CALCIUM. Calcium (Ca2) is a silver-white metallic element of the alkaline-earth group. Ninety-nine percent of calcium in the human body is in bone and teeth. The remaining one percent is in blood and body fluids. In addition to its role in maintaining strength of bone and teeth, calcium is involved in nerve cell function, control of muscle tone, and blood clot formation. Calcium is also necessary in order for many important proteins to properly perform critical metabolic functions throughout the body.
Cells. Calcium concentrations in the fluids outside cells are much larger than calcium concentrations inside cells (the cytosol). Unequal calcium concentrations in the extracellular fluid and cytosol are required for cells to carry out many crucial functions. For example, when a hormone in the blood binds to a receptor on the cell, calcium pours into the cytosol from extracellular fluid. This change in the amount of calcium in the cytosol signals the cell to perform some critical function. The critical function that is triggered depends on the type of cell. (In muscle cells, for example, a nerve signal triggers the release of calcium into the cytosol, allowing muscle contraction to occur.) After the critical function is performed, calcium is rapidly pumped out of the cell, and the calcium concentration in the cytosol returns to the normal (low) level.
Structural. In addition to cellular functions, calcium's more familiar role is a structural one—as a component of bones and teeth. Blood calcium levels are maintained strictly even if calcium has to be taken from bone. Bone mineral (hydroxyapatite) is made up primarily of calcium, phosphate, and carbonate. Bone constantly changes during growth and throughout adulthood. Changes in bone occur through balancing activities of bone-destroying cells (osteoclasts) and bone-forming cells (osteoblasts), which act together to remove and replace bone, respectively. During growth, bone formation generally exceeds destruction, yielding net bone-mass gain in the whole skeleton.
Bone-mass accumulation continues until peak bone mass is achieved, generally during the third decade of life. The age at which peak bone mass is reached varies by gender and differs by skeletal site. Males achieve peak bone mass later than females and gain more bone during puberty than females, resulting in larger bones. Although peak bone mass at all skeletal sites is generally reached by age thirty, bone accumulation is nearly complete by age twenty in the lumbar spine and in portions of the hip for both males and females. Genetic, environmental (for example, physical activity or mechanical "loading" of the skeleton), hormonal, and nutritional factors interact to influence peak bone-mass levels. Failure of an individual to reach the maximum peak bone mass permitted by his or her genetic makeup can be related to low calcium intake or a sedentary lifestyle without adequate physical activity. Parathyroid dysfunction, genetic or nutritional skeletal disorders, or medication use may affect peak bone-mass accumulation and overall bone health adversely. Smoking and excessive alcohol consumption also are likely to be detrimental to skeletal health.
After an individual reaches peak bone mass, net bone gain in the whole skeleton generally does not occur. Agerelated bone loss occurs in both genders, but the rate of bone loss increases with estrogen loss at menopause in females. Age-related bone loss is caused by increased osteoclast (bone-destroying) activity compared to osteoblast (bone-building) activity. Physical activity during adulthood, combined with adequate overall nutrition and calcium intake, can help to maintain bone strength.
Absorption. Calcium absorption across the intestinal wall into the blood occurs by different mechanisms. Two major mechanisms include passive diffusion and active transport. Vitamin D is required for the active transport mechanism but not for the passive diffusion mechanism. The percent of calcium that is absorbed into blood generally decreases with higher calcium intakes; however, the total amount of calcium absorbed is usually greater with higher calcium intakes. The percent of calcium absorbed into blood is highest in infants, spikes again at the start of puberty, then gradually declines with age. The percent of calcium absorbed into blood also increases during the last two trimesters of pregnancy.
Homeostasis. The body keeps tight control (homeostasis) of blood calcium concentration by continuously changing various factors. When blood calcium concentration falls below normal, the parathyroid gland releases parathyroid hormone (PTH). PTH stimulates increased removal of phosphate into urine by the kidneys. This increased phosphate removal triggers the kidneys to keep calcium in the blood rather than excrete it in the urine. PTH also stimulates osteoclasts to remove calcium from bone in order to help restore normal blood calcium concentration. Finally, PTH is involved in making certain that enough vitamin D is present in the intestine to allow for increased calcium absorption from the gut into the blood. PTH decreases to normal once calcium homeostasis is reached. Another hormone, calcitonin, is responsible for stopping bone breakdown by osteoclasts when blood calcium concentration is above normal. Thus, the hormones PTH and calcitonin work together to keep blood calcium concentration within a very narrow range.
Bioavailability. Both dairy products and most dietary supplements provide adequate amounts of calcium. Calcium is present in smaller amounts in grains, fruits, and vegetables. Because grains are eaten in high amounts, however, they are an important source of calcium. Other calcium-rich foods include bok choy (Chinese cabbage), kale, cabbage, and broccoli. Calcium from some foods containing high levels of oxalic acid (spinach, sweet potatoes, rhubarb, beans) or phytic acid (unleavened bread, nuts and grains, seeds, raw beans) is absorbed poorly due to formation of insoluble calcium salts. The ability to enhance dietary calcium intake by consuming calcium-fortified food sources is increasingly common.
Although high protein intake temporarily increases urinary calcium excretion, there is no evidence to indicate that calcium intake recommendations should be adjusted according to protein intake. Although caffeine has a slightly negative impact on calcium retention, the modest calcium loss can be offset by a similarly modest increase in calcium intake. High salt (sodium chloride) intake usually results in increased urinary calcium loss because excretion of sodium and calcium at the kidney are linked. High salt intake triggers increased urinary sodium loss and, therefore, increased urinary calcium excretion. However, as with protein and caffeine, there is no evidence to indicate that calcium intake recommendations should be adjusted according to salt intake.
Dietary requirements and bone mass. Because circulating calcium levels are so strictly controlled, blood calcium concentration is a poor indicator of calcium status. Chronic inadequate calcium intakes or poor intestinal absorption leads to reduced bone mass as PTH acts to maintain homeostatic blood calcium at the expense of skeletal strength. Bone mineral content (BMC) and bone mineral density (BMD) are common measures of bone strength and fracture risk. BMC is measured in grams, the amount of bone mineral at the selected site (for example, whole skeleton, lumbar spine, hip, forearm) and BMD (g/cm2) are calculated as BMC divided by bone area in the region of interest. An adult is defined as osteoporotic by the World Health Organization if his or her BMD is more than 2.5 standard deviations below gender-specific normal young adult BMD. Osteoporosis and related spine, hip, and wrist fractures are major public health concerns.
Recommended daily calcium intakes (measured in milligrams) increase from infancy through adolescence. The rate of calcium accretion relative to body size is greatest during infancy. Infants accrete approximately 140 mg of calcium per day during the first year of life. This need for calcium during the first year of life is reflected in the amount of milk consumed by human milk-fed infants. Although evidence indicates that feeding of formula results in greater bone mineral accretion than human milk feeding during the first year of life, there is no indication that this effect is beneficial either short-or long-term.
