Introduction
Equine nutrition plays a central role in rearing of horses because they are non-ruminant herbivores. Horses rely on the fermentation processes to digest cellulose in their single stomach. Normally, horses feed on forages, concentrates, pellets, and supplements. Given that horses are non-ruminant herbivores, they take small portions of these types of feed several times a day to allow complete digestion of each portion. In this view, the understanding of nutritional requirements and metabolic mechanisms is essential in the formulation of equine nutrition. Horses require calcium, phosphorus, and vitamin D as macronutrients that are important in the formation and the growth of bones.
The proportions of calcium, phosphorus, and vitamin D in diet determine their respective nutritional importance because of metabolic interactions. Breidenbach, Revesz, and Harmeyer state that high doses of vitamin D increase excretion of calcium and phosphorus in horses (106). The statement implies that equine nutrition requires a delicate balance of calcium, phosphorus, and vitamin D because their metabolism interrelates. Therefore, the essay examines nutritional and metabolic importance of calcium, phosphorus, and vitamin D in equine nutrition.
Calcium
Metabolism and functions
Calcium is an essential mineral in the diet of horses because it is a major constituent of bones. Horses usually get calcium from forages and concentrates made from lucerne or manufactured supplements. When ingested, calcium undergoes absorption in the gastrointestinal system and the circulatory system distributes to diverse parts of the body, where bone formation and growth occur. The body distributes absorbed calcium into tissues and organs using blood and extracellular fluids, and removes excess calcium through intestinal secretion, kidneys, and skin (Schryver, Hintz, and Lowe 102).
Horses need calcium in abundance for they require strong bones for them to adapt to their functions. For example, racing horses require enough calcium in their bodies for the formation and maintenance of strong bones because their bones are prone to wear and tear during rigorous performances. Rourke, Kohn, Levine, Rosol, and Torobio state that calcium has structural importance in horses because it forms the skeletal system that supports horses against gravity, protects internal organs from physical damage, and hosts diverse structures in their respective positions (197). Hence, the structural importance of calcium enables horses to have stout bodies with the capacity to perform numerous functions such as racing and transportation.
Calcium also has physiological importance among horses because it causes muscle contraction, regulates locomotion, boosts cardiac functions, and facilitates gastrointestinal peristaltic movements. Regarding muscle contraction, calcium interacts with actin and myosin muscle fibers, which are responsible for the contraction and relaxation of skeletal muscles. Since horses perform strenuous physical activities such as running, they require calcium to aid in the contraction of muscles.
Horses with low levels of calcium in their bodies are usually very weak because skeletal muscles do not have sufficient amounts of calcium to cause contraction of muscles and bring about skeletal movement. In this view, calcium regulates locomotion among horses as high concentration of calcium in the body increases the potential of locomotion, while low concentration of calcium in the body reduces the potential of movement. As calcium causes contraction of muscles, it mediates the contraction of involuntary cardiac muscles, and thus, determines the rate of heart beat. In the gastrointestinal system, calcium facilitates peristaltic movement, which is central to the digestion process. Overall, calcium is important because it mediates contraction of different types of muscles as aforementioned.
In the body, calcium regulates physiological activities such as blood clotting process, functions of transmembrane channels, transmission of impulses, and enzymatic reactions. During the blood clotting process, calcium interacts with clotting factors in the cascade process, which eventually leads to the formation of a blood clot. The absence or low level of calcium causes the loss of blood when an injury occurs because blood takes unusually long period to clot (Rourke et al. 198).
Hence, horses require calcium to speed the blood clotting process and promote healing of wounds in case of injuries. Since transmembrane channels regulate the entry and exit of substances in and out of the cells, calcium takes part in regulating these channels. Primarily, calcium maintains integrity of cell membranes as it allows communication among cells via transmembrane channels. In conjunction with potassium and sodium, calcium also influences the transmission of nerve impulses in the body. Calcium acts as a cofactor, which regulates activity of enzymes in cellular processes, such as cell division, growth, and motility.
Homeostasis of calcium
Horses normally experience frequent instances of hypocalcemia and hypercalcemia when compared to other mammals. Homeostatic mechanism regulates the level of calcium in blood and tissues within a narrow range. According to Rourke et al., calcitonin, vitamin D, and parathyroid hormone are three hormones, which are responsible for homeostatic regulation of calcium in horses (176).
When calcium levels are very high, thyroid gland releases equine calcitonin, which causes the reduction of calcium levels in serum and stimulates storage of calcium in bones. In contrast, a low level of calcium induces the release of parathyroid hormone by the thyroid gland, which causes renal reabsorption, synthesis of vitamin D, and bone resorption (Matsuzaki and Dumont 232). Given that horses often experience imbalances in the levels of calcium, they utilize these hormones in restoring the imbalances and optimizing the functions of calcium in their bodies. Therefore, assessment of the nutrition and metabolism of calcium indicates that it has important structural and biochemical functions in horses.
