Calcium is a very important mineral for horses. Calcium and phosphorus are the most abundant minerals in the body, making up 70% of the total mineral content. Calcium has several important functions in the body. Most people are familiar with calcium being necessary for strong, healthy bones. In fact, 99% of the calcium in the body is contained in the bones and teeth. A horse which is asked to perform athletically must have optimally strong bones to withstand the rigors of training and hard work, whether it be for racing or other performance. In addition to bone formation/strength, calcium is important for several vital metabolic functions in the body including:

• Muscle contraction ("calcium pump")

• Skeletal muscle for locomotion

• Heart muscle to pump blood efficiently

• Smooth muscle, such as within the gastrointestinal tract for normal "gut motility" and digestion

• Nerve conduction

• Other specific metabolic reactions (blood clotting, normal cell membrane function, glandular secretion, temperature regulation, regulating activity of many enzymes, and cellular activity)

The importance of calcium to the performance horse includes normal functioning of metabolism, the conduction of impulses along the nerves to muscle, the contraction of leg and body muscles for exercise, the contraction of the heart muscle for pumping blood, the contraction of the diaphragm for breathing, the functioning of the GI muscle for digestion. Calcium also plays a vital role in maintaining strong and healthy bones, cartilage and joints for peak performance without injury/breakdown.

The Regulation of Calcium
Due to the vital metabolic functions of calcium, its level in the bloodstream is closely regulated by the body. Vitamin D and hormones, such as PTH (parathyroid hormone) and calcitonin, normally maintain blood calcium levels within a narrow range to ensure normal muscle/nerve conduction and metabolic functions. Ideally, a horse's daily calcium requirement should be provided entirely from the diet. If not, bone provides a "storage" form of calcium for the body. When the diet does not provide adequate calcium, the PTH stimulates the mobilization of calcium from bones and into the bloodstream. Parathyroid hormone and calcitonin also prevent high calcium levels in the blood (calcemia) by excreting excess calcium into the urine and reducing absorption from the gut. With long-term dietary calcium deficiency, significant reabsorption of bone can occur, which can lead to decreased bone strength or abnormal bone formation. Thus, the body will sacrifice optimum bone strength formation to maintain the blood calcium levels, if necessary.

Factors Affecting Calcium in the Diet
Calcium derived from the diet is affected by several dietary factors. Other minerals can affect the availability of calcium. For example, excess phosphorus in the diet will complex with calcium in the small intestine, resulting in decreased calcium absorption. Excess calcium in the diet may interfere with the absorption of copper, manganese, zinc, and iron. Excess zinc inhibits calcium and copper absorption. Adequate and balanced amounts of minerals are necessary. For these reasons, indiscriminate supplementation of minerals should be avoided.

An additional factor that can result in a calcium deficiency is oxalates. Some pasture plants contain high concentrations of oxalates. These can complex with calcium and, thus, decrease the calcium available to the horse. Examples of oxalate-containing plants include alfalfa, halogeton, greasewood,
shamrock, rhubarb, panic grasses, lambsquarter, and five hooked bassia. This situation is usually not a problem, but can occur if oxalate-containing plants are the primary forage or if dietary calcium levels are low/marginal.

Problems with Improper Calcium Levels in the Diet
Significant problems can develop from continued improper calcium levels in the diet, such as DOD (developmental orthopedic disease) and "bran disease" (nutritional/secondary hyperparathyroidism). Some race track "breakdown" injuries, as well as lameness and tendinitis, have been attributed to inadequate calcium in the diet of young race horses. Improper calcium levels in the diet are more likely to cause problems for the growing horse than for the mature horse.

Developmental orthopedic disease is a group of conditions which are associated with the abnormal formation of bone and cartilage in the growing horse. It includes physitis/epiphysitis, osteochondrosis, OCD (osteochondritis dissecans), and tendon contractures. Developmental orthopedic disease is actually a multi-factorial disease. It can be associated with several factors in addition to improper calcium levels in the diet, such as genetics, very rapid growth,  protein deficiency in the diet, and improper levels of the trace minerals copper, manganese, and zinc.

Bran disease (nutritional hyperparathyroidism), also called "big head" or Miller's disease, is associated with excess phosphorus in the diet with inadequate calcium. Bran contains high amounts of phosphorus and low amounts of calcium. Grains also are high in phosphorus and low in calcium (see Table 1).

