Minerals (Encyclopedia of Medicine)
The minerals (inorganic nutrients) that are relevant to human nutrition include water, sodium, potassium, chloride, calcium, phosphate, sulfate, magnesium, iron, copper, zinc, manganese, iodine, selenium, and molybdenum. Cobalt is a required mineral for human health, but it is supplied by vitamin B12. Cobalt appears to have no other function, aside from being part of this vitamin. There is some evidence that chromium, boron, and other inorganic elements play some part in human nutrition, but the evidence is indirect and not yet convincing. Fluoride seems not to be required for human life, but its presence in the diet contributes to long term dental health. Some of the minerals do not occur as single atoms, but occur as molecules. These include water, phosphate, sulfate, and selenite (a form of selenium). Sulfate contains an atom of sulfur. We do not need to eat sulfate, since the body can acquire all the sulfate it needs from protein.
The statement that various minerals, or inorganic nutrients, are required for life means that their continued supply in the diet is needed for growth, maintenance of body weight in adulthood, and for reproduction. The amount of each mineral that is needed to support growth during infancy and childhood, to maintain body weight and health, and to facilitate pregnancy and lactation,are listed in a...
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Minerals (Encyclopedia of Science)
Minerals are the natural, inorganic (nonliving) materials that compose rocks. Examples are gems and metals. Minerals have a fixed chemical makeup and a definite crystal structure (its atoms are arranged in orderly patterns). Therefore, a sample of a particular mineral will have essentially the same composition no matter where it is fromarth, the Moon, or beyond. Properties such as crystal shape, color, hardness, density, and luster distinguish minerals from each other. The study of the distribution, identification, and properties of minerals is called mineralogy.
Almost 4,000 different minerals are known, with several dozen new minerals identified each year. However, only 20 or so minerals compose the bulk of Earth's crust, the part of Earth extending from the surface downward to a maximum depth of about 25 miles (40 kilometers). These minerals are often called the rock-forming minerals.
Mineralogists group minerals according to the chemical elements they contain. Elements are substances that are composed of just one type of atom. Over 100 of these are known, of which 88 occur naturally. Only ten elements account for nearly 99 percent of the weight of Earth's crust. Oxygen is the most plentiful element, accounting for almost 50 percent of that weight. The remaining elements are (in descending order) silicon, aluminum, iron, calcium, sodium, potassium, magnesium, hydrogen,...
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Minerals (Encyclopedia of Children's Health)
Minerals are inorganic nutrients. That is, they are materials found in foods that are essential for growth and health and do not contain the element carbon. The minerals that are relevant to human nutrition are water, sodium, potassium, chloride, calcium, phosphate, sulfate, magnesium, iron, copper, zinc, manganese, iodine, selenium, and molybdenum. Cobalt is a required mineral for human health, but it is supplied by vitamin B12. There is some evidence that chromium, boron, and other inorganic elements play some part in human nutrition, but their role has not been proven.
Minerals should be provided by a normal, healthy diet. In special cases, additional mineral supplements may be called for. Preterm (low birth weight) infants have special needs for calcium, phosphorus, and sodium, as well as extra needs for vitamin D. Iron supplements may also be recommended.
The amount of each mineral that is needed to support growth during infancy and childhood, to maintain body weight and health, and to facilitate pregnancy and lactation, are listed in a table called the Recommended Dietary Allowances (RDA). This table was compiled by the Food and Nutrition Board, a committee that serves the United...
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Minerals (Encyclopedia of Nursing & Allied Health)
Minerals are naturally occurring inorganic substances that are obtained from food and perform a range of important functions in the body. Minerals are categorized as major minerals, or macronutrients, which are present in the body in amounts greater than five grams; and trace minerals, which are present in amounts below five grams. Trace minerals are sometimes called micronutrients.
The major minerals consist of calcium, phosphorus, potassium, sulfur, sodium, chloride, and magnesium. Sodium, potassium, and chloride are sometimes grouped together as electrolytes. An electrolyte is a substance that breaks down into ions when it is dissolved in a suitable medium and thus becomes a conductor of electricity. Each of the major minerals aids in maintaining the body's fluid, electrolyte, and acid-base balance as well as having specific functions.
CALCIUM. Calcium is the most abundant mineral in the human body; 99% of it is stored in the bones and teeth. Calcium maintains bone structure and helps regulate blood calcium levels. This mineral is also necessary for the transport of electrical ions across cell membranes. Inadequate calcium intake during childhood and adulthood can result in osteoporosis, in which there is loss of bone substance. Many Americans do not get enough calcium in their diets. Good dietary sources of calcium include milk, broccoli, mustard greens, kale, cheese, and sardines. The recommended dietary allowance (RDA) of calcium for adults is about 800 mg.
PHOSPHORUS. Phosphorus is also an abundant mineral. Most of the phosphorusbout 80%hat occurs in the body is combined with calcium in the bones and teeth. Phosphorus plays a role in the energy metabolism of cells; helps maintain the body's acid-base balance; and is needed for tissue growth and renewal. Animal products that are high in protein, such as milk, cottage cheese, and steak, are excellent sources of phosphorus. Deficiencies of phosphorus are rare except in patients taking antacids for long periods of time. The RDA of phosphorus for adults is 800 mg.
MAGNESIUM. About 50% of the body's magnesium is in the bones, with the remainder in the cells of the muscles and soft tissues. Magnesium functions in the operation of enzymes and aids in the metabolism of calcium, potassium, and vitamin D. Magnesium deficiency can result from a low intake of the mineral, from diarrhea, and from alcoholism. Magnesium deficiency can cause hallucinations and has been associated with heart problems. Good dietary sources of magnesium include spinach, oysters, baked potatoes, and sunflower seeds.
Magnesium is used in a number of over-the-counter preparations as an antacid and laxative. The most common uses of magnesium in clinical medicine include treatment of tachycardia (excessively rapid heartbeat), and depletion of electrolytes (chloride, potassium, and sodium). It is also used to manage premature labor. The RDA of magnesium is 350 mg for men, 280 mg for women.
