Dietary supplementation for horses is a vexed area where the scarcity of scientific studies, owners’ desires to do the best for their animals and enthusiastic advertising by manufacturers all conspire to make it difficult for anyone to know the best thing to do.
There is nothing inherently dodgy or implausible about vitamins and trace elements. They are essential to life. Strictly speaking, a vitamin is a substance that has to be obtained from the diet because the organism involved is unable to manufacture, or synthesise, it in its body. For example, vitamin C or ascorbic acid is a vitamin for people but not for horses. Horses synthesise this substance in their guts; people do not, and depend on a dietary supply. A trace element is, as its name suggests, a chemical element or nutrient that is required by the body in minute, or trace, amountsIt has long been known that dietary deficiencies can lead to illness (Wikipedia 2010a), although it was not until the early part of the 20th century that all the vitamins were isolated and described. It is also indisputable that until fairly recent times, vitamin and trace element deficiencies were common in humans even in the developed countries, and in less fortunate parts of the world this is still so.
The discovery of vitamins, and the ability to manufacture them commercially, soon led to a highly profitable industry selling supplements. Once the preserve of the whole-food shop, dazzling arrays of supplements are now de rigueur almost anywhere that food and, indeed, medicines are sold. The beliefs that routine supplementation is good for everyone, that most diets are deficient and that many people have special needs for extra micronutrients have all been promoted way beyond the actual evidence, to the point where many people not only take multivitamins routinely, but take them well in excess of the recommended daily doses. Unfortunately, recent studies are showing that when it comes to vitamins and trace elements it is possible to have too much of a good thing, and substances that are vital for health in small doses can be dangerous in larger amounts. This is hardly surprising: these substances are biologically active, and all biologically-active substances are likely to have side effects as well as main effects. For people living in developed countries, the scientific evidence at present is that there are no proven benefits to routine supplementation, that modern foodstuffs are not deficient in nutrients, that most people in industrialised countries can get all of their nutritional needs from food, and that high doses of vitamins probably do more harm than good (Hall 2008; Novella 2009; Skeptvet 2009, Carroll 2010).
Might this be the case also in horses?
Equine nutritional supplements have been something of a growth industry over the last 20 or 30 years. Your average tack shop or feed store now incorporates an impressive display of vitamins, minerals and other feed additives and there are hundreds of vitamin and mineral mixes sold for horses, even though the recommended daily allowances and maximum and minimum safe levels for horses are not at all well known (Merck 2008). In the last few months I’ve seen advertising that suggests that all soils, in the UK at least, are deficient in minerals, that all pasture and hay is therefore deficient, and that all horses therefore need to be given supplements. It is very difficult to find any hard evidence on this subject. In temperate climates there are localised areas of nutrient deficiencies in the soil, and these can affect the health and growth of ruminant livestock such as sheep and cattle depending on how intensively the land is managed (Whitehead 2000), but I’ve not been able to find any comparable data for horses that is independent and doesn’t come from somebody selling supplements. If any reader can point me to any, please do.
It appears that vitamin and mineral deficiencies in the horse are actually rather rare (Merck 2008) and generally follow some other problem such as starvation or malabsorption because of age or illness. Horses’ bodies manufacture vitamin A from beta-carotene and vitamin C from sucrose in the diet. All the B vitamins and vitamin K are synthesised by the bacteria in the equine gut. Vitamin D is produced by the action of ultraviolet light on the horse’s skin. In fact the only vitamin needed by the horse that has to come from its diet is vitamin E (Kerrigan 1986). There are adequate levels of the essential trace elements in most pastures and hay for a horse doing light to moderate work, and not otherwise stressed (Wikipedia 2010c). A horse who has reasonable access to sunlight and green, growing pasture or sun-cured forage is unlikely to be deficient in any vitamins. Mineral deficiencies can arise, but these are rarely a consequence of the pasture a horse is in and more commonly a result of faulty feeding. The trace elements most likely to be deficient are calcium, phosphorus, copper, sodium chloride (salt) and selenium (Merck 2008).
It is more common for horse rations to contain an excess of certain nutrients, and this can lead to direct toxicity or to induced deficits of other minerals. Phosphorus, iron, copper, selenium and vitamin A are the substances most commonly fed to excess (Merck 2008).
In horses, the best-known example of a nutritional disorder is nutritional secondary hyperparathyroidism, also known as big-head disease or bran disease. It is caused by a diet containing too much phosphorus and too little calcium, and was prevalent in western Europe when many horses were fed large amounts of wheat bran. Although the link between bran in the diet, calcium deficiency and bone weakness was established by the veterinary surgeon Dr Varnell in 1860, the condition was still occasionally being diagnosed in the 1980s in horses fed excessive amounts of grain (Hintz 1987). It is still common in horses grazing certain tropical grasses, where chemicals called oxalates in the grasses combine with calcium so that it cannot be absorbed by the horse’s gut.
Selenium and vitamin E deficiencies have been considered as possible risk factors in horses prone to exertional rhabdomyolysis (tying up or azoturia) but the evidence is by no means unequivocal: if your horse suffers from this condition it is sensible to have a vet blood-test your horse and supplement, if necessary, under expert supervision. This is definitely not one to guess at yourself as even a slight excess of selenium can cause irreversible damage including loss of mane and tail hair and sloughing of the hooves (Merck 2008), and large overdoses can cause death (Wikipedia 2010b).