Calcium accretion continues in childhood, and maximal accretion occurs during puberty. Children of ages one to eight years accrete 60 to 200 mg of calcium per day. Peak calcium accretion occurs during puberty for both males (mean age 14.5 years) and females (mean age 12 years). Accordingly, calcium intake requirements are highest during adolescence.
Calcium retention and bone turnover decline after menarche in females, but the amount of calcium women need does not change because the percentage of calcium absorbed into the blood decreases. In males, bone mineral accretion occurs until mean age 17.5 years. Evidence from clinical trials indicates that calcium supplementation in children can increase BMD, but the effect occurs primarily among populations who usually have low calcium intake, is not apparent at all skeletal sites, and probably does not persist when supplementation is stopped. Apparently the benefit is short-term only.
Dietary calcium requirements decline for both males and females once adulthood is reached and remain constant throughout the reproductive years. Intestinal calcium absorption, however, also decreases with age. At the end of the reproductive years (approximately age fifty), bone-mass loss occurs in both males and females. Bone-mass loss is particularly pronounced in females during the first few years following menopause. The bone loss that occurs with the loss of estrogen at menopause cannot be reversed simply through increased calcium intake. Reductions in age-related bone loss through calcium supplementation have been demonstrated in postmenopausal women, but the effects vary by skeletal site, usual calcium intake, and postmenopausal age. Because of the reduction in intestinal calcium absorption with age in all individuals and the potential of increased calcium intake to offset bone loss due to estrogen depletion, increasing the amount of calcium in one's diet is recommended for all individuals over fifty years of age.
Maternal calcium requirements increase during the third trimester of pregnancy in accordance with fetal growth needs and to prepare for lactation, and the mother's intestinal calcium absorption efficiency increases in order to meet her increased need for calcium. If this need for more calcium is not met, the mother's skeleton will be depleted to meet the calcium demands of the fetus. Furthermore, calcium loss from the mother's skeleton occurs during lactation and cannot be prevented by calcium supplementation. However, evidence indicates that maternal bone density is recovered to prelactation levels within approximately six months after the recurrence of menses.
Toxicity. Calcium toxicity is uncommon but can occur if too much calcium is taken in through dietary supplements. In susceptible individuals, excess calcium intake can lead to the formation of kidney stones (renal calcium deposits); however, dietary calcium is not a common cause of kidney stones. Hypercalcemia from ingestion of large quantities of calcium supplements is rare but the resulting kidney problems and ramifications to cell function affect major tissues and organs. In the United States, the maximum daily calcium intake judged likely to pose no adverse health effects—Tolerable Upper Intake Level (UL)—is set at 2,500 mg per day for all ages beyond one year of age. There are insufficient data to determine a UL for calcium for infants less than one year of age.
Summary. Changes in dietary calcium requirements throughout the lifespan reflect concurrent alterations in growth rate, intestinal absorption efficiency, and reproductive and estrogen status. Because calcium plays vital roles in critical cell responses, plasma calcium levels are strictly homeostatically controlled at the expense of skeletal integrity, if necessary. Homeostatic control of circulating calcium involves PTH, vitamin D, and calcitonin. Appropriate lifestyle choices (for example, physical activity) and adequate calcium nutrition promote optimal bone-mass accretion during growth and young adulthood, possibly resulting in reduced current and future fracture risk. Dairy products and dietary supplements provide similarly adequate amounts of calcium to the body. Grains, fruits, and vegetables contain smaller amounts of calcium, and calcium absorption from foods high in oxalic acid or phytic acid is limited. Calcium-enriched products such as bread and fruit juice are becoming increasingly important sources of dietary calcium.
See also Dairy Products; Lactation; Milk, Human; Nutrition; Phosphorus and Calcium; Trace Elements.
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Wosje, Karen S.. "Calcium." Encyclopedia of Food and Culture. 2003. Encyclopedia.com. (September 30, 2016). http://www.encyclopedia.com/doc/1G2-3403400104.html
Wosje, Karen S.. "Calcium." Encyclopedia of Food and Culture. 2003. Retrieved September 30, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3403400104.html
Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.
Calcium is an alkaline earth metal. The alkaline earth metals make up Group 2 (IIA) of the periodic table, a chart that shows how the elements are related. They include beryllium, magnesium, strontium, barium, and radium. The alkaline earth metals are more chemically active than most metals. Only the alkali metals in Group I (IA) are more reactive.
Calcium compounds are common and abundant in the Earth's crust. Humans have used calcium compounds for hundreds of years in construction, sculpture, and roads.
Calcium metal was not prepared in a pure form until 1808 when English chemist Humphry Davy (1778-1829) passed an electric current through molten (melted) calcium chloride.
Metallic calcium has relatively few uses. However, calcium compounds are well known and widely used. They include chalk, gypsum, limestone, marble, and plaster of paris.
Group 2 (IIA)
Alkaline earth metal
Discovery and naming
It is impossible to say when humans first knew about or used compounds of calcium. Whenever they used limestone to build a structure, they were using a compound of calcium. Limestone is the common name for calcium carbonate (CaCO3). Whenever humans built a statue or monument out of marble, they were using calcium carbonate in another form. Ancient Egyptians and early Greeks used mortar, a cement-like material that holds stones and bricks together. Early mortar was made by roasting or heating limestone for long periods of time. Water was then mixed with the powder, which would then dry to form a strong bond.
Humphry Davy | English chemist
H umphry Davy (1788-1829) was a major contributor to the field of electrochemistry. This is the science involving the relation of electricity to chemical changes. He is best known for discovering calcium, sodium, potassium, magnesium, strontium, and barium. He also discovered nitrous oxide and two types of lamps.
Davy grew up in Cornwall, England, in a poor family. His father, who died when Davy was a boy, had lost money in unwise investments, so Davy helped his mother pay off the debts. He disliked being a student, though he liked reading about science. In later life, he said he was happy he did not study too hard because he had more time to think on his own.
With no money for further education, the 17-year-old Davy began to work for a surgeon-pharmacist. He also started learning on his own about other subjects that interested him, such as geography, languages, and philosophy. He even wrote poems that later earned him the respect and friendship of William Wordsworth, Samuel Coleridge, and other leading English poets of his time!
At 19, Davy read a chemistry book by the famous French scientist Antoine-Laurent Lavoisier (1743-94). That book convinced him to concentrate on chemistry. For the rest of his life, Davy's career was marked by brilliant scientific explorations in chemistry and electrochemistry.
Davy discovered nitrous oxide after testing the effects of hydrogen and carbon dioxide on himself. (He liked to use himself as a human guinea pig!) Nitrous oxide is a gas consisting of nitrogen and oxygen. While studying nitrous oxide gas, he discovered that its effects often made him feel very happy or very sad. The feeling of happiness eventually gave nitrous oxide another name: laughing gas. Most importantly, though, Davy recognized that it could be used as an anesthetic. An anesthetic is a chemical used to dull pain during minor surgery.
