamin supplements assesses if their horse needs some extras and if so which elements and compounds do they actually require in larger quantities. To feed safe and efficiently it is essential to have a general understanding of the role of these elements and compounds in the body and the minimum requirements for your horse’s well-being.  In the previous edition of H&P the dietary minerals were reviewed.  This article will continue with the vitamins.

Origin of vitamins

The name vitamins is obtained from “vital amines” as it was originally thought that these substances were all amines. This is now known not to be as vitamins have a range of structures. Vitamins are classified as either water-soluble or fat soluble organic compounds that can be naturally found in small amounts in plant and animal-derived foodstuffs. Organisms require small amounts of vitamins for proper function of the body and any deficiency can leads to metabolic and physical disorders.

In an optimal environment horses can obtain most of their vitamins from fresh grass and other plants and in the case of vitamin K and B complex vitamins, additional amounts can be supplied by microbial synthesis in the intestine tract. But when horses are stabled, have limited excess to pasture or are kept on poor quality pastures they generally need some extra vitamins (and minerals). The requirements can also increase when horses are breeding, pregnant, lactating, growing, ageing, exercising or in poor health. When vitamins cannot be synthesized by the body or they cannot be made in adequate amounts, they need be supplied by the diet and/or supplementary sources.

Supplements (as well as the premix in complete hard feeds) can include synthetic and/or natural sources of vitamins. When we talk about natural vitamins we refer to vitamins that are taken from the original source and retain in their organic complex which may include number of other structures/complexes such as phytochemicals. Synthetic vitamins are initially made in the laboratory where they isolated the active compound from foodstuffs, which also makes them cheaper than natural ones. In the vitamin supplement industry for humans as well as pets and horses there is an ongoing “synthetic versus natural” debate about which source is healthier. For both sides there are studies and arguments that can confirm one or the other. It depends largely on the (biologically) activity and absorption of the vitamins. Also the difference in price will determine which ones are used in the supplements and premixes. Ideally, more research is needed to have a closer look at some of these vitamins and the differences between the sources in the uptake action and responses in the body of the horse. In this article the focus lies on the biological role of vitamins in the body and quantities recommended for horses.

 So which vitamins are important and how much does a horse require?

The National Research Council published a report (Nutrient Requirements of Horses 2007) that estimated the vitamin requirements of horses by using several response variables (e.g. prevention of specific deficiency symptoms, maximising tissue stores, and optimisation of various biological functions). The report only estimated requirements for vitamins A, D, E, thiamine (vitamin B1) and riboflavin (vitamin B2). Why only four vitamins are estimated will be explained below.