As for the known toxic and harmful effects of overdoses: vitamin A in excess can damage the bones and skin and cause developmental problems in unborn foals, and too much vitamin D can cause calcium to be laid down in the blood vessels, heart and soft tissues as well as bone weakness and weight loss. It can be fatal. Iodine toxicity, resulting in an enlarged thyroid gland (goitre) can occur in horses fed too much dried seaweed. Too much potassium can cause heart problems. Feeding magnesium to excess – possible if you give your horse a fortified diet and also a magnesium-based ‘calmer’ – may interfere with calcium absorption and lead to a deficiency of that mineral. Iron excess can interfere with the metabolism of copper. Too much copper can cause problems with the uptake of selenium and iron (Merck 2008).
Current veterinary advice is that a horse in light to moderate work who is fed a balanced diet with plenty of good-quality forage (including hay or haylage and decent grazing), who spends at least 4 hours a day outdoors in order to metabolise vitamin D from sunlight and who is not in extremely hard work is, if otherwise healthy, unlikely to need any supplementation except for common salt, sodium chloride, which may be given as a free-access lick or added to the feed (Merck 2008). There are, however, circumstances in which it is possible for a horse to suffer from deficiencies. The most obvious is the fat horse or pony who needs to be on a severely restricted diet in order to control his weight. He may not be getting enough food to fulfil his needs for vitamin precursors (the substances that are converted to vitamins in the digestive tract) or trace elements, especially if his diet is based on soaked or poor-quality forage. Phosphorus deficiency can occur in horses eating poor-quality forage and no grain. It leads to weakening of the bones and low-grade lameness, but the owner is likely to notice the horse eating soil or other non-foods before any clinical signs develop. Very old horses may need supplements to compensate for the reduced efficiency of their digestive processes. Horses kept permanently stabled or rugged may have vitamin D deficiency as a result of insufficient sunlight on the skin. Horses in very hard work that makes them sweat a lot will benefit from electrolyte supplements to compensate for the sodium and potassium lost in sweat. If they are also fed high-fat diets they may need extra vitamin E. Horses who are chronically stressed, or ill, may have higher than normal requirements for some vitamins and trace elements. Pregnant and lactating mares may need supplements too (Wikipedia 2010c).
If your horse does have a clinically significant deficiency, he might need a higher level of supplementation than he’d get in an all-purpose vitamin and mineral mix, so it is worth having a proper test and just giving him what he really needs. In fact, if you have any suspicion that your horse may have a vitamin or trace element deficiency, it is far better for your bank balance and his health and soundness to have him blood-tested, and to adjust his diet accordingly.
For obvious reasons, it is best to have this test done by your vet and not by a company who is selling supplements and who has a vested interest in finding a deficiency. Nutritional deficiencies are easily and inexpensively diagnosed using blood serum analysis and this is the only reliable way of doing it. I wouldn’t personally be tempted by alternative diagnostic techniques such as hair analysis (Barrett 2008) or applied kinesiology (Barrett 2009) as they haven’t been shown to work in independent tests and in many cases the person or company offering the technique is also selling supplements. Do-it-yourself diagnosis and treatment also has its risks. I have heard of owners giving their nervous horses magnesium and vitamin B supplements ‘because they are essential for the proper working of the nervous system’ – and so they are, but nervous behaviour isn’t necessarily the result of a disordered nervous system, and the horse may not actually be deficient in those substances. Even if these owners don’t cause any actual harm with their oversupplementation, they are spending money on products that will do no more than produce vitamin-rich and mineral-rich urine. There are cheaper ways to improve the nutritional status of your fields, if that is the problem.
In conclusion, though most horses are unlikely to need supplements, some may benefit from them. And if your horse is one of those who would benefit, it would seem most sensible and economical to find out what he actually needs, given that supplements cost money and that some are dangerous in large doses.
By Alison Averis
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Barrett s (2009). Applied Kinesiology: phony muscle testing for ‘allergies’ and ‘nutrient deficiencies’. www.quackwatch.com/01/QuackeryRelatedTopics/Tests/ak.html
Carroll R (2010). Vitamins. www.skepdic.com/vitacon.html
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Hintz HF (1987). The nutrition and feeding of horses. In Veterinary notes for horse owners. Ed by P D Rossdale. 17th Edition. Stanley Paul, London.
Kerrigan, R (1986). Practical horse nutrition. Adelaide, Australia.
Merck & Co. inc. (2008) Nutrition: horses. www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/toc_182600.htm
Novella S (2009). Another negative study of vitamins. www.sciencebasedmedicine.org/?p=372
Skeptvet (2009). Orthomolecular medicine – big talk, little evidence, real risk. www.skeptvet.com/Blog/2009/08/orthomolecular-medicine-big-talk-little-evidence-real-risk/
Whitehead DC (2000). Nutrient elements in grassland. CABI Publishing, Wallingford.
Wikipedia (2010a). Vitamins www.en.wikipedia.org/wiki/vitamins
Wikipedia (2010b). Selenium. www.wikipedia.org/wiki/selenium
Wikipedia (2010c). Equine nutrition. www.en.wikipedia/wiki/Equine_nutrition