If growing horse diets containing bran or grains do not provide sufficient calcium, elevated PTH levels can occur. Under this circumstance, calcium is mobilized from bone to maintain the blood calcium levels. Bone is demineralized and replaced by weak, fibrous connective tissue that appears as swellings, often over the bones in the head or at the growth plates of the legs and neck.

Low Calcium in the Bloodstream
Although there are mechanisms to maintain normal calcium levels in the bloodstream, these can be overwhelmed under some circumstances. If calcium is lost in significant amounts through sweating and not replaced soon by oral intake, low blood calcium level or "hypocalcemia" may result. Under these circumstances, calcium cannot be mobilized quick enough from the bone to maintain normal levels. This can be a serious problem for a horse. Some examples of
specific problems associated with hypocalcemia include:

• Transport/stress tetany

• "Thumps"

• Exhausted horse syndrome; colic

Transport/stress tetany is associated with horses on calcium deficient/marginal diets which are stressed or transported long distances. Excessive sweating results in calcium losses. The signs include muscle twitching, muscle spasms, and tetany (stiff, rigid limbs).

"Thumps," or synchronous diaphragmatic flutter, is sometimes seen in endurance horses or horses that have sweated for prolonged time periods and when electrolytes have not been adequately replenished. Sweat contains chlorine, sodium, potassium, calcium, phosphorus, and magnesium as well as water. Low blood calcium, potassium, and chlorine result in an increase in neuromuscular irritability and hypersensitivity of the phrenic nerve. With thumps, heart beats stimulate the hypersensitive phrenic nerve to fire. This results in a contraction of the diaphragm seen at the flank each time the heart beats.

The exhausted horse syndrome is a complex metabolic derangement seen in endurance horses or horses exerting themselves in tough conditions or beyond their abilities/condition. These horses are severely dehydrated with multiple electrolyte deficiencies and signs of exhaustion. Colic can occur as part of this syndrome.

These conditions are all very serious and warrant immediate veterinary evaluation and treatment. Prevention of these conditions includes adequate conditioning before competition/exertion, optimal nutrition, and replacement of lost electrolytes and water during the ride/stress/transport/exertion.

Calcium & Management of HYPP
Hyperkalemic Periodic Paralysis, HYPP,  is an inherited condition associated with some Quarter Horses. Dietary management is an important part of controlling the disease. Supplementation with calcium carbonate as an alkalinizer in the diet may be beneficial by promoting potassium excretion with hydrogen ion retention by the kidneys.

Requirements for Calcium in the Diet
The first thing to address in balancing a diet for calcium and phosphorus is to ensure sufficient amounts of calcium and phosphorus are available to meet the horse's needs. The calcium to phosphorus ratio is also important, but secondary. An excellent book containing NRC nutrient tables and recommended dietary levels is Equine Clinical Nutrition: Feeding and Care by Lon D. Lewis. Nutrient recommendations contained in this book suggest providing growing horses with 15-20% higher calcium than those recommended by the NRC to maximize bone strength and mineral content, while decreasing the risk of DOD. These suggestions are also supported by a study (B.D. Nielsen et al., Texas A&M) of young Quarter Horse geldings beginning race training. Additional dietary calcium (25% above NRC requirements) at the onset of training resulted in increased bone density compared to controls receiving NRC requirements.

Providing optimal calcium levels and an overall balanced diet will help to maximize the performance and bone/joint strength of the horse. This is especially important during training/conditioning of the young horse. When evaluating a specific feeding program, it is ideal to analyze nutrient
content of hay and grain. Although not as accurate, estimates can be obtained from charts based on averages for the type of hay and grain being fed. Commercial grains and feed products provide nutritional information on the label.

The biggest variation in calcium content of the diet hinges on whether grass or alfalfa hay is fed, as these forage sources usually differ significantly in calcium content. Remember, growing horses may need additional supplementation, but since both excesses and deficiencies can cause problems, a specific assessment of nutrient content of the diet is recommended. Feed companies may provide assistance and feedstuff analysis. Consulting with an equine nutritionist and/or veterinarian may be helpful, especially if a specific nutritional problem has occurred.

Conclusion
Without needed amounts of calcium in the diet, metabolic functions and structural support may be compromised. On the other hand, too much dietary calcium can also lead to serious consequences. Horse owners need to evaluate dietary levels of minerals and supplement as needed. Sufficient levels and proportions of minerals in horse diets are needed to facilitate optimum growth and/or performance.

Dr. Harrison is a board certified specialist in large animal internal medicine at Good Thunder Veterinary Service, a veterinary practice devoted exclusively to equine, at New Carlisle, Ohio.

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