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The Importance of Magnesium in Horse Diets

Reprinted from December 29, 2011

By Dr. Kathleen Crandell

Magnesium is a vital macromineral, and it is becoming increasingly recommended by veterinarians for various treatments in the horse. So why do horses require magnesium, and how does it fit for therapeutic use?

Magnesium is involved in numerous functions in the body. To start, magnesium is one of the major minerals in bone. Up to 60% of the body’s magnesium is found in the skeleton, with only 30% of that available for mobilization during times when it is needed elsewhere in the body. Magnesium is involved in over 300 enzyme reactions, including the generation of cellular energy and decoding of genetic information. It works in concert with calcium in nerve transmission and muscle contraction. The role of magnesium in muscle relaxation is the key to understanding subclinical symptoms that would indicate supplementation may be necessary.

The body attempts to maintain a balance of magnesium inside and outside cells. There is actually very little magnesium found in the extracellular fluid (about 1%), which is why testing blood for magnesium deficiency is relatively inaccurate. Gross deficiency of magnesium could be life-threatening but is rarely seen in horses. The current uses of supplemental magnesium are aimed at resolving subclinical deficiencies, the very ones that are difficult to test for but whose signs are recognizable.

Magnesium is found in varying quantities in forages and grains. Concentrate feeds may or may not have additional magnesium, depending on the amount in the other ingredients. The amount of indigestible fiber and presence of oxalates will affect availability of magnesium in the forages. Supplemental magnesium can be either inorganic (magnesium sulfate or magnesium oxide) or organic (chelated magnesium). Magnesium oxide is perhaps the most commonly used source and has an approximate 50% absorption rate. The advantage of magnesium oxide is that the body will not absorb it if there is no deficiency, so it is difficult to overdose a horse on it. Magnesium sulfate (Epsom salt) is also highly available to the horse for absorption, but it also has an effect of drawing water into the bowel and causing diarrhea, so it is not recommended for daily use.

Magnesium helps protect against inflammation and free-radical damage. What shows promise in the field of veterinary medicine is the link to the protective role of magnesium against damage from endotoxins. Horses with colic that results in endotoxin release or laminitis are known to often have low blood magnesium levels, and there is hope that treatment with magnesium during these critical times may decrease the amount of damage that occurs.

Magnesium may play a role in insulin resistance and equine metabolic syndrome. The link of magnesium deficiency within the cells and diabetes in the human has been made but has yet to be scientifically confirmed in the horse. What scientists do know is that insulin may modulate movement of magnesium from intracellular to extracellular tissues and that blood magnesium levels rise following a meal high in starch or sugar, indicating that magnesium is involved with the action of insulin to clear the glucose from the blood. If magnesium is low in the cell, there will be impaired carbohydrate metabolism and reduced insulin response (insulin resistance). Although there have been anecdotal reports in horses of supplemental magnesium improving insulin resistance, reducing the amount of neck fat associated with the disease and reducing the risk of laminitis, a recent research study found no advantage to feeding a magnesium and chromium supplement to insulin-resistant horses.

Within the muscle, calcium and magnesium work antagonistically, calcium causing muscle contraction and magnesium inducing relaxation. If there is not enough magnesium, muscles tend to spasm. Although the presence of low magnesium in the muscle tissue may stem from a genetic disorder rather than dietary quantities, there are reports of horses that have responded to magnesium supplementation for treatment of chronic tying-up.

However, the role of magnesium in nerve excitability has been established as a problem when synchronous diaphragmatic flutter occurs. Synchronous diaphragmatic flutter involves spasmodic contractions of the diaphragm and is usually seen in endurance horses with electrolyte imbalances. The condition is also known as thumps. Treatment with calcium and magnesium has been found to speed recovery.

Severe magnesium deficiency effects such as seen in grass tetany in cows are rare in horses but have been documented.

Magnesium is found in calming supplements because of its role in both nerve excitability and muscle contraction, and the suspicion that nervousness may be caused by suboptimal levels of magnesium in the diet.

Just as calcium and magnesium have a balancing act within the body, so should it be in the diet. The two minerals are ideally kept within a ratio of 2.5:1 to 3:1, calcium to magnesium. The typical diet of the horse will usually fall into a suitable range of calcium and magnesium; if supplementation with magnesium is warranted, the amount of calcium in the diet should be considered.

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