In 1808, Davy invented the carbon arc lamp. He had proposed using carbon as the electrode material instead of metal. (Electrodes are conductors used to establish electrical contact with a nonmetallic part of a circuit.) With carbon electrodes, he made a strong electric current leap from one electrode to the other. This created an intense white light. Davy's invention marked the beginning of the era of electric light. Arc lamps are still used today.
Using his knowledge of electricity, Davy built a large battery which he used to break down substances most scientists thought were pure elements. In 1807, he discovered the element potassium. He created this by using electrolysis. Electrolysis produces chemical changes by passage of an electric current through an electrolyte. An electrolyte is a nonmetallic electric conductor. Within a week he isolated sodium in a similar way. Then in 1808, he used a slightly modified method to isolate calcium, magnesium, barium, and strontium. Davy was only 29 by the time he had discovered all of these elements!
Davy later invented the miner's lamp (now known as the Davy lamp). He learned that methane was the mine gas that caused explosions. But he realized it ignited only at high temperatures. So he designed a lamp in which the flame was surrounded by wire gauze. This reduced the heat and prevented flammable gases from igniting. This made coal-mining safer by reducing the number of explosions in mines.
Davy was rewarded by many honors and medals for his discoveries and inventions. He died of a stroke in 1829 at the age of 49.
Another calcium compound used by early civilizations was plaster of paris. Plaster of paris is made by heating gypsum, or calcium sulfate (CaSO4), to remove the water that makes it crystallize. Water was added and it hardened into a brittle, cement-like substance. Until recently, it was most often used to make casts to protect broken bones. However, it has largely been replaced by fiberglass, which is lighter, yet stronger. The first mention of plaster of paris to protect broken bones can be found in a book written by Persian pharmacist Abu Mansur Muwaffaw in about 975 A.D.
By the 1700s, chemists had learned a great deal about calcium compounds. They knew that limestone, gypsum, marble, and many other commonly occurring compounds all contain a common element. They called the element calx. That word comes from the Latin term for lime. In 1807, Davy isolated the new element.
Davy invented a system for melting compounds of elements that were difficult to separate by usual methods. He passed an electric current through the compound, causing the compounds to break into parts. One of those parts was calx. He created the name calcium by adding the suffix -ium to calx; -ium is the ending used for almost all metallic elements. Davy was also able to produce free sodium, potassium, strontium, magnesium, and barium.
Calcium is a fairly soft metal with a shiny silver surface when first cut. The surface quickly becomes dull as calcium reacts with oxygen to form a coating of white or gray calcium oxide.
Calcium's melting point is 850°C (1,560°F) and its boiling point is 1,440°C (2,620°F). It has a density of 1.54 grams per cubic centimeter.
Calcium is a moderately active element. It reacts readily with oxygen to form calcium oxide (CaO):
Calcium reacts with the halogens—fluorine, chlorine, bromine, iodine, and astatine, The halogens are the elements that make up Group 17 (VIIA) of the periodic table. Calcium also reacts readily with cold water, most acids, and most nonmetals, such as sulfur and phosphorus. For example, calcium reacts with sulfur:
Occurrence in nature
Calcium is the fifth most common element in the Earth's crust. Its abundance is estimated to be about 3.64 percent. It is also the fifth most abundant element in the human body.
Calcium does not occur as a free element in nature. It is much too active and always exists as a compound. The most common calcium compound is calcium carbonate (CaCO3). It occurs as aragonite, calcite, chalk, limestone, marble, and travertine, and in oyster shells and coral.
Shellfish build their shells from calcium dissolved in the water. When the animals die or are eaten, the shells sink. Over many centuries, thick layers of the shells may build up and be covered with mud, sand, or other materials. The shells are squeezed together by the heavy pressure of other materials and water above them. As they are squeezed together, the layer is converted to limestone. If the limestone is squeezed even more, it can change into marble or travertine.
Six naturally occurring isotopes of calcium exist: calcium-40, calcium-42, calcium-43, calcium-44, calcium-46, and calcium-48. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.
Radioactive isotopes of calcium have also been made. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.
Two radioactive isotopes of calcium are used in research and medicine. Calcium-45 is used to study how calcium behaves in many natural processes. For example, it can be used to see how various types of soil behave with different kinds of fertilizers. The calcium-45 is used as a tracer in such studies. A tracer is a radioactive isotope whose presence in a system can easily be detected. The isotope is injected into the system at some point. Inside the system, the isotope gives off radiation. That radiation can be followed by detectors placed around the system. Calcium-45 can also be used as a tracer in the study of glassy materials, detergents, and water purification systems.
Both calcium-45 and calcium-47 can be used to study how calcium is used in the body. A doctor may think that a person's body is not using calcium properly in making bones or regulating nerve messages. The doctor can use calcium-45 or calcium-47 to find out more about this problem. The radioactive isotope is injected into the person's bloodstream. Then its path can be followed by the radiation it gives off. The doctor can then tell if the calcium is being used normally in the body.
Pure calcium metal can be made by the same method used by Davy. An electric current is passed through molten calcium chloride:
There is not much demand for pure calcium. Most calcium is used in the form of limestone, gypsum, or other minerals that can be mined directly from the earth.
Shellfish build their shells from calcium dissolved in the water.
Calcium metal has relatively few uses. It is sometimes used as a "getter." A getter is a substance that removes unwanted chemicals from a system. Calcium is used as a getter in the manufacture of evacuated glass bulbs. Calcium is added to the bulb while it is being made. It then combines with gases left in the glass in the final stages of manufacture. Calcium is also used as a getter in the production of certain metals, such as copper and steel. The calcium removes unwanted elements that would otherwise contaminate the metal.
Calcium is also used to make alloys. An alloy is made by melting and mixing two or more metals. The mixture has properties different from those of the individual metals. An alloy of calcium and cerium is used in flints found in lighters (the elements that create sparks).
The starting point for the manufacture of most calcium compounds is limestone. Limestone occurs naturally in large amounts in many parts of the world. It is usually mined from open-pit quarries. A quarry is a large hole in the ground from which useful minerals are taken.
Limestone is first heated to obtain lime, or calcium oxide (CaO):
Lime is one of the most important chemicals in the world. It usually ranks in the top five chemicals produced in the United States. In 1996, about 19 billion kilograms (42 billion pounds or 21 million tons) of lime was produced in the United States.
Lime is used in the production of metals. It is used during the manufacture of steel to remove unwanted sand, or silicon dioxide (SiO2), present in iron ore:
The product formed in this reaction, calcium silicate (CaSiO3), is called slag.
