Minerals

Chapter 6. Minerals



































































TABLE 6-1 MINERAL DEFICIENCIES, EXCESSES, AND MAJOR DIETARY SOURCES
M ineral D eficiency E xcess S ources
Calcium Rickets, osteomalacia, nutritional secondary hyperparathyroidism Impaired skeletal development; contributes to other mineral deficiencies Dairy products, poultry and meat meals, bone
Phosphorus Same as for calcium deficiency Causes calcium deficiency Meat, poultry, fish
Magnesium Soft tissue calcification, enlargement of long bone metaphysis, neuromuscular irritability Dietary excess unlikely; absorption is regulated according to needs Soybeans, corn, cereal grains, bone meals
Sulfur Not reported Not reported Meat, poultry, fish
Iron Hypochromic microcytic anemia Dietary excess unlikely; absorption is regulated according to needs Organ meats
Copper Hypochromic microcytic anemia, impaired skeletal growth Inherited disorder of copper metabolism causes liver disease Organ meats
Zinc Dermatoses, hair depigmentation, growth retardation, reproductive failure Causes calcium and copper deficiency Beef liver, dark poultry meat, milk, egg yolks, legumes
Manganese Dietary deficiency unlikely; impaired skeletal growth, reproductive failure Dietary excess unlikely Meat, poultry, fish
Iodine Dietary deficiency unlikely; goiter, growth retardation, reproductive failure Dietary excess unlikely; goiter Fish, beef, liver
Selenium Dietary deficiency unlikely; skeletal and cardiac myopathies Dietary excess unlikely; necrotizing myocarditis, toxic hepatitis and nephritis Grains, meat, poultry
Cobalt Dietary deficiency unlikely; vitamin B 12 deficiency, anemia Not reported Fish, dairy products





Minerals are inorganic elements that make up only about 4% of an animal’s total body weight; nonetheless, the essential minerals must be present in the diet to sustain life and maintain health.


CALCIUM AND PHOSPHORUS


Calcium and phosphorus are usually discussed together because their metabolism and the homeostatic mechanisms that control their levels within the body are closely interrelated. Calcium is a principal inorganic component of bone. As much as 99% of the body’s calcium is found in the skeleton; the remaining 1% is distributed throughout the extracellular and intracellular fluids. Phosphorus is also an important component of bone. Approximately 85% of the body’s phosphorus is found in inorganic combination with calcium as hydroxyapatite in bones and teeth. Most of the remaining portion of this mineral is found (in combination with organic substances) in the soft tissues.

The calcium in bone provides structural integrity to the skeleton and also contributes to the maintenance of proper blood calcium levels through ongoing resorption and deposition. The calcium in bone tissue is not in a static state but is constantly being mobilized and deposited as bone growth and maintenance take place and as the body’s needs for plasma calcium fluctuate. The level of circulating plasma calcium is strictly controlled through homeostatic mechanisms and is independent of an animal’s dietary intake of calcium. Circulating calcium has essential roles in nerve impulse transmission, muscle contraction, blood coagulation, activation of certain enzyme systems, maintenance of normal cell-membrane permeability and transport, and cardiac function.

Phosphorus that is present in bone is found primarily in combination with calcium in the compound called hydroxyapatite. Like calcium, this phosphorus lends structural support to the skeleton and is also released into the bloodstream in response to homeostatic mechanisms. The phosphorus that is found in the soft tissues of the body has a wide range of functions and is involved in almost all of the body’s metabolic processes. It is a constituent of cellular deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), several B-vitamin coenzymes, and the cell membrane’s phospholipids, which are important for regulating the transport of solutes into and out of cells. Phosphorus is also necessary for the phosphorylation reactions that are part of many oxidative pathways for the metabolism of the energy-containing nutrients. Phosphorus is a component of the high-energy phosphate bonds of adenosine triphosphate (ATP), adenosine diphosphate, and cyclic adenosine monophosphate.



When the blood calcium level is normal, PTH secretion is inhibited through a negative feedback mechanism, and calcitonin, a hormone produced by the parafollicular cells (C cells) of the thyroid gland, is released. Calcitonin functions to reduce blood calcium levels by acting primarily to increase osteoblastic activity and decrease osteoclastic activity in bone tissue. The end result is a decrease in calcium mobilization from the skeleton. Calcitonin is also released in response to hypercalcemia and the release of certain hormones, such as gastrin. Under normal physiological circumstances, PTH and active vitamin D are the most important regulators of calcium homeostasis, with calcitonin playing a more minor role. However, calcitonin may be of increased importance in the normal homeostatic mechanisms of calcium regulation during growth, pregnancy, and lactation.


Foods vary greatly in their calcium content. Dairy products and legumes contain high amounts, but cereal grains, meat, and organ tissues contain very little. The bioavailability of calcium in a food is influenced not only by the source of the mineral, but also by other dietary constituents and the animal’s life stage (see Chapter 13, pp. 111-113). Phosphorus, on the other hand, is widely distributed in foods. Foods that contain both phosphorus and calcium include dairy products and legumes. Fish, meats, poultry, and organ meats are also very rich sources of phosphorus. However, these foods are very deficient in calcium, and so their inclusion in the diets of dogs and cats must be balanced by a dietary source of calcium to ensure that an adequate calcium:phosphorus ratio is still maintained.


MAGNESIUM


Although magnesium is a macromineral, its amount in the body is much lower than that of calcium and phosphorus. Approximately 60% to 70% of the magnesium found in the body exists in the form of phosphates and carbonates in bone. Most of the remaining magnesium is found within cells, and a very small portion is present in the extracellular fluid. In addition to its role in providing structure to the skeleton, magnesium functions in a number of metabolic reactions; a magnesium-ATP complex is often the form of ATP that is used as a substrate in many of these processes. As a cation in the intracellular fluid, magnesium is essential for the cellular metabolism of both carbohydrate and protein. Protein synthesis also requires the presence of ionized magnesium. Balanced in the extracellular fluids with calcium, sodium, and potassium, magnesium allows muscle contraction and proper transmission of nerve impulses.

Magnesium is widespread in food sources and is abundant in whole grains, legumes, and dairy products. Naturally occurring magnesium deficiency is not common in dogs and cats. However, excess magnesium in the diets of cats has been implicated as a risk factor for struvite urolithiasis (see Section 5, pp. 362-365 for a complete discussion).
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Jul 31, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on Minerals

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