By Dr. Mariette van den Berg, B. AppSc. (Hons), MSc., PhD (Equine Nutrition)

Similar to the trends observed in human health, the dietary supplements market for horses has grown significantly over the last 25 years. This has resulted in an overwhelming array of supplements available to horse owners/caregivers. Just visit your local horse store, flick through any horse magazine, or scrolling through your social media and you will find any number of advertisements for equine supplements.

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 the case 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 intestinal 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 in the table that follows on the next page.
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.


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... To read the complete article you need to be a subscriber