Another important use of lime is in pollution control. Many factories release harmful gases into the atmosphere through smokestacks. Lining a smokestack with lime allows some of these gases to be captured. The lime is known as a scrubber. Lime captures one harmful gas, sulfur dioxide (SO2), which is a contributor to acid rain (a form of precipitation that is significantly more acidic than neutral water, often produced as the result of industrial processes):
Calcium sulfite (CaSO3) is a solid that can be removed from the inside of the smokestack.
"In the limelight"
At one time, lime was used as a source of light in theaters. When lime is heated to a high temperature, it gives off an intense white light. Pots of hot lime were often used to line the front of the stage. The light the pots gave off helped the audience see the performers. As a result, the performers were said to be "in the limelight." That phrase is still in use today, but lime is no longer used as a source of light in theaters.
Lime is also used in water purification and waste treatment plants. When water combines with water, it forms slaked lime, or calcium hydroxide (Ca(OH)2):
Slaked lime traps impurities present in the water as it forms. It carries the impurities with it as it sinks to the bottom of the tank.
Lime is used to make more than 150 different industrial chemicals. Some examples of these chemicals with their uses are:
Milk is a good source of calcium.
calcium alginate: thickening agent in food products such as ice cream and cheese products; synthetic fibers
calcium arsenate (Ca3(AsO4)2): insecticide
calcium carbide (CaC2): used to make acetylene gas (for use in acetylene torches for welding); manufacture of plastics
calcium chloride (CaCl2): ice removal and dust control of dust on dirt roads; conditioner for concrete; additive for canned tomatoes; provides body for automobile and truck tires
calcium cyclamate (Ca(C6H11NHSO4)2): sweetening agent (cyclamate), no longer permitted for use because of suspected cancer-causing properties
calcium gluconate (Ca(C6H11O7)2): food additive; vitamin pills
calcium hypochlorite (Ca(OCl)2): swimming pool disinfectant; bleaching agent; deodorant; algicide and fungicide (kills algae and fungi)
calcium permanganate (Ca(MnO4)2): liquid rocket propellant; textile production; water sterilizing agent; dental procedures
calcium phosphate (Ca3(PO4)2): supplement for animal feed; fertilizer; commercial production of dough and yeast products; manufacture of glass; dental products
calcium phosphide (Ca3P2): fireworks; rodenticide (kills rats); torpedoes; flares
calcium stearate (Ca(C18H35O2)2): manufacture of wax crayons, cements, certain kinds of plastics, and cosmetics; food additive; production of water resistant materials; production of paints
calcium tungstate (CaWO4): luminous paints; fluorescent lights; X-ray studies in medicine
Calcium is essential to both plant and animal life. In humans, it makes up about two percent of body weight. About 99 percent of the calcium in a person's body is found in bones and teeth. Milk is a good source of calcium. The body uses calcium in a compound known as hydroxyapatite (Ca10(PO4)6(OH)2) to make bones and teeth hard and resistant to wear.
Calcium has many other important functions in the human body. For example, it helps control the way the heart beats. An excess (too much) or deficiency (not enough) of calcium can change the rhythm of the heart and cause serious problems. Calcium also controls the function of other muscles and nerves.
"Calcium (revised)." Chemical Elements: From Carbon to Krypton. 2006. Encyclopedia.com. (September 30, 2016). http://www.encyclopedia.com/doc/1G2-3427000027.html
"Calcium (revised)." Chemical Elements: From Carbon to Krypton. 2006. Retrieved September 30, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3427000027.html
As the most plentiful mineral in the body, calcium plays a key role in the development and maintenance of bones and teeth. Calcium enables the contraction of muscles, including the function of the body's most important muscle, the heart. It is also essential for normal blood clotting, proper nerve impulse transmission, and the appropriate support of connective tissue.
Almost every segment of the population—women, children, teenagers, men, unborn babies, and the elderly—benefit from calcium in their daily diet. The mineral is an important dietary supplement for those who are undergoing significant periods of bone growth, such as in childhood, during pregnancy , and while breast-feeding.
Calcium is an effective weapon for the aging population as they combat osteoporosis . A condition that simply means "porous bones," osteoporosis attacks bones when they are their most vulnerable. As the body ages, bones lose more calcium, and it becomes vital to supplement the diet with calcium in order to encourage bone growth and prevent or slow down the process of osteoporosis.
While the body relies on the presence of calcium for many of its everyday functions, the number of reasons why the mineral should be supplemented in the diet are numerous. Calcium is beneficial to everyone, but re-search has shown that women may benefit more than others. A study in the October 1999 issue of the journal Obstetrics & Gynecology found that pregnant women who do not get enough calcium in their diet can increase the bone mineral content of their fetus by about 15% by taking 1,300 mg of a calcium supplement per day during their second and third trimesters. For those women who already consume enough calcium, the additional supplements do not have this effect. Additional research shows that calcium deficiencies lead to preeclampsia during pregnancy, causing high blood pressure, swelling, and weight gain greater than 1 lb (0.5 kg) per day. The risk of preeclampsia developing lowers by 45–75% for women who receive calcium supplementation.
Premenstrual syndrome (PMS) is another condition women face that may be alleviated by the use of calcium supplements. Researchers at the National Institute of Mental Health (NIMH) concluded that those women who took 1,200 mg of calcium per day reduced their overall PMS symptoms by more than 50%. In the study, calcium supple-mentation led to the reduction of psychological PMS symptoms (such as mood swings) by 45%, food cravings by 54%, and bloating and water retention by 36%.
A 1999 study reported that researchers have found that increasing the amount of daily calcium consumed by women may reduce their risk of stroke . Those women in the Nurses' Health Study who took more than 400 mg of calcium daily were at the lowest risk for a stroke, while those who consumed more than 600 mg each day did not have an increased benefit. Researchers believe that the risk of stroke is reduced by calcium from decreased cholesterol levels, or by stopping the formation of blood clots that cause strokes.
For elderly postmenopausal women, the prevention of osteoporosis becomes critical. In order to maintain bone mass during this time, a study conducted in 1999 concluded that a low-dose hormone replacement therapy (HRT) combined with calcium and vitamin D supple-mentation is an effective therapeutic option for prevention of osteoporosis. Estriol, which is used in HRT, appears to be helpful in controlling menopausal symptoms. Results from research regarding this use of estriol on bone density have been contradictory, according to the Alternative Medicine Review, with the results showing the most effectiveness coming from Japanese studies.
Calcium alone is frequently prescribed with estrogen at the beginning of menopause to treat or prevent osteoporosis. This therapy is recommended to guard against the increased loss of calcium in the bones due to increasing age. As bones lose more calcium they become dense and brittle, and more vulnerable to the attack of osteoporosis. This condition is most common in people over 70, and in women after menopause, where it may increase the risk of broken hips, ribs, and pelvis, and the weakening of other bones. Increased physical exercise is also important for bone strengthening.
