Magnesium (Mg) is a macromineral that holds an enormous range of importance in the body. Stored largely in bones (60%) and muscle tissue (30%), it is critical for the normal functioning of the musculoskeletal system, heart muscle, the nervous and endocrine systems and brain function. It is a component of well over 300 biochemical reactions throughout the body, including cell division and enzymatic systems that facilitate all energy generation, such as carbohydrate metabolism and protein synthesis. Often a forgotten electrolyte, magnesium is needed for proper blood acid/base balance, body water homeostasis, muscle contraction and to prevent cramping, particularly important for performance horses.
Magnesium is found in flaxseed, brans and, in varying amounts, in all green forages and some grains. Young, rapidly growing spring grass with its high-water content will likely be low in magnesium (and high in potassium, which interferes with Mg absorption). Outright dietary magnesium deficiency is unlikely in most average forage-based diets. However, many horses experience a borderline deficiency due to lower actual magnesium absorption rates that range between 40 and 60% combined with the fact that Mg is used up or excreted and should be replenished on a daily basis. Although rare, a true magnesium deficiency can result in soreness, muscle tremors, ataxia (incoordination) and, if left untreated, collapse and death. Blood tests for magnesium levels are often inaccurate as magnesium is tightly controlled by the kidneys with most being stored in the bone and soft tissue, and circulating magnesium represents less than 1% of total body stores. Borderline, mild or moderate magnesium deficiencies are more likely to occur and may display symptoms as behavioral or performance changes including tight muscles, reluctance to work, irritability, fatigue, nervousness and inattention. With difficult testing but recognizable symptoms, magnesium supplementation is often advocated as an option to resolve subclinical deficiencies.
Supplementation with magnesium is known to support certain muscle or behavioral issues as inadequate levels of this mineral at any age may cause problems of nervousness and muscular weakness. Common supplement levels of Mg for mature, average-size horses range from 1 to 10 grams daily depending on the issue needing to be addressed. It is often recommended to try supplementation for 3 to 4 weeks. If behavior and muscle condition improves, symptoms were likely due to a magnesium deficiency. If there is no change, the issue is most likely not caused by inadequate magnesium. The form of the mineral matters when choosing a supplement. Magnesium citrate is an excellent choice as it is readily absorbed due to its high bioavailability. Magnesium oxide, magnesium aspartate and magnesium gluconate are other common options. Magnesium sulfate, also known as Epsom salt, is often used. However, it is both generally unpalatable and can have a strong laxative effect and potentially cause digestive irritation and diarrhea due to its high effectiveness at drawing water into the intestines.
Reasonable supplementation of magnesium to healthy, mature horses is generally considered safe with no known long-term side effects. The small intestine is the primary site of absorption and takes in what is needed. Excess magnesium is efficiently processed by the kidneys and excreted in urine. The current daily recommendation per the National Research Council (NRC) is 15 mg per kg of body weight. For an 1,100 lb (500 kg) horse this would be 7.5 g (7,500 mg) per day. Some life stages, such as horses in heavy training or lactating mares, may require above these recommendations. The NRC’s maximum tolerable concentration is 0.8% equating to about 80 g in the total diet for an 1,100 lb horse in light exercise consuming 2% of his body weight. Horses may respond to magnesium supplementation individually and likely will have varying maintenance dosages that may fluctuate depending on season, workload and stress. It is recommended to start with the lowest dose and follow label directions, as well as to be aware of other contributing amounts from feeds and supplements. Any dietary changes, including magnesium supplementation, should be done under the supervision of your veterinarian.
Magnesium and calcium are minerals that work together in many bodily functions. Horses receiving excessive calcium in their diets can develop magnesium deficiency symptoms, likely caused by elevated calcium levels competing for or blocking magnesium absorption. Interestingly, excess magnesium can cause similar issues as it inhibits calcium and too little calcium can cause nervousness. However, deficiencies of Mg are far more common than deficiencies of calcium. Excessive levels of phosphorus and potassium can also interfere with Mg absorption. As is the case for all minerals, balance is key. Ideal calcium to magnesium ratio is about 2:1. Most typical equine diets will be around this range. The ratio may be slightly higher or lower, but it is important that magnesium does not exceed calcium and calcium does not far exceed magnesium. Horses showing symptoms of muscular problems, hypersensitivity or irritability on diets that may have adequate total magnesium but excessive amounts of calcium may be candidates for magnesium supplementation to help balance the ratio. For horses that are on an alfalfa hay or mixed grass/alfalfa hay diet and experiencing muscle problems or irritability, it may help to try supplementation with magnesium.