SODIUM. Sodium is a mineral that plays an important role in the proper functioning of nerves and muscles. It is also an important component of intracellular fluid. Sodium deficiency does not occur with a normal diet, but may result from illness or injury. Too much sodium in the diet may raise blood pressure and cause hypertension. Salt is the main source of sodium in the diet, but table salt is not the most significant source of sodium. Most sodium in the average American's diet comes from processed and fast foods. The RDA of sodium is between 100 and 3300 mg.
POTASSIUM. Potassium helps maintain fluid and electrolyte balance in the body. Potassium is found in a variety of foods; however, potassium deficiency can result from illness, injury, or treatment with diuretics. The best sources of dietary potassium are fresh fruits and vegetables, especially bananas, potatoes, and raisins. The RDA of potassium is between 1875 and 5625 mg.
CHLORIDE. Chloride helps maintain fluid balance in the body. It is an essential component of the hydrochloric acid in the gastric fluid required for digestion. Chloride deficiency can result from repeated vomiting, diuretic therapy, or kidney disease. The RDA of chloride is between 1700 and 5100 mg.
SULFUR. Sulfur occurs in the body in such other compounds as thiamine and proteins. It helps to maintain the structure of skin, hair, and nails, and functions in oxidation/reduction reactions. Sulfur deficiency is a relatively unusual condition, because the body's need for sulfur is satisfied by the amino acids contained in foods high in protein.
The trace minerals, or micronutrients, include iron, iodine, zinc, fluoride, selenium, chromium, and copper. Even though these elements are present in very small amounts in the human body, they serve many important functions.
IRON. Iron is a component of hemoglobin in red blood cells and myoglobin in muscle cells. It helps these compounds to hold and carry oxygen throughout the blood and the muscles. Iron also aids in enzyme activity and cell synthesis. Lack of iron in the diet can cause iron-deficiency anemia, which is the most common nutrient deficiency in the world. Symptoms include tiredness, weakness, and a tendency to feel cold. Animal foods such as meat, poultry, and fish are excellent sources of iron. Vitamin C also helps promote the absorption of iron. The RDA of iron is 10 mg for men, 18 mg for women.
IODINE. Iodine is a mineral that is needed for the hormone thyroxine, which plays a part in energy metabolism. Iodine deficiency causes an enlargement of the thyroid gland in the neck, which is known as a goiter. A deficiency in pregnant women can also result in mental and physical retardation known as cretinism. Iodine can be found in seafood, foods grown on land, and bakery products. The RDA of iodine is 150 micrograms.
ZINC. Zinc is needed in only very small amounts, but it functions in nearly every organ of the body. It plays a role in the immune system, sperm production, taste perception, and wound healing. Inadequate intakes of zinc can result in poor growth and appetite as well as poor taste acuity. Too much zinc can impair the absorption of iron and copper in the body. Sources of zinc include meat, shellfish, poultry, legumes, and whole grains. The RDA of zinc is 15 mg.
SELENIUM. Selenium is a relatively rare nonmetallic trace element; there is less than 1 milligram of selenium in the average human body. The selenium is concentrated in the liver, kidneys, and pancreas; and in males, in the testes and seminal vesicles. It also activates thyroid hormone, which regulates the body's metabolism. Selenium can be found in a variety of foods; good sources of it include brewer's yeast, wheat germ, wheat bran, kelp (seaweed), shellfish, brazil nuts, barley, and oats. Selenium is most widely recognized as a substance that speeds up the metabolism of fatty acids and works together with Vitamin E (tocopherol) as an antioxidant. Antioxidants are organic substances that are able to counteract the damage done by oxidation to human tissue. The RDA of selenium is between 0.05 and 0.2 micrograms.
FLUORIDE. Fluoride has not been proven to be an essential mineral, but it does play a role in forming bones and teeth. Fluoride is most readily available from fluoridated drinking water. Too much of this element can cause a discoloration of the teeth known as fluorosis, but adequate fluoride consumption throughout life will help protect against dental caries. The RDA of fluoride is between 1.5 and 4.0 mg.
CHROMIUM. Chromium is closely associated with the hormone insulin, which regulates blood glucose levels. Chromium is usually depleted during food processing, which increases the chance for a deficiency if fast foods are eaten very often. Good sources of chromium include liver, whole grains, cheese, and nuts. The RDA of chromium is between 0.05 and 0.2 mg.
COPPER. Copper helps to form hemoglobin and collagen in the body as well as enzymes. Copper deficiency can impair growth and development, but is rarely encountered. Copper toxicity is also rare, but can occur from too much supplementation. Copper can be found in cherries, legumes, whole grains, seafood, nuts, and organ meats. The RDA of copper is 2 mg.
OTHER MICRONUTRIENTS. There are other trace minerals found in the body including boron, molybdenum, cobalt, and nickel. These minerals are all important to the body's health, but they are readily available in a normal diet. Deficiencies of these micronutrients are extremely rare.
Acid-base balancehe balance between the acidity and alkalinity of body fluids.
Antioxidant substance that works to counteract the damage done by oxidation to human tissue. Dietary antioxidants include the trace mineral selenium.
Electrolyten element or compound that dissociates in water and acts as a conductor of electricity.
Hemoglobin protein found in red blood cells that carries oxygen from the lungs to the tissues of the body.
Inorganicertaining to chemical compounds that are not hydrocarbons or their derivatives.
Myoglobin form of hemoglobin found in muscle tissue.
Trace elementn element that is required in only minute quantities for the maintenance of good health. Trace elements are also called micronutrients.
Vitamin and mineral supplementation has become a very common practice in the general population, due in part to aggressive advertising and marketing of dietary supplements. While vitamin and mineral supplements are beneficial to those whose diets are lacking in certain nutrients, extremely high doses of some minerals can have toxic effects. For example, too much iron can cause tissue damage and infection. High levels of magnesium can cause depressed deep tendon reflexes, fatigue, and sleepiness. High levels of selenium have been associated with tooth decay.
On the other hand, care should be taken to meet the body's needs for higher levels of mineral intake during pregnancy and periods of high physical or emotional stress (surgery, trauma, etc.).