On the other hand, although calcium supplementation is useful in lowering the risk of osteoporosis in Western women, more research is needed to determine why the rates of osteoporosis are low in some Eastern societies with low-calcium diets . There is evidence that osteoporosis, like coronary artery disease, is primarily a problem in Western societies. In addition, accumulating evidence that a diet high in fruits and vegetables helps to prevent fractures suggests that the level of calcium in the diet is not the only nutritional factor involved in osteoporosis.
Calcium has been shown to be beneficial to the colon. Among those people taking calcium supplements, research points to a modest reduction in the recurrence of polyps in their colons. Colon polyps are benign tumors that often turn cancerous. Researchers think that calcium binds to carcinogens, preventing abnormal cell growth.
Stemming from its active role in building bone density throughout the body, calcium may prove particularly beneficial for strengthening of the jawbone. Dental researchers at the State University of New York at Buffalo report that calcium supplementation may prevent periodontal disease as it builds a strong jawbone. Periodontal, or gum, disease is an infection caused by bacteria that deposits in pockets between the teeth and gums, and is the leading cause of tooth loss in the United States. As the infection progresses, the jawbone that holds a tooth in place is eventually destroyed, causing the tooth to loosen and fall out. The researchers contend that calcium's overall bone-building role would equal a stronger jawbone that would better fight off gum disease .
While supplements of calcium can be found in many forms, research has shown a promising benefit if it is obtained from dairy foods, rather than supplements or leafy greens—calcium in the form of dairy may actually prevent weight gain. Those in the study who consumed at least 1,000 mg of calcium a day (equaling about 3 cups, or 750 ml of skim milk), gained 6–7 lb (2.7–3.2 kg) less over two years than those with low-calcium diets. Researchers of Purdue University speculate that calcium probably prevents weight gain by increasing the breakdown of body fat and decreasing its formation. It is important to note, however, that dairy products should be consumed in moderation, as other research conducted has indicated that dairy products are not necessarily a good source of absorbable calcium. In addition, other studies indicate that women are often reluctant to increase their intake of dairy products because they dislike milk, suffer from lactose intolerance, or fear that they will gain too much weight.
Calcium is proving essential to those children around the world who are stricken by rickets. Rickets is a deficiency condition in children that affects developing cartilage and newly formed bone throughout the body, causing severe deformities. Often thought to be a result of the inadequate intake of vitamin D from dietary sources or lack of exposure to sunlight, research reported in 2000 has found that children with rickets respond well to calcium supplementation. While rickets is still rare in most developed countries, it is becoming more common in the United States due to lower milk consumption by children; and it remains a problem in many other parts of the world. Researchers conclude that effective treatment for the condition is calcium supplementation alone, or in combination with vitamin D. Osteomalacia, or the adult form of rickets, also responds to calcium supplementation.
Evidence is accumulating in the United States that women are not the only group at risk for insufficient dietary levels of calcium. Children and adolescents are also at risk, according to a 2001 report from the National Institutes of Health. Researchers found that "only 13.5% of girls and 36.3% of boys ages 12 to 19 in the United States get the recommended daily amount (RDA) of calcium, placing them at serious risk for osteoporosis and other bone diseases" in their adult years. The report listed increased consumption of soft drinks and decreased consumption of milk as contributing to the problem.
Calcium may be supplemented in the diet in a variety of ways. Numerous foods are rich in calcium, including dairy products (such as milk, yogurt, and cheese) and leafy green vegetables like turnip greens, broccoli, kale, and collards. Canned salmon, sardines, shrimp, and tofu are also high in calcium. More foods are being fortified with calcium, making it easier to ensure the proper amount of the mineral is consumed. Calcium-fortified foods range from cranberry juice cocktail, cereal and waffles, to orange juice and flour. With almost every segment of the population consuming too little calcium, researchers recommend calcium-fortified foods to increase daily calcium intake.
While the types of food calcium may be obtained from continues to increase, most people still lack enough of the essential mineral. For those who are not getting enough calcium from foods, supplements are an acceptable alternative. The chemical form of calcium supplements come in five varieties: carbonate, citrate, lactate, phosphate, chelate, and citrate malate. The supplements are available as tablets, syrup, or suspension form. Calcium supplements should be stored at room temperature and away from moisture and sunlight. It should not be stored in the bathroom, and the liquid forms should not be frozen.
Experts state that calcium is best absorbed from the citrate malate form, or the type of calcium found in some juices, but they recommend calcium carbonate for the overall amount of calcium it offers and its affordability. Calcium carbonate can be found in antacids, and it is absorbed better when taken with meals. Food slows down the time it takes substances to travel through the gut, giving the calcium more time to be absorbed. Absorption is key for the proper functioning of calcium. Sufficient levels of vitamin D and hydrochloric acid in the stomach, and the presence of other minerals, such as magnesium and phosphorous are essential for quick absorption.
The body may also be better able to absorb calcium when taken along with ingredients extracted from chicory root. Research indicates that Raftilin inulin and Raftilose oligofructose, both extracts from chicory root, are dietary fibers that are not digested in the stomach or the small intestine. Instead, they are fermented by Bifidobacteria in the colon—beneficially leading to increased calcium absorption throughout the body, with emphasis on bone tissue. Additionally, Oligofructose improves the texture and mouthfeel while improving taste and fruit flavors in low-fat yogurts. Inulin is used for fat replacement and fiber enrichment of reduced-fat and fat-free sour cream and whipped topping.
There are many ways to ensure calcium is part of a daily diet, but it is important that the recommended daily allowance (RDA), or appropriate dosage of the mineral be followed. The RDA of calcium for adults is 800 mg; pregnant women and young adults should be certain their intake equals 1,200 mg per day. Adults over 50 should increase their intake to 1,000 mg per day with supplements that include vitamin D.
Calcium supplements may be taken with a large glass of water during or after a meal. Tablets in chewable form must be chewed thoroughly before swallowing, and effervescent tablets should be diluted in cold water or juice before taking. It is recommended that other medications be taken two hours after any calcium supplement. The simultaneous intake of calcium may interfere with the absorption of other drugs. Do not take more than 500 mg of calcium at one time for the best absorption of the mineral.
When adding calcium supplements to the diet, it is recommended that it not be taken within one to two hours of eating bran, or whole grain cereals or breads. Large amounts of alcohol or caffeine containing beverages or tobacco should be avoided. Large amounts of calcium, phosphates, magnesium, or vitamin D in medication or dietary supplements should not be taken unless directed by a physician. Those with diarrhea , stomach trouble, parathyroid disease, sarcoidosis, or kidney stones should consult with their physician before taking calcium.