Magnesium is required for proper muscle function. For actual muscle contraction, calcium and magnesium work harmoniously albeit antagonistically. Calcium contracts the muscle and magnesium relaxes it. Muscle cells—myocytes—allow calcium to enter the cell through the cell membrane, raising the intracellular calcium threshold to activate muscle contraction. When the contraction is finished, intracellular magnesium pushes the calcium back out of the cell, relaxing the contraction. This action reaction happens rapidly over and over. However, in instances where there is deficient magnesium within the cell, calcium can leak back in causing a stimulatory effect or spasm that inhibits the relaxation of the muscle cell. Because of this simple yet critical internal relay, supplemental magnesium is often advocated for horses that may display myopathy issues—diseases that primarily cause damage to muscle—including muscle tension or tremors, tying up, sore backs, and those that are easily fatigued or generally “thin skinned.” Calcium and magnesium may be out of balance with the likely culprit being excessive dietary calcium. Magnesium supplementation may help to relax tense, tight muscles.
For athletic horses, magnesium plays several important roles. Magnesium is a cofactor, a helper molecule that assists in biochemical transformation, in most enzymatic processes needed to make the proteins necessary to build muscle, like insulin-like growth factor (IGF-1). Without enough magnesium, protein synthesis is impaired. It lessens or delays the buildup of lactic acid in muscle tissue often associated with fatigue and increases oxygen delivery to muscle tissue important for optimal performance and muscle recovery. Urine and sweat are the major avenues for magnesium excretion. Equestrian sports that result in high sweat losses may require magnesium supplementation to replace losses. Athletic horses may require 1.5 to 2 times over maintenance requirements to compensate for heavy sweating that can account for up to 20% of Mg losses, particularly important for those with disciplines with an endurance component. Muscles use adenosine triphosphate (ATP)—the major cellular energy unit that converts energy from food into useable fuel. Magnesium is a required cofactor in the production of ATP. Muscle tissue has limited quantities of ATP that need to be replenished during exercise; magnesium helps to recycle it. Significant electrolyte imbalances, including the depletion of calcium and magnesium, seen in fatigued or dehydrated horses paired with heavy sweating, such as seen in racing or endurance disciplines, may run into a specific health issue called synchronous diaphragmatic flutter (SDF), commonly called “thumps,” which involves spasmodic contractions of the diaphragm much akin to hiccup-like contractions synchronously with the heartbeat. Acute treatment is aimed at correcting dehydration and electrolyte imbalances often with running fluids, an electrolyte drench or intravenous calcium.
Magnesium is heavily involved in the nervous system, and low magnesium levels may result in nerve excitability and hypersensitivity. Supplemental magnesium may support neuromuscular and behavioral sensitivities. Magnesium helps to calm the nervous system by stabilizing the membranes of nerve cells, regulating minerals involved in nerve transmissions, promoting production of serotonin, a major mood hormone charged with feelings of well-being and is a cofactor for gamma amniobutyric acid (GABA), a primary neurotransmitter that inhibits nervous system activity. These inhibitory actions of magnesium support brain relaxation. Magnesium supplementation in humans is documented to have calming effects on a variety of brain-receptor types. It is often used in part of a holistic approach to anxiety, depression, irritability, migraines or pre-menstrual syndrome. Magnesium sulfate delivered intravenously to horses is so effective that it has historically been used as an anesthetic and sedative. Several factors are involved in supporting relaxed behavior in horses not the least of which is their basic personality. Used anecdotally in calming supplements for years, supplemental magnesium may support positive behavior in some horses, according to a recent equine study. For nervous, anxious horses or excitable horses, a surprisingly high number respond quickly to filling in simple nutritional gaps with basic, essential nutrients like magnesium. Magnesium supplementation for edgy horses may provide noticeable benefits regarding spookiness and overreaction to sound, movement or touch. Horses that respond favorably to supplementation were possibly deficient in magnesium, and, once receiving optimal levels, are able to have normal muscular and neurological functioning that may be exhibited as a calmer attitude. A forage-based diet made up primarily of pasture or grass hays may also be beneficial for nervous horses. Cutting out starch-heavy grains may have a positive influence on calmness. If more calories are needed to maintain body condition, using beet pulp or a beet-pulp based feed, rice bran and oils or fats can substitute calories for part or all of the grain.