Health care team roles
Professional dietitians and other nutrition experts are primarily responsible for recommending mineral supplementation when it is necessary and for educating consumers on the dangers of excess supplementation. They also play a role in educating the public on the benefits of eating a well-balanced diet in order to receive adequate amounts of the various minerals.
Dentists and dental hygienists should instruct patients about the importance of dietary calcium and fluoridated water to healthy teeth.
Physicians, registered nurses, and pharmacists should instruct patients about the possible side effects of certain medicationsparticularly diuretics, antihypertensives, and some types of laxativesthat may cause electrolyte imbalance. Emergency room personnel should be knowledgeable about mineral deficiencies and mineral toxicities in the differential diagnosis of such symptoms as cardiac arrhythmias, seizures, disorientation, muscle twitching, and muscle weakness.
Baron, Robert B., MD, MS. "Nutrition." Current Medical Diagnosis & Treatment 2001. Edited by Lawrence M. Tierney, Jr., MD, et al. New York: Lange Medical Books/McGraw-Hill, 2001.
Mahan, Kathleen L., and Sylvia Escott-Stump. Krause's Food, Nutrition, and Diet Therapy. 10th ed. Philadelphia: W. B. Saunders Company, 2000.
The Merck Manual of Diagnosis and Therapy. Edited by Mark H. Beers, MD, and Robert Berkow, MD. Whitehouse Station, NJ: Merck Research Laboratories, 1999.
Russell, Percy J., and Anita Williams. The Nutrition and Health Dictionary. New York: Chapman & Hall, 1995.
Sizer, Frances S., and Eleanor N. Whitney. Nutrition: Concepts and Controversies, 7th ed. Belmont, CA: Wadsworth Publishing Company, 1997.
Committee on the Scientific Evaluation of Dietary Reference Intakes. Institute of Medicine (1997) Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride. Washington, DC: National Academy Press, 1997.
Nutrition Hotline, American Dietetic Association. 216 West Jackson Blvd., Suite 800, Chicago, IL 60606. (800) 366-1655.
Lisa M. Gourley
Minerals (World of Earth Science)
The term mineral is often used to denote any material that occurs naturally in the ground, including oil and natural gas. However, mineralogists and geologists restrict its use to naturally occurring solids having specific chemical compositions. For example, all solid forms of pure silica (SiO2) are minerals, including natural glass and quartz, but coal is not a mineral because it has no definite and universal chemical composition.
Solids produced by living thingsones, shells, pearls, and the likere a special case. Scientists usually consider these objects non-minerals even when they have definite a chemical composition, as do the calcium carbonate (CaCO3) shells of marine animals. The distinction is more professional than physical; mineralogists study minerals, but biologists study shells and bones, so shells and bones must not be minerals. However, biological solids that have been completely rearranged at the atomic level are officially regarded as minerals. For example, graphite and diamond formed by metamorphosis of coal are minerals.
Because solidity is part of the definition of a mineral, substances may change from mineral to non-mineral or vice versa by melting or solidifying. Liquid water has a definite chemical composition (H2O) but is not considered a mineral because it is not solid; ice, however, is a mineral. Magma or molten lava are not minerals because they have no definite, universal composition and are liquids; solidified, they become mixtures of specific minerals.
The atoms making up a mineral may be arranged either randomly, like mixed marbles in a bag, or in an orderly pattern, like squares on a chessboard. If a mineral's atoms show long-range organization, the mineral is termed crystalline. The objects commonly called crystals are crystalline minerals of relatively large size that happen to have developed smooth faces. Many crystals, however, are too small to see with the naked eye, and most have imperfectly developed faces or none at all. Most rocks consist of chunks of several crystalline minerals fused together. In some rocks, such as granite, these individual pieces are large enough to see, while in others, such as slate, they are too small.
If a mineral's atoms are randomly arranged it is termed an amorphous mineral or a mineraloid. The most common amorphous mineral is glasshe solid formed by cooling magma or molten lava so quickly that its atoms do not have time to organize into crystals. Molten lava quenched in air or water, or intrusive magma cooled rapidly by contact with rock form glasses. All glasses are metastable; that is, they tend to lapse into crystalline form, much as water molecules in cold vapor organize themselves into snowflakes. In the case of glasses, this spontaneous crystallization process is termed devitrification. The processes of devitrification causes glasses to be rare in proportion to their age. Most natural glasses date from the last 60 or 70 million years, a mere tenth of the time since the beginning of the Cambrian Period. The remainder have devitrified.
Because oxygen, silicon, and other elements may be present in any ratio in a glass, depending on the composition of the original melt, some mineralogists do not consider glasses minerals and restrict the term mineral to naturally occurring crystals. For the remainder of this article, the term mineral will be used in this restricted sense.
Earth's crust and mantle consist almost entirely of minerals, yet the number of known minerals is less than 3,000. Two factors limit the number of possible and actual minerals. First, a crystal's atoms must be arranged in some periodically repeating, three-dimensional pattern, but only a finite number of such patterns exists. Second, there are only a few score naturally occurring elements, many of which are rare and eight of whichxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium, in order of decreasing commonnessomprise
98.5% of Earth's crust by weight. Oxygen alone makes up approximately 47% of the crust by weight (over 90% by volume), and silicon makes up approximately another 27%. The number of minerals that can form is therefore finite, and many of those that could theoretically form do so rarely.
The atoms of the two most common elements on earth, silicon and oxygen, readily arrange themselves into tetrahedra (four-sided pyramids) having a silicon atom at the center and an oxygen atom at each point. This unit is the silicate radical, (SiO4)4/sup>. Silicate radicals can link into sheets, chains, or three-dimensional frameworks by sharing oxygen atoms. If every oxygen atom participates in two tetrahedra, then the overall ratio of silicon to oxygen is 1:2, and the resulting chemical formula is that of silica, SiO2. Minerals built mostly of silica are termed silicate minerals. The mineral quartz is pure crystalline silica; other silicate minerals result when atoms of elements other than silicon are introduced at regular intervals. For example, some of the tetrahedra in the silicate framework may be centered on aluminum atoms rather than silicon atoms. In this case, atoms of other elements (usually calcium, potassium, or barium) must be present to balance the ionic charges in the framework. The silicate minerals having this particular structure are the feldspars, which make up approximately 60% of the earth's crust by volume.