Phosphorus
Metabolism and functions
Phosphorus is an important mineral that horses require for the growth and development of healthy bones and teeth. Calcium combines with phosphorus in the development of strong bones and teeth, which enable horses to adapt to their functions of racing and chewing forages respectively. Owing to the importance of phosphorus in the growth and development of horses, industries that manufacture feeds incorporate phosphorus as a macronutrient. Since horses need strong bones, they require supplements of calcium and phosphorus in their diets. Schryver, Hintz, and Craig recommend that mature horses need an average of 17g of phosphorus daily for maintaining physiological functions in their bodies (1257).
Hence, daily dietary intake of horses must contain appropriate amounts of phosphorus, which meet physiological functions. Horses that consume phosphorus in their diet regularly have strong skeletal structure because phosphorus is a constituent of bones and teeth (Lawrence 209). Evidently, phosphorus is an essential macronutrient for its deficiency leads to poor development of bones and teeth.
Phosphorus is an important constituent of biochemical energy in the form of adenosine triphosphate (ATP). The performance horses utilize ATP in the generation of the required horsepower. Lawrence asserts that horses require sufficient energy in form of ATP for muscle contraction, which they obtain from carbohydrates (206). ATP is a biochemical energy that contains phosphorus, and its function is to transfer energy from one cell to another cell during contraction of muscles. Additionally, phosphorus is an important component of cyclic adenosine monophosphate (cAMP), a chemical messenger that mediates a number of cellular activities.
Adenyl cyclase is an enzyme that converts ATP into cAMP with the loss of pyrophosphate. Matsuzaki and Dumont state that cAMP regulates the activity of the parathyroid gland in the production of parathyroid hormone, which in turn regulates the level of calcium in the body (227). In this view, phosphorus indirectly regulates activity of the parathyroid gland and metabolism of calcium in the body.
Homeostasis of phosphorus
The metabolism of phosphorus occurs normally like calcium or any other mineral in the body of horses. The intake of phosphorus through diet or supplements increases the level of phosphorus in blood after absorption in the intestines. The blood transports phosphorus to bones, teeth, and tissues, where their metabolism occurs. Horses excrete excess phosphorus in urine and feces. Hence, diet, renal excretion, and fecal excretion provide homeostatic mechanisms that maintain the level of phosphorus in the body at the required level. The homeostatic mechanisms show that renal or urinary excretion of phosphate is proportional to dietary intake of phosphorus (Schryver, Hintz, and Craig 1261).
The direct relationship between renal excretion and dietary intake of phosphate indicates that kidneys play a central role in the homeostatic mechanisms. According to the findings of an experiment, the endogenous fecal excretion of phosphorus among horses is not proportional to the dietary intake of phosphorus (Schryver, Hintz, and Craig 1261). The findings show that endogenous fecal execration does not play a significant role in phosphorus homeostasis.
Vitamin D
Metabolism and functions
As one of the vitamins that horses utilize, metabolism of vitamin D is unique. Once ingested, vitamin D is inactive, and therefore, goes into the liver where hepatic enzymes convert it into an active form of vitamin D called 25-OH-D3 through the process of hydroxylation. DeLuca and Schnoes report that 25-hydroxylase is a hepatic enzyme that activates vitamin D by hydroxylating its 25-crabon (625). Without hydroxylation in the liver, vitamin D remains inactive in the body, and thus, becomes physiologically deficient. In this view, the liver plays a central role in the metabolism and the use of vitamin D.
The active form of vitamin D, 25-OH-D3, then circulates in the bloodstream and moves to the kidneys where further hydroxylation takes place. When the concentration of 25-OH-D3 increases, parathyroid hormone induces the synthesis of 25-hydroxyvitamin-1α-OH hydroxylase in the kidneys, which catalyzes conversion of 25-OH-D3 into 1α,25-(OH)2-D3. The hydroxylation of 25-OH-D3 in the kidney gives an active form of vitamin D, 1α,25-(OH)2-D3, which performs diverse cellular and systemic functions. According to DeLuca and Schnoes, 24-hydroxylase degrades both 1α,25-(OH)2-D3 and 25-OH-D3 into 1α,24,25-(OH)2-D3, and 24,25-OH-D3 respectively (642). Therefore, horses excrete vitamin D in the form of 1α,24,25-(OH)2-D3, and 24,25-OH-D3 through the kidneys under the control of parathyroid hormone.
Vitamin D is an important vitamin because it regulates metabolism of calcium and phosphorus. In its regulatory function, vitamin D enables intestines to absorb calcium and phosphorus from the consumed food. Given that concentration and electrical gradient prevent the absorption of calcium and phosphorus in the intestines, vitamin D alters the property of intestinal membranes and allows the movement of these minerals against the concentration and electrical gradient (Breidenbach, Revesz, and Harmeyer 105).