Vitamin A Vitamin A is a fat-soluble vitamin that plays an important role in vision, bone growth, reproduction, cell divisions and cell differentiation (in which the cell becomes part of the brain, muscle, lungs, blood or other specialised tissue). It also helps regulating the innate and adaptive immune response to infection.  There are two principle forms of vitamin A that can be found in foods. Vitamin A in animal-derived foods is called preformed vitamin A and is absorbed in the form of retinol. Retinol is one of the most usable (active) forms of vitamin A and can be converted in the body into other vitamin A forms such as retinal and retinoic acid. Vitamin A that is found in plants (as well as colourful fruits and vegetables) is called provitamin A carotenoid. They can be converted in the body into retinol. Common provitamin A carotenoids in plants are beta-carotene, alpha-carotene and beta-cryptoxanthin. Horses obtain most vitamin A as beta-carotene which is more efficiently converted into retinol by the body than the other provitamin forms. Fresh green grass and plants contains the highest concentration of beta-carotene. Conserved forages such as mature hay generally contains less beta-carotene, but concentrations depend on degree of maturity, conditions at harvest and length of storage. The most common forms of vitamin A used in supplements are retinyl-palmitate and –acetate. The international unit (IU) is used to express vitamin activity of different sources on a comparable basis. Depending on the age, sex, workload, stage of gestation, pregnancy, lactation and growth, the minimum daily vitamin A requirement for horses is estimated to be between 30 -60 IU/kg body weight (BW)Deficiency in vitamin A can lead to night-blindness and can affect immunity and reproduction. Too much vitamin A has been reported to result in bone fragility, excessive growth of bone, developmental orthopaedic diseases in growing horses and birth defects. Beta-carotene toxicity has not been reported in horses.
Vitamin D Vitamin D is a fat-soluble vitamin that is needed for the health and to maintain strong bones. It helps the body with the absorption of calcium from the intestine and reabsorption of calcium from the kidney and influences mobilisation and accumulation of calcium and phosphorus from the bone. Vitamin D plays also a role in muscle contraction, cell growth and differentiation, the central nerve system and immune system. There are several forms of vitamin D, but the two major vitamins are D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D2 is produced by invertebrates, fungus and plants in response to UV irradiation. Vitamin D3 can be found in animal cells. Vitamin D3is made in the skin when 7-dehydrocholesterol reacts with UVB ultraviolet light. Horses that are maintained in a practical setting with some exposure to UV light appear to produce adequate amounts of vitamin D. Horse that have limited exposure to sunlight (kept indoors etc) must receive some in the diet.However, general horse feeds are relatively low in vitamin D. The most common supplement form for horses is Vitamin D3, cholecalciferol. The NRC recommends minimum intake of 6.6 IU/kg BW for maintenance, breeding and exercising. Growing horse require between 22-14 IU/kg BW, which depends on the stage of growth. Deficiency in vitamin D can cause rickets, a disease characterized by bone deformities. Excess level of vitamin D is associated with calcification of soft tissue.
Vitamin E Vitamin E is the collective name for a group of fat-soluble compounds with distinctive antioxidant activities. Vitamin E and selenium partner and play an important role as anti-oxidants, protecting body tissues from damaging effects of oxidation.  It is also plays and essential role in maintaining proper function of the muscular, nervous, circulation, reproduction and immune systems. There are eight natural occurring chemical forms (alpha, beta, gamma and delta-tocopherol and alpha, beta, gamma and delta-tocotrienol). They all vary in biological activity and of these, alpha-tocopherol has the highest bioavailability. Fresh green grass and plants contain adequate levels of vitamin E, but concentrations dramatically drop when plants are processes (e.g. cutting, baling, and storage). Horses that have limited excess to fresh pasture benefit from vitamin E supplementation. Common forms of vitamin E supplements used by horse feed and supplement manufacturers are esters of the alpha-tocopherol such as alpha-tocopherol acetate. There is an increased interest in the use of natural vitamin E.  Natural vitamin E appears to have 36 % greater bioavailability than the synthetic form. The difference between the two sources is the (isomer) structure and uptake action. The natural source appears to be better transported to and retained in cells and tissues than the synthetic source.Food stuffs with the highest concentration of vitamin E are vegetable oils (e.g. soya bean oil), followed by nuts and seeds including whole grains. Depending on the age, sex, work load, stage of gestation, pregnancy, lactation and growth, the minimum daily vitamin E requirement for horses is estimated to be between 1-2 IU/kg BWDeficiency in vitamin E can cause impaired immunity, muscle degeneration, spinal cord degeneration and equine motor neuron disease (EMND).
Thiamine Thiamine is also known as vitamin B1 and is one of the B vitamins, a group of water-soluble vitamins that participate in many of the chemical reactions in the body. Thiamine (vitamin B1) helps the body cells covert carbohydrates into energy. It also essential for proper functioning of the heart, muscles and nervous system. Thiamine can be found in cereals, whole grains (especially wheat germ), protein supplements such as soya bean meal and cotton seed meal and brewer’s yeast.  In supplements thiamine is generally supplied as thiamine hydrochloride or mononitrate. Recommended levels of thiamine are 0.11-0.125mg/kg BW for moderate to heavy working horses and 0.06mg/kg BW for all others.  Deficiency can cause anorexia, ataxia, slow heart rate and muscle twitching.
Riboflavin Riboflavin or Vitamin B2is also member of the water-soluble B vitamins. It is important for body growth and red blood cell production and helps in releasing energy from carbohydrates. Riboflavin can be found in higher levels in green legumes such as lucerne and clover. Horses are able to synthesise riboflavin in the intestine (microbes). Depending on the age, sex, work load, stage of gestation, pregnancy, lactation and growth, the minimum daily vitamin riboflavin requirement for horses is estimated to be between 0.04-0.05 mg/kg BW.Although not reported in horses, deficiency in other species resulted in poor hair coat, dermatitis, anaemia, reduced fertility and poor growth.