When atoms of elements other than silicon unite with oxygen to form the basic building block of a mineral, nonsilicate minerals result: carbonates from carbon (e.g., calcite [CaCO3]), sulfates from sulfur (e.g., anhydrite [CaSO4]), phosphates from phosphorus (e.g., apatite [Ca5(PO4)3F]), and the oxide minerals, in which O2/sup> alternates with positively charged ions (e.g., spinel [MgAl2O4]). Other mineral groups do not involve oxygen at all, including the halides (e.g., salt [NaCl]), the sulfides (e.g., pyrite [FeS2]), and the native elements (pure sulfur, carbon, gold, etc.).
Although for simplicity's sake chemical formulas have been identified with mineral species in the preceding paragraph, the identity and properties of a mineral depend not only on what kinds of atoms compose it but on the arrangement of these atoms in space. Diamond and graphite, for instance, both consist entirely of carbon atoms and so have the same chemical formula (C), but differ in structure. A mineral's structure, in turn, depends partly on its chemical formula and partly on its history, that is, on the changes in pressure, temperature, and chemical context through which it has passed in reaching its present state. A simple example of a mineral structure recording process is the production of glass by rapid cooling of molten silica. To hold a piece of glass is to know a small, specific piece of history; this silica must have cooled rapidly. The dependence of mineral formation on time and temperature is exactly analogous to cookery. Indeed, geologists routinely speak of how the formation of minerals in large bodies of cooling magma is influenced by the "baking" of the magma. Minerals are therefore studied not only for their directly useful properties but for what their very existence reveals about the history of the earth.
See also Crystals and crystallography; Minerology; Obsidian
Minerals (Encyclopedia of Food & Culture)
MINERALS. Living organisms appear to selectively concentrate certain elements from the environment while rejecting others. The adult human body contains approximately thirty-five elements. Four of these (hydrogen, oxygen, carbon, and nitrogen) constitute 99 percent of the atoms in the body. As a comparison, the most abundant elements in the Earth's crust are oxygen (67 percent), silicon (28 percent), and aluminum (8 percent). The remaining 1 percent of the elements in the human body (with the exception of sulfur) are the inorganic or mineral constituents of the body and thus form the ash when the body is "burned." Seven of the remaining elements, sodium, potassium, calcium, magnesium, phosphorus, sulfur, and chloride, together represent about 0.9 percent of the body's weight. The seventeen others make up the remaining 0.1 percent, some of which, but not all, are considered nutritionally essential. These elements appear in the body at measurable concentrations but may not perform an essential biological function. Cadmium is one such example. The newborn infant is virtually free of this element, but gradually accumulates cadmium by ingestion and inhalation, such that over a lifetime an average person living in an industrial society accumulates milligrams of this element. Not only does cadmium appear to serve no essential function in the body, it is also likely to be undesirable and potentially detrimental.
Most experts agree that thirteen mineral elements are nutritionally essential. These are minerals that when deficient consistently result in an impairment of a function that is prevented or cured by supplementation. There still is some question about seven others (Table 1).
The functions of mineral elements are structural, osmotic, catalytic, and signaling. Calcium plays the most obvious role as structural component of bone but also participates in many examples of cell signaling. Sodium, chloride, and potassium constitute the majority of minerals whose function is to maintain osmotic and water balance and membrane electrical potentials. The micro-mineral elements listed in Table 1 have historically been classified as "trace" elements primarily because they occurred at levels below past methods for detection. In general, these minerals function as biocatalysts. Iron is the most prominent example because a deficiency of iron is probably the most common nutritional deficiency on earth (anemia afflicts more than 15 percent of the world's population). Copper and zinc are the prototypical biocatalysts because virtually all of their known functions involve either catalytic or structural roles in many different enzymes. Copper is unique in that all of the known deficiency symptoms in experimental animal models can be explained on the basis of failure of known enzymes. Zinc deficiency, on the other hand, presents symptoms that are not directly attributable to any of the fifty or more enzymes in which it is found. Selenium, manganese, and molybdenum are also constituents of enzymes. Deficiency symptoms for selenium and manganese have been well characterized but a nutritional deficiency of molybdenum has not been satisfactorily demonstrated. The most compelling reason to include molybdenum among the thirteen nutritionally essential elements is because of its presence (and thus function) in several important enzymes. Some microminerals serve a very narrow range of biological functions. Iodine and cobalt are exclusively constituents of thyroid hormones and vitamin B12, respectively. No other role has been identified for these elements.
|Known nutritionally essential minerals|
|Element||Amount in 70-kg Human (g)||Function|
|Calcium||1,200||Component of bones; signal transduction in hormonal action, muscle contraction, blood clotting; and structural role in proteins|
|Phosphorus||700||Component of bone Necessary for activation of high energy intermediates|
|Potassium||240||Osmotic, electrolyte, and water balance|
|Chloride||120||Osmotic, electrolyte, and water balance|
|Sodium||120||Osmotic, electrolyte, and water balance|
|Magnesium||35||Activation of ATPases, kinases, and other enzymes|
|Iron||4.0||Catalytic redox reactions, oxygenation, and O2-carrying proteins|
|Zinc||2.0||Catalytic as a Lewis acid and structural function for some metalloenzymes|
|Copper||0.1||Catalytic in redox reactions some involving iron|
|Selenium||0.020||Structural and catalytic component of peroxidases, especially glutathione peroxidase. Provides antioxidant protection|
|Iodine||0.015||Component of thyroid hormones|
|Molybdenuma||0.012||Structural component of enzymes, especially xanthine oxidase and sulfite oxidase|
|Manganese||0.015||Catalytic role in enzymes involved in cartilage formation|
|Cob||0.001||Structural component of vitamin B12|
Abbreviations: ATPase, adenosine triphosphatase.
aBiochemical evidence only that it is essential.
bEssential only as a component of vitamin B12.