Magnesium is needed for the metabolism of the three major energy sources: carbohydrates, fats and protein. In carbohydrate metabolism, magnesium supports the regulation of blood sugar and is required for both proper glucose utilization and insulin signaling. The hormone insulin, a major blood sugar regulator, may facilitate the movement of magnesium from intracellular to extracellular tissues. Magnesium blood levels rise following a meal with high starch or sugar content, suggesting that magnesium is involved with the action of insulin to clear glucose from the blood. If cellular magnesium is low, carbohydrate metabolism may be impaired with a reduced insulin response. Magnesium improves insulin sensitivity thereby improving the body’s use and storage of glucose. A deficiency may be linked to a slowed insulin response indicative of insulin resistance. Magnesium and its effects on insulin resistance is well researched in humans with many scientific papers written on the topic. Magnesium deficiency is known to play a significant role in insulin resistance in humans, and other species. Research is expanding in the equine realm with preliminary studies in horses found supplementing with this mineral to have beneficial results.
When insulin levels are elevated, fat storage increases, leading to higher insulin levels secreted. Systemic inflammation contributes to a wide spectrum of diseases including equine metabolic syndrome. Deficient concentrations of magnesium interfere with insulin signaling, are associated with increased cytokine production and systemic inflammation. Omega-3 fatty acids are known to help maintain normal inflammatory pathways. A study at Colorado State University using a comprehensive omega-3 fatty acid supplement and Platinum Metabolic Support (magnesium and chromium supplement) helped to maintain blood glucose levels within normal ranges in horses.
Equine idiopathic headshaking (HSK) is a perplexing disorder in horses. Confusing to diagnose and difficult to treat, it is a medical condition recognized by veterinarians with ongoing research being performed to best diagnose and treat this mysterious syndrome. The exact etiology of headshaking is unknown. However, hypersensitivity of the trigeminal nerve, inflammation of the trigeminal ganglia or an immune-mediated reaction may be causative factors of the disorder. The trigeminal nerve is responsible for facial sensation. As a main cause of headshaking is hypersensitization of the trigeminal nerve, anything to reduce or calm the firing of this nerve may be beneficial. Magnesium can have a calming effect and may help reduce hypersensitivity of the trigeminal nerve. Supplementation with higher than normal doses may be advantageous but should only be done under veterinary care and monitoring. When in combination with boron, magnesium absorption is enhanced. A study done at the University of California, Davis found that a supplement combination of magnesium with boron had a 64% reduction in headshaking when compared to a hay-only diet. There are a variety of management strategies that exist to help horses suffering from HSK, including limiting exposure to sunlight, wearing nose nets while exercising, aggressive insect repelling, antihistamines and other veterinary therapies. It is generally recommended to start with a single treatment and continue long enough to know if that treatment has effect on symptoms.
Magnesium is found in flaxseed, brans and, in varying amounts, in all green forages and some grains. Young, rapidly-growing spring grass with its high-water content will likely be low in magnesium (and high in potassium, which interferes with magnesium absorption).
Outright dietary magnesium deficiency is unlikely in most average forage-based diets. However, many horses experience a borderline deficiency due to lower actual magnesium absorption rates that range between 40 and 60 percent combined with the fact that magnesium is used up or excreted and should be replenished on a daily basis. Although rare, a true magnesium deficiency can result in soreness, muscle tremors, ataxia (in-coordination) and, if left untreated, collapse and death.
Blood tests for magnesium levels are often inaccurate as magnesium is tightly controlled by the kidneys with most being stored in the bone and soft tissue, and circulating magnesium represents less than 1 percent of total body stores. Borderline, mild or moderate magnesium deficiencies are more likely to occur and may display symptoms as behavioral or performance changes, including tight muscles, reluctance to work, irritability, fatigue, nervousness and inattention.
With difficult testing but recognizable symptoms, magnesium supplementation is often advocated as an option to resolve sub-clinical deficiencies.
by Emily Smith, MS,