Thus, a deficiency of vitamin D causes physiological deficiency of calcium and phosphorus. Essentially, absorption of calcium and phosphorus would not take place despite their intake in the diet. Vitamin D also is important in the formation and development of bones because it promotes mineralization process. DeLuca and Schnoes explain that vitamin D has the capacity to alleviate osteomalacia and rickets because it avails calcium to mineralization sites in the skeletal system (649). In this view, vitamin D mediates deposition and resorption of bones, the processes that are central to the homeostasis. In tissues, vitamin D is very important because it regulates cell division by inhibiting proliferation of cells and enhancing differentiation.
Homeostasis of vitamin D
The level of vitamin D in the body is subject to homeostatic mechanism that is under the regulation of calcium, phosphorus, and parathyroid hormone. DeLuca and Schnoes explain that when the level of calcium in the body is low, parathyroid hormone elicits synthesis of vitamin D, which in turn elicits intestines to increase the absorption of calcium (632). Moreover, synergistic effect of vitamin D and parathyroid hormone increases plasma calcium by increasing bone resorption and reabsorption of calcium in the kidneys. When the level of phosphorus is low in the body, there is stimulation of vitamin D synthesis, which is independent of parathyroid hormone.
In contrast, when the levels of calcium and phosphorus are high, the level of parathyroid hormone goes down, and hence, no synthesis of vitamin D. The resultant effect is decreased reabsorption of calcium and phosphorus in the kidneys and increased mineralization of bones. DeLuca and Schnoes observe that vitamin D acts as a hormone that mobilizes calcium and transports phosphorus (632). Thus, the homeostatic mechanism shows that vitamin D plays a central role in the metabolism of calcium and phosphorus in horses.
Calcium, Phosphorus, and Vitamin D Interactions
The influence of calcium
The intake of calcium and phosphorus in various forms of diets leads to interactions that influence their respective metabolism. Calcium intake influences the metabolism of phosphorus in the body in terms of excretion and retention. Schryver, Hintz, and Craig report the findings that when ponies consume a diet with high calcium, the absorption of phosphorus increases (1260). The findings indicate that calcium increases the absorption of phosphorus, but decreases its excretion among ponies. Schryver, Hintz, and Lowe explain that renal excretion of phosphorus decreases because a high level of calcium stimulates bone formation, which increases the requirement of phosphorus (103). Therefore, it is evident that calcium influences the metabolism of phosphorus in terms of excretion, retention, and uses among ponies.
The influence of phosphorus
The level of phosphate in the diet influences the metabolism of calcium in horses. A study done among ponies fed with a diet that has high levels of phosphorus and adequate amounts of calcium shows that phosphorus inhibits absorption and decreases the retention of calcium (Schryver, Hintz, and Lowe 103). Assessment of each pony shows that the level of plasma phosphorus negatively correlates with the level of plasma calcium. High plasma phosphorus or high phosphorus diet increases turnover rate of bones as the processes of bone resorption and deposition hastens. Schryver, Hintz, and Lowe found out that an increase in plasma phosphorus increases the rate of bone resorption by 83% and increases the rate of calcium deposition by 40% (103).
The interaction of calcium and phosphorus causes secondary hyperparathyroidism among horses. The deficiency of calcium coupled with high plasma phosphorus predisposes horses to secondary hyperparathyroidism (Hintz and Cymbaluk 258). The finding implies that the diet with high phosphorus influences metabolism of calcium during bone resorption and calcium deposition. Overall, phosphorus decreases retention of calcium and hastens the turnover rate of bones.
The influence of vitamin D
In horses, the metabolism of calcium and phosphorus is under the influence of vitamin D. High dosage of vitamin D increases renal excretion of calcium and phosphorus in horses. The findings of a study show that toxic levels of vitamin D causes a twofold increase in the rate of renal excretion of calcium and a 20-fold increase in the rate of phosphorus excretion (Breidenbach, Revesz, and Harmeyer 106).
The findings imply that vitamin D has significant influence on the levels of calcium and phosphorus, and thus, an important factor to consider in equine nutrition. Despite the fact that vitamin D stimulates intestines to absorb calcium and phosphorus, high level of vitamin D is toxic because it increases excretion of calcium and phosphorus. Owing to the influence of toxic level of vitamin D, equine nutrition should have minimal amounts of vitamin D, which are only sufficient to promote absorption of calcium and phosphorus. The increased excretion of calcium and phosphorus in response to high concentration of vitamin D is under the influence of parathyroid hormone.
Conclusion
Calcium, phosphorus, and vitamin D are equine macronutrients, which have important roles in the growth and development of horses. Calcium and phosphorus are major constituents of bones and teeth. Given that horses perform extraneous activities such as race performance and transportation, they require strong bones. The metabolism of calcium and phosphorus is under hormonal regulation of parathyroid hormone. Vitamin D also aids in the formation of bones because it mobilizes deposition and resorption of calcium and phosphorus. Homeostatic mechanisms of these minerals indicate that their levels in the body influence their metabolism. Overall, horses require a regulated dietary intake of calcium, phosphorus, and vitamin D at the appropriate proportions for them to have optimal metabolism and functions.
Works Cited
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