The remaining mineral elements are those that occur in significant concentrations in the human body and most probably serve an important biological function. However, consistent findings regarding deficiency symptoms and specific biochemical functions have not been reported. Fluorine is a unique example of a mineral that currently has no definitive biological function but because it appears beneficial to dental health, it is a recommended nutrient.
Calcium and Phosphorus
Approximately 99 and 85 percent of the total calcium and phosphorus, respectively, in the human body are found in bone. Both ions leave the bone and are deposited back each day representing normal metabolic activity or "turnover" of bone. The remaining 1 percent of calcium is found in both extracellular and intracellular pools and is absolutely critical for normal body function such as muscle contraction and nerve activity. Although very rare, a sudden drop in extracellular concentrations of calcium (<50 percent) can lead to an emergency situation such as tetany or convulsions. Nerve cells bathed in hypocalcemic fluid spontaneously "fire," leading to uncontrolled nerve activation and muscle spasm. The majority of the extracellular calcium is in chemical equilibrium with bone. Approximately 30 percent is under hormonal control by several hormones, parathyroid hormone, vitamin D, and thyrocalcitonin. As a result, the concentration of extracellular calcium is remarkably constant. Blood levels of phosphorus fluctuate much more and appear to be determined in large part by urinary excretion.
The absorption of calcium from the diet is dependent on a number of dietary and physiological factors. Vitamin D is synthesized in skin when exposed to ultra-violet irradiation [290 to 315 nanometers of ultraviolet (UV) light]. Sunscreen lotions [Sun Protection Factor (SPF) 8] can reduce this synthesis as much as 90 percent. Inadequate sunlight exposure was most likely the cause of calcium deficiency rickets observed at the turn of the century in countries at northern latitudes. A change in dietary calcium absorption in humans appears to take several weeks to accomplish but accounts for the ability of humans to tolerate diets that provide relatively little calcium (200 to 400 mg/day). This activation process becomes less potent with age and may account in part for the increased calcium requirements with age.
Dietary factors affecting the absorption of calcium are well known. They include chelating organic acids such as oxalic and phytic acid. The former is the most potent and is responsible for the markedly diminished "availability" of calcium found in spinach.The amount of calcium contained by a food is only an approximation of the amount of calcium that is ultimately "available." Estimated fractional absorption (percent of intake absorbed into the body) of calcium from these foods ranges from 5 percent for spinach to 61 percent for broccoli. Vegetables of the Brassica family such as broccoli and cabbage appear to contain little oxalate and thus contain calcium that exhibits higher bioavailability than dairy products. Milk and dairy products have relatively high calcium content as well as relatively high fractional absorption (30 percent), resulting in the highest amount of calcium per serving. Lactose in milk enhances the absorption of calcium in infants but its effect in adults is less clear. Other dietary factors affect the retention of dietary calcium but have little impact on its absorption. For example, high intakes of either sodium or protein are thought to result in increased urinary losses of calcium. Protein increases renal calcium loss by increasing acid load while sodium increases losses via shared renal transporters. Both of these conditions may affect calcium balance and ultimately the requirements for this nutrient. The bone loss associated with chronic calcium losses or negative calcium balance may ultimately lead to weakened bones or osteoporosis. Calcium supplements may adversely affect the bioavailability of iron.
Calcium deficiency occurs primarily as rickets or osteomalacia in young children. Bones are deformed (bowed legs) and weak due to inadequate calcification of the protein matrix of bone. This deficiency can arise as a result of too little dietary calcium (relatively rare) or inadequate vitamin D synthesis. Historically, the latter has been the major cause brought about primarily because of reduced exposure to sunlight. It is conceivable, however, that dietary factors such as oxalates and cultural customs (clothing) may interact to play a role in the development of rickets especially since recent cases have been reported in areas of the world near the equator where sunlight should not be limiting. Calcium deficiency does not appear to be a primary cause of osteoporosis. This condition is characterized not by inadequate bone mineralization but by a loss of total bone both protein matrix and mineral. Bones weaken and become susceptible to fracture.
Sodium and Chloride
Total body sodium is approximately one-tenth of that of calcium. One-third of body sodium is found in bone but its metabolic significance is unknown. Sodium and chloride constitute the major cation and anion, respectively, in the extracellular fluid of humans. Sodium is the primary determinant of the osmotic pressure of the extracellular fluid and as such is the main determinant of extracellular fluid volume. The sodium ion concentration changes less than 3 percent day in and day out despite dramatic fluctuations in sodium intake. This is a reflection of a very tightly controlled and highly regulated system to maintain constant osmotic pressure. Through most of human evolution, the availability of dietary salt has been very highly restricted. Much of dietary sodium (and chloride) were derived from sources such as meat and vegetables, which contain very low levels. Consequently, humans and other mammals have evolved physiological mechanisms that permit sodium conservation under extreme conditions. This physiological conservation system comprised of pressure receptors, renal renin, lung angiotensinogen, adrenal aldosterone, and vassopression all makes dietary requirements extremely difficult to assess. For example, the Yanomamo Indians in Northern Brazil have been found to excrete as little as 1 mEq/day of sodium (Na) per day. This reflects a dietary consumption of approximately 60 mg salt per day (over 100 times less than that which is normally consumed in Western populations). At the other extreme are the northern Japanese, who consume nearly 26 grams of salt each day. These regions of Japan have unusually high incidences of cerebral hemorrhage, most likely related to the high incidence of hypertension. Other areas of the world such as Northern Europe and the United States consume approximately 10 g/day or less of salt. The sodium and potassium contents of some selected foods are shown in Figure 1. It is apparent that many "unprocessed" foods contain very little sodium. Estimates of sodium intake suggest that over 85 percent of the sodium consumed in Western diets is sodium added during processing. This is clearly illustrated by the progressively higher sodium content of peas (fresh, frozen, and canned) and perhaps more important, the dramatic reduction in potassium content. The net result is a reversal of the naturally low sodium to potassium ratio found in all fresh plants.