Calcium is typically well tolerated by those who add it to their diets, but if the mineral is taken in high levels it can cause several side effects, including: nausea, vomiting , loss of appetite, constipation , stomach pain , thirst, dry mouth , increased urination, and weakness. While these side effects are rare, it is even more unlikely to experience the life-threatening symptoms of an irregular or very slow heart beat. If these dangerous symptoms appear while taking calcium, use of the mineral should be discontinued and emergency treatment should be sought. An overdose of a calcium supplement may lead to confusion, irregular heartbeat, depression , bone pain, or coma.
It is important that all over-the-counter (OTC) or prescription medications are reviewed with a physician before beginning calcium supplement.
According to the Complete Guide to Prescription & Nonprescription Drugs, the following are some of the drugs that may cause possible interactions if taken with calcium:
- calcium-containing medicines
- oral contraceptives
- digitalis preparations
- diuretics, thiazide
- iron supplements
- nalidixic acid
- para-aminosalicyclic acid (PAS)
The Editors of Time-Life Books. "Essential Vitamins and Minerals." In The Medical Advisor: The Complete Guide to Alternative & Conventional Treatments. Richmond, VA: Time-Life Inc., 1996.
Griffith, H. Winter. "Calcium Supplements." In Complete Guide to Prescription & Nonprescription Drugs, 1999 Edition. New York: The Berkley Publishing Group, 1998.
"Calcium May Help Prevent Colon Polyps." Environmental Nutrition 22, no. 2 (February 1999): 1.
"Calcium May Help Prevent Gum Trouble." Tufts University Health & Nutrition Letter 17, no. 5 (July 1999): 6.
"Calcium May Reduce Stroke Risk in Women." Stroke (September 1999).
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Gulliver, Pauline, and Caroline C. Horwath. "Assessing Women's Perceived Benefits, Barriers, and Stage of Change for Meeting Milk Product Consumption Recommendations. " Journal of the American Dietetic Association 101 (November 2001): 1354–1357.
Head, Kathleen A., n.d. "Estriol: Safety and Efficacy." Alternative Medicine Review 3, no. 2 (April 1998). <http://www.thorne.com>.
Hegsted, D. Mark. "Fractures, Calcium, and the Modern Diet." American Journal of Clinical Nutrition 74 (November 2001): 571.
Liebman, Bonnie. "Calcium Supplements: The Way to Go." Nutrition Action Healthletter 25, no. 3 (April 1998): 5.
Marion, Matt. "Health Bulletin." Men's Health 14, no. 10 (December 1999): 32.
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Wallace, Phil. "NIH Says Calcium 'Crisis' is Affecting Young People." Food Chemical News 43 (December 17, 2001): 27.
Food and Drug Administration, Office of Consumer Affairs, HFE–88, Rockville, MD 20857.
Rebecca J. Frey, PhD
Kapes, Beth; Frey, Rebecca. "Calcium." Gale Encyclopedia of Alternative Medicine. 2005. Encyclopedia.com. (September 30, 2016). http://www.encyclopedia.com/doc/1G2-3435100144.html
Kapes, Beth; Frey, Rebecca. "Calcium." Gale Encyclopedia of Alternative Medicine. 2005. Retrieved September 30, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3435100144.html
Calcium is one of the most important elements in the diet because it is a structural component of bones, teeth, and soft tissues and is essential in many of the body's metabolic processes. It accounts for 1 to 2 percent of adult body weight, 99 percent of which is stored in bones and teeth. On the cellular level, calcium is used to regulate the permeability and electrical properties of biological membranes (such as cell walls), which in turn control muscle and nerve functions, glandular secretions, and blood vessel dilation and contraction. Calcium is also essential for proper blood clotting .
Because of its biological importance, calcium levels are carefully controlled in various compartments of the body. The three major regulators of blood calcium are parathyroid hormone (PTH), vitamin D , and calcitonin. PTH is normally released by the four parathyroid glands in the neck in response to low calcium levels in the bloodstream (hypocalcemia). PTH acts in three main ways: (1) It causes the gastrointestinal tract to increase calcium absorption from food, (2) it causes the bones to release some of their calcium stores, and (3) it causes the kidneys to excrete more phosphorous, which indirectly raises calcium levels.
Vitamin D works together with PTH on the bone and kidney and is necessary for intestinal absorption of calcium. Vitamin D can either be obtained from the diet or produced in the skin when it is exposed to sunlight. Insufficient vitamin D from these sources can result in rickets in children and osteomalacia in adults, conditions that result in bone deformities. Calcitonin, a hormone released by the thyroid, parathyroid, and thymus glands, lowers blood levels by promoting the deposition of calcium into bone.
Most dietary calcium is absorbed in the small intestine and transported in the bloodstream bound to albumin, a simple protein . Because of this method of transport, levels of albumin can also influence blood calcium measurements. Calcium is deposited in bone with phosphorous in a crystalline form of calcium phosphate.
Deficiency and Toxicity
Because bone stores of calcium can be used to maintain adequate blood calcium levels, short-term dietary deficiency of calcium generally does not result in significantly low blood calcium levels. But, over the long term, dietary deficiency eventually depletes bone stores, rendering the bones weak and prone to fracture. A low blood calcium level is more often the result of a disturbance in the body's calcium regulating mechanisms, such as insufficient PTH or vitamin D, rather than dietary deficiency. When calcium levels fall too low, nerve and muscle impairments can result. Skeletal muscles can spasm and the heart can beat abnormally—it can even cease functioning.
Toxicity from calcium is not common because the gastrointestinal tract normally limits the amount of calcium absorbed. Therefore, short-term intake of large amounts of calcium does not generally produce any ill effects aside from constipation and an increased risk of kidney stones . However, more severe toxicity can occur when excess calcium is ingested over long periods, or when calcium is combined with increased amounts of vitamin D, which increases calcium absorption. Calcium toxicity is also sometimes found after excessive intravenous administration of calcium. Toxicity is manifested by abnormal deposition of calcium in tissues and by elevated blood calcium levels (hypercalcemia). However, hypercalcemia is often due to other causes, such as abnormally high amounts of PTH. Usually, under these circumstances, bone density is lost and the resulting hypercalcemia can cause kidney stones and abdominal pain. Some cancers can also cause hypercalcemia, either by secreting abnormal proteins that act like PTH or by invading and killing bone cells causing them to release calcium. Very high levels of calcium can result in appetite loss, nausea , vomiting, abdominal pain, confusion, seizures, and even coma.
Requirements and Supplementation
Dietary calcium requirements depend in part upon whether the body is growing or making new bone or milk. Requirements are therefore greatest during childhood, adolescence, pregnancy, and breastfeeding. Recommended daily intake (of elemental calcium) varies accordingly: 400 mg for infants 0–6 months, 600 mg for infants 6–12 months, 800 mg for children 1–10 years, 1,200 mg for ages 11–24 years, and 800 mg for individuals over 24 years of age. Pregnant women require additional calcium (RDA 1,200 mg). Many experts believe that elderly persons should take as much as 1,500 mg to help prevent osteoporosis , a common condition in which bones become weak and fracture easily due to a loss of bone density. Dairy products, meats, and some seafood (sardines, oysters) are excellent sources of calcium. Spinach, beet greens, beans, and peanuts are among the best plant-derived sources.