Niacin Niacin is also known as vitamin B3, nicotinic acid and vitamin PP. Niacin assists with proper function of the digestive system, skin and nerve system. It also plays a role in the conversion of food to energy. Niacin is present in most common horse feeds but concentrations and digestibility may vary depending on the food source. Niacin is also produced by microbial fermentation in the caecum of the horse. In supplements niacin is generally supplied in a nicotinic acid form.  Niacin requirements for horses have not yet been determined. In addition, deficiency and toxicity in horses have also not been described. Deficiency in other species caused severe metabolic disorders and lesions of the skin.
Pantothenic acid Pantothenic acid or vitamin B5 is involved in numerous metabolic pathways including carbohydrates, fats, proteins and steroid hormones. Like niacin it is widely distributed in common horse feeds. Common supplemental form of the vitamin is calcium pantothenate. Pantothenic acid requirements are not estimated for horses and deficiency and toxicity in horses have also not been reported.
Vitamin B6 Vitamin B6 is a water-soluble vitamin and is part of the vitamin B complex. There are several forms of the vitamins. Pyridoxine is the form that is commonly given as vitamin B6 supplement. Vitamin B6 plays important role in many reactions of amino acid metabolism, glycogen utilization and the synthesis of epinephrine (adrenaline) and norepinephrine and metabolism of fats. Horses can synthesise vitamin B6 in the intestinal tract. Vitamin B6 requirements for horses are not determined and deficiency and toxicity in horses have not been described.
Biotin Biotin is also known as vitamin B7 or vitamin H (the H comes from Haar und Haut; German words for hair and skin) and is also member of the B complex vitamins. Biotin is necessary for cell growth, the production of fatty acids and the metabolism of fats and amino acids. It also plays and important role in the citric acid cycle which is a process by which biochemical energy is generated during aerobic respiration. Biotin can be found in plant material and is manufactures in small amounts by the gut microflora.  Many horse owners will be familiar with biotin as a supplement for hooves. Although results vary, supplementation of biotin over period of time has shown to improve hoof quality and growth. Optimal biotin levels are determined yet and no deficiency or toxicity of biotin has been reported in horses.
Vitamin B12 Vitamin B12 is a water-soluble vitamin and exists in several forms and contains the mineral cobalt. Therefore compounds with vitamin B12 are collectively called “Cobalamins”. Vitamin B12 is required for proper red blood cell formation, neurological function, and DNA synthesis. It also needed for the production of propionate, which is a major energy source derived from the fermentation of carbohydrates. Vitamin B12 appears to be synthesised in adequate by microbes in the intestine of the horse. No requirements, deficiency and toxicity have been recorded for vitamin B12.
Folate Folate is a general term for folic acid (synthetic) and naturally occurring folate. Folate gets its name from the Latin word “folium” which means leaf. It can be found in green forage and legume seeds. Folate helps produce and maintain new cells, it is especially important during period of rapid cell division and growth such as during pregnancy. It is needed to make red blood cells and prevent anaemia.  Folate is also essential for the metabolism of amino acids. Microbes in the digestive tract of the horse are able to synthesise folic acid. Folate deficiency in horses has not been recorded. In other species it has been reported to cause anaemia and lower birth weight.
Vitamin K Vitamin K is a fat-soluble vitamin that plays an important role in blood clotting. It also participates in the activation of other proteins and is involved in metabolism pathways in bone and other tissue. There a number of vitamin K form found in green plant material (vitamin K1; phylloquinone). Others are manufactured by the microflora of the horse (vitamin K2; menaquinone). There are also synthetic forms (e.g. vitamin K3; menadione) that are used in supplements and premixes. Menadione is metabolized to an active form in the body.  There are no set requirements for vitamin K for horses. Intake of fresh grass and vitamin K2 produced by intestinal bacteria will probably meet the requirements. Some vitamin K antagonist such as dicumarol (chemical substance of plant and fungal origin) and other drugs can reduce vitamin K metabolism causing a deficiency. Excess vitamin K appears to have no adverse effects.
Vitamin C Most people are familiar with vitamin C, but not many know the function and requirements for horses. Vitamin C is also known as L-ascorbic acid and plays important role as a biological antioxidant within the redox (oxidation reduction) system. It is also needed for synthesis of collagen, carnitine and norepinephrine. Most animal (including horses), unlike humans, are able to synthesize vitamin C from glucose. Vitamin C supplementation may be beneficial for horses in poor-health. Common supplement form for horses are ascorbic acid, ascorbyl palmitate and calcium ascorbyl-2-monophosphate. Deficiency and toxicity have not been described.
Choline Choline is a water-soluble “essential nutrient” and is usually grouped with the B complex vitamins.  It is an important component of many other biologically molecules, such as acetylcholine (neurotransmitter), lecithin and other phospholipids. Choline can be synthesised in the liver from amino acid methionine. It can also be found in free or bound form in foodstuffs. The form most commonly used by supplement and feed manufactures is choline hydrochloride. There are no requirements determined for choline. Deficiency in horses has not been recorded. It appears to only rarely occur when growing young animals are fed a diet deficient in methionine and/or choline. It can cause fatty liver which may lead to cirrhosis. There are no reports on the toxicity levels of choline for horses.

Minimum vitamin requirements are only estimated for vitamin A, D, E, thiamine and riboflavin by the NRC. There are some studies that published dose administration data of some of the other vitamins without reporting adverse effects (deficiency or toxicity), but they are generally inconclusive and are not able to suggest optimal levels. Ideally, more research into vitamin requirements is required to gain more knowledge about deficiency, optimal levels and toxicity. In addition, due to the increased interest in using vitamins (and minerals) as health foods to improve quality of life, which has been adapted from human research, it is probably also necessary to have a closer look at these compounds and the potential health benefits in horses.

© MB Equine Services 2014


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