A deficiency of sodium normally does not occur even in areas where salt is scarce. The abnormal loss of sodium and other electrolytes, however, could occur under conditions of extreme sweat loss, chronic diarrhea and vomiting, or renal disease, all of which produce an inability to retain sodium. Acute episodes of diarrhea or vomiting resulting in a loss of 5 percent of body weight could lead to shock. The most important therapy under these circumstances is to restore sodium and water or circulatory volume. Chloride deficiency has been reported in infants consuming low-sodium chloride formulas. They show signs of metabolic alkalosis, dehydration, anorexia, and growth failure. Potassium depletion most notably affects cardiac function where either elevations or reductions in serum potassium can cause arrythmias.
Magnesium is an important intracellular ion involved in many enzymatic reactions of food oxidation and cell constituent synthesis. Approximately 60 percent of total body magnesium is found in bone, where approximately half can be released during bone resorption. Magnesium food sources are widely distributed in plant and animal products with the highest content found in whole grains and green (high chlorophyll) leafy vegetables. Refining wheat with the removal of the germ and outer layers may remove nearly 80 percent of the magnesium from wheat. Meats and most fruits and vegetables are poor sources of magnesium. The absorption of magnesium appears to be unrelated to the absorption of calcium (that is, is independent of vitamin D) and is relatively unaffected by food constituents. Phytate and phosphates, however, may adversely affect magnesium availability by forming insoluble products although their practical significance is unclear. Experimental magnesium deficiency has been produced in humans. Urinary magnesium drops virtually to zero while plasma levels are relatively well preserved. The change in urinary excretion reflects a "urinary threshold" for magnesium. After continued deficiency, however, neuromuscular activity is affected, ultimately leading to tremors and convulsions. Serum and urinary calcium levels are profoundly reduced and not restored by parathyroid hormone administration. It was concluded that magnesium is essential for the mobilization of calcium from bone. A deficiency of magnesium under normal conditions is unlikely but may occur with the presence of other illnesses such as alcoholism or renal disease.
Over 65 percent of body iron is found in hemoglobin, the respiratory pigment used to transport oxygen within and between tissues. One-third of body iron is a "storage" form that can be mobilized during times of need. The amount of "storage" iron may vary greatly with age and gender. Food sources of iron are complicated by numerous factors that affect the bioavailability of dietary iron. Non-heme sources of iron are found in plant and vegetable products and the absorption from these sources (versus heme found in meat products) is generally lower and influenced to a greater extent by total diet composition. Vitamin C is probably the most signficant enhancer of non-heme iron absorption, while plant phenolics such as tannins found in teas and phytates found in cereals are some of the most potent inhibitors. None of these factors, however, affect the absorption of heme iron found in meats. Iron status can markedly affect the amount of iron absorbed from a mealow status increases iron absorption. The effect is most pronounced for non-heme iron, changing over fourfold compared to 50 percent for heme iron. Although iron status can influence absorption, the most important determinant of iron availability is the composition of the diet. It is clear that non-heme iron absorption is markedly affected by the characteristics of the food with which it is eaten and that there are clear differences in the nature of absorption of heme and non-heme iron. Iron deficiency is seldom related to iron intake per se. Major causes of anemia (too little hemoglobin) include blood loss and/or diets containing either no enhancers (such as meat or ascorbic acid) or high levels of inhibitors. Infection can also change iron metabolism significantly such that much of the anemia in the world is due to chronic infection. The losses for iron for both men and women are known precisely but the amount of dietary iron requirement depends on the overall diet.
Zinc is present in all tissues and performs both structural and catalytic functions in many different enzymes. Unfortunately, changes in the activities of these enzymes are not sufficient to explain the pathological effects of experimental zinc deficiency. Experimental animals refuse to eat experimental diets that are very low in zinc. Human zinc deficiency was demonstrated nearly two decades ago in the United States. Young children from 6 months to 5 years of age showed low amounts of zinc in the hair relative to other groups. Hair zinc and taste acuity were restored after three to five months of zinc supplementation. Earlier studies also revealed zinc deficiency in regions of Iran and Egypt. It is very difficult to assess zinc status in humans. Serum zinc is not adequate to assess nutritional status. In experimental situations, serum zinc falls remarkably (<50 percent) following a low zinc intake without immediate (or apparent) ill effects. In 1974, a Recommended Dietary Allowance (RDA) of 15 mg/day was established for zinc. (It was not until 1974 that we had enough information to estimate an RDA for zinc, at which time the value was established at 15 mg. The RDA presented in 1989 gives 15 mg per day for adults. The 2001 Institute of Medicine value is 11 mg per day.) Approximately 70 percent of zinc consumed by most people is derived from animal products. Cereals contain appreciable zinc but the availability varies considerably. Several plant compounds interfere with the absorption of zinc. The most prominent of these is phytates (inositol hexa-and pentaphosphate). These inhibitors most likely contribute to the natural incidence of dietary zinc deficiency observed in humans.
Although the importance of copper deficiency in animals has been recognized since the 1930s, it is still not possible to establish an RDA for copper in humans because of the uncertainty regarding the quantitative requirements. There is no doubt that copper is an essential nutrient for humans. Current estimates of the minimum copper requirement are between 0.4 and 0.8 mg/day. Copper is critical for the function of several enzymes, especially blood ceruloplasmin. The activity of this enzyme in blood falls dramatically in experimental animals soon after giving copper-deficient diets and is thought to be a good indicator of copper depletion even in humans. Ceruloplasmin is essential for iron absorption (it catylizes the oxidation of Fe2/sup> to Fe3/sup> required for binding of iron to the blood transport protein, transferrin) and explains the anemia observed in copper deficiency. In contrast to zinc, all of the symptoms of a copper defeciency under experimental conditions can be explained by changes in various enzymes that require copper. Two inherited diseases associated with abnormal copper metabolism have been observedne (Menkes' disease) is associated with copper deficiency, while the other (Wilson's disease) is a disease of excessive copper accumulation. Excessive intake of zinc can precipitate a copper deficiency. An example of zinc-induced copper deficiency has been reported in humans and is attributed to a reduction in the absorption of copper. Excessive zinc may induce intestinal proteins that bind copper and thereby prevent its transfer from the intestine into the body.