Calcium absorption is affected by many factors, including age, the amount needed, and what foods are eaten at the same time. In general,
|Supplement||Elemental calcium by weight||Comment|
• Most commonly used
• Less well absorbed in persons with decreased stomach acid (e.g., elderly or those on anti-acid medicines)
• Natural preparations from oyster shell or bone meal may contain contaminants such as lead
• Least expensive
• Better absorbed, especially by those with decreased stomach acid
• May protect against kidney stones
• More expensive.
|Calcium phosphate||38% or 31%||
• Tricalcium or dicalcium phosphate
• Used more in Europe
• Absorption similar to calcium carbonate
• Used intravenously for severe hypocalcemia
• Well absorbed orally, but low content of elemental calcium
• Very expensive
• Available as syrup for children
• Low content elemental calcium.
|Calcium lactate||13%||• Well absorbed, but low content elemental calcium.|
|source: Gregory, Philip J. (2000) "Calcium Salts." Prescriber's Letter. Document #160313.|
calcium from food sources is better absorbed than calcium taken as supplements. Children absorb a higher percentage of their ingested calcium than adults because their needs during growth spurts may be two or three times greater per body weight than adults. Vitamin D is necessary for intestinal absorption, making Vitamin D–fortified milk a very well-absorbed form of calcium. Older persons may not consume or make as much vitamin D as is optimal, so their calcium absorption may be decreased. Vitamin C and lactose (the sugar found in milk) enhance calcium absorption, whereas meals high in fat or protein may decrease absorption. Excess phosphorous consumption (as in carbonated sodas) can decrease calcium absorption in the intestines . High dietary fiber and phytate (a form of phytic acid found in dietary fiber and the husks of whole grains) may also decrease dietary calcium absorption in some areas of the world. Intestinal pH also affects calcium absorption—absorption is optimal with normal stomach acidity generated at meal times. Thus, persons with reduced stomach acidity (e.g., elderly persons, or persons on acid-reducing medicines) do not absorb calcium as well as others do.
Calcium supplements are widely used in the treatment and prevention of osteoporosis. Supplements are also recommended, or are being investigated, for a number of conditions, including hypertension , colon cancer , cardiovascular disease, premenstrual syndrome, obesity , stroke , and preeclampsia (a complication of pregnancy). There are several forms of calcium salts used as supplements. They vary in their content of elemental calcium, the amount effectively absorbed by the body, and cost. Whatever the specific form, the supplement should be taken with meals to maximize absorption.
Calcium is one of the most important macronutrients for the body's growth and function. Sufficient amounts are important in preventing many diseases. Calcium levels are tightly controlled by a complex interaction of hormones and vitamins . Dietary requirements vary throughout life and are greatest during periods of growth and pregnancy. However, recent reports suggest that many people do not get sufficient amounts of calcium in their diet. Various calcium supplements are available when dietary intake is inadequate.
see also Minerals; Osteomalacia; Osteoporosis; Rickets.
Donna Staton Marcus Harding
Berkow, Robert, ed. (1997). The Merck Manual of Medical Information, Home Edition. Whitehouse Station, NJ: Merck & Co.
National Research Council (1989). Recommended Dietary Allowances, 10th edition. Washington, DC: National Academy Press.
Olendorf, Donna; Jeryan, Christine; and Boyden, Karen, eds. (1999). The Gale Encyclopedia of Medicine. Farmington Hills, MI: Gale Research.
Food and Nutrition Board (1999). Dietary Reference Intakes for Calcium, Phosphorous, Magnesium, Vitamin D, and Fluoride. Washington, DC: National Academy Press. Available from <http://www.nap.edu>
Gregory, Philip J. (2000) "Calcium Salts." Prescriber's Letter Document #160313. Available from <http://www.prescribersletter.com>
Staton, Donna; Harding, Marcus. "Calcium." Nutrition and Well-Being A to Z. 2004. Encyclopedia.com. (September 30, 2016). http://www.encyclopedia.com/doc/1G2-3436200052.html
Staton, Donna; Harding, Marcus. "Calcium." Nutrition and Well-Being A to Z. 2004. Retrieved September 30, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3436200052.html
The average human body contains just over 1 kg of calcium, more than 99% of it in the skeleton (and teeth). Here it is mostly in the form of complex phosphate salts forming the rigid structures that allow bone to fulfil its essential supportive role. Skeletal calcium is not, however, inert. Bone contains cells that lay down new bone and resorb old bone and the regulated activities of these cells, made possible by the extensive blood supply that bone receives, ensure that skeletal calcium actively turns over. Beyond middle age, the rate of bone deposition fails to keep pace with its resorption and the disparity can become severe enough to cause osteoporosis, when the bones become fragile and fracture easily. In addition to its structural role, the skeleton serves also as a reservoir from which calcium can be mobilized if necessary.
Calcium absorption from the small intestine and excretion from the kidneys are also regulated to ensure that the concentration of calcium in the plasma is very precisely controlled, probably more tightly than any other component of plasma. The need for such precise calcium homeostasis is underscored by the serious consequences that follow deviations from the norm. Excessively low plasma calcium levels (hypocalcaemia) are particularly dangerous because they evoke spontaneous activity in both nerves and muscles, causing muscle spasms that can become so severe as to obstruct the airway. Conversely, with too high a plasma calcium level (hypercalcaemia), nerves and muscle can become less active, leading to weakness. The longer term consequences of aberrant plasma calcium regulation can include skeletal problems and kidney stones.
Three agents are principally responsible for plasma calcium regulation, acting directly or indirectly at the three sites where the amount entering or leaving the blood can be influenced — bone, kidneys, and intestine.
Parathyroid hormone is a peptide released from the parathyroid glands in the neck in direct response to any fall in the plasma calcium concentration. In bone it enhances calcium resorption and transfer into the blood. In the kidneys it both reduces calcium excretion and promotes formation of the active metabolite of vitamin D3, which in turn enhances intestinal absorption. Thus parathyroid hormone helps to restore plasma calcium levels to normal.
Vitamin D (cholecalciferol) is not only a component of the diet (extra is added to cereals and dairy products) but also is synthesized in the skin in the presence of sunlight. After modification in the liver, vitamin D3 is further modified to its active form in the kidneys, a step that is stimulated largely via parathyroid hormone, and hence in turn by a fall in the plasma calcium concentration. The active metabolite of vitamin D3, 1,25-dihydroxycholecalciferol (calcitriol) is a hormone that stimulates calcium uptake from the small intestine and mobilization of calcium from bone, both serving to reverse the fall in plasma calcium that triggered formation of the hormone initially. Defects in any of the pathways leading to formation of 1,25-dihydroxycholecalciferol give rise to rickets.