Approximately 80 percent of total body iodine (20 milligrams) is found in the thyroid gland. All of the iodine that leaves this gland does so as a component of the thyroid hormoneshyroxine and triiodothyronine. In fact, all of the functional significance of iodine is as a component of these hormones. Iodine deficiency represents the most common cause of preventable mental deficits in the world's population. Since most of the world's iodine is found in the oceans, coastal areas are not deficient. However, mountainous areas such as the Himalayas, European Alps, and the mountains of China, as well as the flooded river valleys of Asia, areas where leaching of iodine from soils has occurred for eons, produce iodine-deficient crops and plants. Iodine deficiency during pregnancy causes cretinism, a diet-related birth defect that is characterized by permanent mental retardation and severe growth stunting. In young children and adults, iodine deficiency results in enlarged thyroid glands or goiter. Although various foods such as cassava, cabbage, and turnips contain goitrogens, substances that interfer with iodine metabolism, their practical signficance is not clear. Cassava, the dietary staple in regions of Africa and other areas, may be the exception, especially when not well cooked. The cyanide released by the ingestion of this plant is transformed and ulitmately leads to an inhibition of the uptake of iodine by the thyroid. Goiter was once common in areas of the United States near the Great Lakes and westward to Washington State, but the introduction of iodized salt almost competely eliminated goiter in these areas by the 1950s. The minimum requirement for iodine to prevent goiter is approximately 1 μg/kg/day whereas the recommended intake is nearly twice this amount.
Although selenium was first recognized as a toxic trace element for livestock, it is now clear that selenium is an essential nutrient for all animals. During the 1930s, livestock grazing in parts of the Great Plains of North America were found to contract a disease characterized by hair loss, lameness, and death by starvation. The cause of this disease was excess selenium obtained from the plants grown in soils containing high selenium concentration. In fact, selenium, more than any other essential trace element, varies greatly in its concentration in soils throughout the world. Plants accumulate selenium from soils but are not thought to require selenium for growth. Although human toxicity was not observed in affected regions in the United States, endemic selenium poisoning has been observed in high-selenium regions of China where the symptoms included loss of hair and nails. China also possesses regions of very low selenium where, in fact, humans have been diagnosed with selenium deficiencyeshan disease (cardiomyopathy) and Keshaneck disease (degenerative joint disease). Although other factors may be involved, selenium deficiency is clearly a predisposing factor. Selenium functions as part of several important enzymes. The most prominent is a soluble enzyme, glutathione peroxidase, whose function is to reduce hydrogen peroxide and organic (lipid) peroxides, thus preventing the oxidative destruction of cell membranes. Selenium is incorporated into the enzyme as the amino acid selenocysteine by reactions that are unique to selenium. Together with vitamin E, selenium, as a structural component of glutathione peroxidase, forms an antioxidant defense against oxidative stress. The requirement for selenium has been estimated by various methods. On the basis of intakes in regions of China with and without deficiency disease, approximately 20 μg/day is considered an adequate amount to prevent deficiency. The estimated safe and adequate selenium intake suggested by the U.S. National Research Council ranged from 50 to 200 μg/day in 1980. An amount to maintain the highest serum glutathione peroxidase activity appears to be 70 and 55 μg/day for an average man or woman, respectively, which became the Recommended Dietary Allowance (RDA) in 1989. In 1996, the World Health Organization recommended 40 and 30 μg/day for men and women, respectively. Intakes greater that 400 μg/day are considered to be the maximum safe level. Selenium is thus an example of a nutrient that possesses a relatively narrow range of intakes that are safe and that meet requirements.
Normal body content of manganese is very lowpproximately 15 milligrams or very similar to iodine. In contrast to iodine, manganese deficiency has not been observed in humans but has occurred naturally in chickens and experimentally in many other species. Manganese is required by several enzymes, which may or may not be inolved in the symptoms of a manganese deficiency. Symptoms include impaired growth, skeletal abnormalities, and defects in lipid and carbohydrate metabolism. The role of manganese in the synthesis of the mucopolysaccharide component of bone and cartilage is the most crucial whereas mineralization of bone appears to be independent of manganese. Excessive manganese will interfere with iron absorption. Under conditions of iron deficiency, manganese absorption is increased. Both iron and manganese appear to share a common site for absorption. The recommendations for manganese intake are based on estimates of normal dietary intakes of 2 to 5 mg/day. This amount is thought to be sufficient to replace the 50 percent of body manganese that is lost every 3 to 10 weeks.
Chromium is one of the most intriguing and potentially important trace elements because it appears to influence the action of a critical hormone, insulin. Unfortunately, the definitive role of chromium in this regard awaits further study. Decreased sensitivity of peripheral tissues to insulin appears to be the primary biochemical lesion in experimental chromium deficiency. Impaired glucose tolerance has been attributed to chromium deficiency in several experimental models. Also, several patients receiving total parenteral nutrition have responded to chromium supplementation in the predicted manner, that is, improved glucose tolerance. These findings have established chromium as an essential nutrient for humans but the specific deficiency symptoms in those who receive enteral feeding have not emerged. Overt chromium deficiency is very unlikely under normal conditions due to the small amounts of chromium needed. Moreover, a marginal deficiency is very difficult to identify due to the lack of reliable markers for diagnoses concerning chromium. Currently, there is little or no evidence that chromium supplements are either warranted or effective. Even the recommended intakes for adults (50 to 200 μg/day) are uncertain due to the lack of reliable methods for assessment.
Fluoride is not generally considered to be an essential element for humans. It is, however, considered beneficial in that normal intakes appear to reduce the incidence of dental caries. The mechanism of this benefit is thought to be due to incorporation of fluoride into the mineral matrix of tooth enamel, thus producing a more resistant mineral apatite crystal. Over 99 percent of the fluoride found in the body is found in bones and teeth as a component of this mineral apatite crystal. An unusually high intake of fluoride causes permanently discolored or mottled teeth, a condition identified in children drinking water with 2 to 3 parts of fluoride per million. The level of fluoride commonly maintained in municipal water supplies is 1 part per million.
Silicon and Nickel
Silicon is the most abundant mineral in the Earth's crust. It is thus surprising that a need for silicon in biological systems has not been more prominent. Limited research conducted since 1974 has indicated a role for silicon in the development of mature bones in chickens and rats. A human requirement has not been established but estimates in the range of 10 to 20 mg/day have been suggested. Most likely intakes of this magnitude occur under normal conditions. Nickel deficiency has been experimentally produced in several species. Growth depression and changes in iron metabolism have been described. Nickel has been discovered in the enzyme urease from bacteria, fungi, yeasts, algae, plants, and invertebrates. Many other enzymes exist for which nickel is apparently a component. Thus, it is likely that nickel plays an essential functional role in higher organisms, including humans.
Molybdenum is an essential component of at least three important enzymes found in animals and humans. A deficiency of one of these enzymes, sulfite oxidase, can have severe consequenceseizures and severe mental retardation in infancy. This deficiency has arisen in patients with genetic mutations in cofactor synthesis but not as a primary molybdenum deficiency. The dietary requirements of molybdenum cannot be given, or even approximated, for any animal species including humans. A deficiency of molybdenum has not been observed under natural conditions for any species. Despite this, the biochemical role of molybdenum as a component of several enzymes establishes it as an essential nutrient for humans.
See also Assessment of Nutritional Status; Calcium; Dietary Assessment; Dietary Guidelines; Fluoride; Food, Composition of; Fruit; Iodine; Iron; Malnutrition; Nutrients; Nutrition; Sodium; Trace Elements; Vegetables; Vitamins.
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Charles Chipley W. McCormick
Minerals (World of Forensic Science)
Minerals have played many important roles in the world of forensic science, from forensic geology used in criminal identification and crime scene investigations, to forensic toxicology and the study of poisons.
Historically, metal-based mineral poisons were commonly used as murder weapons, with arsenic a favorite. In fact, arsenic was often referred to as "inheritance powder" for its efficacy in hastening the demise of wealthy relatives. In the eighteenth century, the Dutch physician Hermann Boerhaave (1668738) was the first expert witness to use basic forensic toxicological methods as the basis for testimony at a murder trial. In this case, Mary Blandy was encouraged by her fiancé to use a powdered preparation in order to get the money from her father's estate (he was very much alive at the time). She dutifully put the white substance into her father's food; he became ill. The servants became suspicious. One of the servants found the white powder and took it to a local apothecary for examination, where the hypothesis was arsenic. The servant relayed her concerns to her employer, who dismissed them, and not long after, was dead. Mary was tried for murder, and four medical toxicologists served as expert witnesses. They noted that the appearance of Mr. Blandy's organs at autopsy was suggestive of arsenic poisoning. Boerhaave reported that he had taken some of the white powder saved by the servant, treated it with a hot iron and smelled it (not a safe test for poisons, by any means). The smell was that of arsenic. Equally important was the testimony of the servant, who was able to describe the white powder that she had observed Mary putting into her father's food. Mary Blandy was found guilty of murder, sentenced to death, and hanged shortly thereafter. This trial set the stage for development of forensic toxicological methods for detection of metal-based (and other) poisons.
In 1911, a forensic method for determining the quantity of metal-based poisons in internal organs was developed by the English physician William Willcox, who was particularly interested in arsenic poisoning. He ran several tests for arsenic, and then used this method to determine how much arsenic was in each of the internal organs of Elizabeth Barrow, a victim of murder by poisoning. His method was used as the basis for far more sophisticated toxicological testing, which can now determine the amount of arsenic down to the microgram (one one-millionth of a gram) in both the human body and in soil.
After the middle of the twentieth century, thallium, a new metal-based poison, was popular for use in rat poison. Although it was banned from commercial use in 1984, it remained readily available in rat poison for at least another decade. In August 1991, Robert Curley developed a barrage of confusing symptoms and was repeatedly hospitalized. The cluster of symptoms included uncontrollable vomiting, abrupt hair loss, numbness of the extremities, general weakness, and burning skin. Shortly before his death in September 1991, he became combative, agitated, and aggressive; at that point, heavy metal exposure was hypothesized. A battery of tests revealed markedly increased thallium levels in his system.
Curley worked in a chemistry laboratory at Wilkes University in Wilkes-Barre, Pennsylvania. Five bottles of thallium salts were found in a stockroom there, although none of his coworkers became ill or evidenced any signs of accidental thallium exposure. Upon Curley's death, an autopsy was performed; it revealed extremely high thallium levels, confirming intentional poisoning, and leading to a ruling of homicide. During the investigation, the Curley home was examined, and several thermoses tested positive for thallium. Curley's widow reported that her husband brought iced tea to work in the thermoses daily. Curley's widow and her daughter by a previous marriage were found to have slightly elevated thallium levels, but they were well below the toxic range. Curley's widow sued the university for wrongful death. Upon further investigation, it was found that she had collected more than one million dollars from a car accident involving her first husband, and had also gained nearly three hundred thousand dollars in life insurance proceeds after Curley's death. At that point, she became a suspect, and the local criminal authorities requested exhumation of the body in order to perform more sophisticated testing.
Frederic Reiders, of National Medical Services, agreed to run forensic toxicology tests on Curley's hair shafts, toenails, fingernails, and skin. From the length of the victim's hair, Reiders was able to create a timeline extending 329 days before Curley's death. He used atomic absorption spectrophotometry to record thallium levels at different times. The surprising conclusion was that Robert Curley had been systematically exposed to thallium, through ingestion, for a period of nine months before his death. There was a sharp spike several days before his death, indicating intentional poisoning. Hair from other parts of his body, as well as the skin, fingernail, and toenail samples, all supported the conclusions reached by Reiders after testing the head hair. It was further determined that the valleys, corresponding to drops in thallium level, occurred whenever Curley was away from home (or in the hospital). When confronted with conclusive evidence, Curley's widow plea-bargained and confessed to poisoning her husband in an effort to gain his life insurance proceeds.
As testing for metal-based poisons has become progressively more conclusively detectable, the criminal use of these substances as a "murder weapon" has dramatically decreased in favor of plant-based toxins.
SEE ALSO Chemical and biological detection technologies; Energy dispersive spectroscopy; Food poisoning; Gas chromatograph-mass spectrometer.