Calcitonin is the third, and least important, calcium-regulating hormone. It is released from cells within the thyroid gland in response to an increase in plasma calcium and to several other factors, including gastrin, a hormone released during feeding and therefore heralding a potential rise in plasma calcium. Calcitonin serves to reverse any such rise by inhibiting bone resorption.
Clinical disorders of calcium regulation can arise for a variety of reasons, related not only directly to excess or deficiency of the relevant hormones, but also to conditions affecting kidney function and intestinal absorption; there can also be defects in the signalling proteins responsible for mediating the effects of parathyroid hormone on its target tissues. Conditions disturbing acid–base homeostasis can alter the concentration of free calcium ions in the blood: alkalinity increases, and acidity decreases their binding to proteins in the plasma.
It is ironic that the insolubility of calcium phosphate that allows it to form so stable a structure in bone was probably also the ultimate cause, in evolutionary terms, of calcium coming to fulfil its other indispensible role as a dynamic regulator of cellular activity. The energy economy of every cell is now dominated by the transfer of phosphate groups, and since calcium phosphate is so insoluble, it is likely that cells have long (in evolutionary terms) been required to actively extrude calcium. Every cell now maintains a very low free calcium ion concentration in its cytoplasm, some 10 000 times or so lower than that in either the plasma or the enclosed calcium stores within the cell. These very steep calcium concentration gradients are maintained by using energy, generated from the metabolism of the cell, to actively export calcium from the cytoplasm, either out of the cell or into the internal stores. There are two benefits of this active calcium transport. Firstly, it allows the energy economy of the cell to operate free of the risk that the key intermediates will be precipitated by calcium. Secondly, it provides steep, ready-made gradients down which calcium can rapidly flow into the cytoplasm when appropriate physiological stimuli cause the opening of calcium ion channels in either the plasma membrane or the membranes of the intracellular stores. Rigorously controlled leaking of calcium through such channels is ubiquitous in the regulation of cellular activity. The fertilization of an egg, every beat of the heart or contraction of any other muscle, release of transmitters from nerve endings — myriads of physiological responses — all are regulated by transient increases in cytoplasmic calcium ion concentration brought about by appropriate stimuli from outside the cell, that cause calcium channels to open, and allow movement down the gradient into the cell. The ensuing increase in cytoplasmic calcium concentration is detected by specific calcium-binding proteins, the most abundant of which is calmodulin. The change in shape of these proteins that follows their binding of calcium allows them to interact specifically with their targets within the cell; these include enzymes, ion channels, and muscle fibres. The intense scrutiny to which calcium channels have been subjected in recent decades has revealed their structures and the stimuli that control their opening (which range from changes in voltage to extracellular and intracellular messenger molecules); it is also beginning to establish the molecular mechanisms underlying their behaviour. Despite the diversity of behaviours, one feature that appears to be shared by all calcium channels is their regulation by cytoplasmic calcium ion concentration itself: each seems to be subject to feedback inhibition by calcium, a mechanism that probably serves to prevent intracellular calcium from rising to levels that could be toxic. This function can fail in sick cells — an excessive influx of calcium is known for example to be destructive to neuronal function when brain cells are damaged by lack of oxygen.
As well as these crucial roles in cellular function and in bone, ionized calcium in the blood plasma is one of several factors necessary for the clotting process: its chemical removal by the addition of citrate solution allows donor blood to be kept fluid for transfusion.
C. W. Taylor
See also blood; body fluids; cell; ion channels; neuromuscular junction; parathyroid glands; synapse.
COLIN BLAKEMORE and SHELIA JENNETT. "calcium." The Oxford Companion to the Body. 2001. Encyclopedia.com. (September 30, 2016). http://www.encyclopedia.com/doc/1O128-calcium.html
COLIN BLAKEMORE and SHELIA JENNETT. "calcium." The Oxford Companion to the Body. 2001. Retrieved September 30, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O128-calcium.html
Calcium is a chemical element and member of the alkaline-earth metals group. In its pure form, calcium is a silvery-white substance. Calcium is one of the most abundant substances on Earth, making up about 3.64 percent of the Earth's crust. It is also the fifth most abundant element in the human body. Calcium is necessary for good health. It is essential to muscle contraction and is needed for the transmission of nerve impulses, the clotting (thickening) of blood, and to maintain healthy membranes (thin layers covering cells and organs through which materials, usually liquids and gases, can pass).
Calcium also helps regulate contractions of the most important muscle in the body, the heart. This was discovered in 1882, when British physician Sydney Ringer (1835-1910) showed that a heart would continue to beat in a solution of salt, calcium, and other chemicals. Large amounts of calcium are needed for a developing baby during pregnancy and for the production of mothers' milk. Most of the calcium in the body—about 99 percent—is contained in the bones and teeth. The remaining one percent circulates in the bloodstream where, as American biochemist Elmer McCollum proved in the early 1900s, it is essential for muscle contractions. Bones get their strength and rigidity from calcium, which makes up 70 percent of their weight.
Calcium was not known as an element until the early 1800s, when chemists trying to prove the existence of unknown metals in natural compounds began using the newly discovered phenomenon of electricity to break them apart. English chemist Sir Humphry Davy, a pioneer in the field of electrochemistry, first isolated elemental calcium in 1808 by electrolyzing a mixture of lime and mercuric oxide.
Natural calcium compounds are found most frequently in rocks and minerals. Calcium carbonate is the most abundant of these, making up over 40 percent of the content of limestone. (In fact, calcium's name comes from the Latin word "calx," or limestone). Marble, dolomite, seashells, pearls, and coral also contain large amounts of calcium carbonate. Today, the compound is used in toothpastes and antacid medicines, and is also an ingredient in white paint.
Another important compound of calcium is carbide, which was discovered by German chemist Friedrich Wohler. In 1892 American scientist T. L. Willson produced calcium carbide by combining lime with carbon and heating the mixture. The result was a hard, brittle crystal that, when exposed to water, yielded calcium hydroxide and acetylene, a flammable gas used in welding. Calcium acetate is used in the production of plastics, and calcium hypochlorite is a bleaching agent and disinfectant.
When it is deficient (lacking) in the diet, calcium is released from the bones to maintain the level needed by the rest of the body. Over time, too little calcium can cause osteoporosis, a progressive weakening of the bones. Another bone disease called rickets can occur if the body does not have enough vitamin D to aid in calcium absorption. Rickets has plagued mankind for a long time. Archaeologists have found the bones of humans dating to about 50,000 b.c. that showed evidence of rickets.
Calcium is found in most plants and animals and is necessary for their health. Calcium phosphate is often used a fertilizer to enrich calcium-poor soil. Good sources of calcium in foods are tofu, milk and other dairy products, leafy green vegetables, sesame seeds, seaweeds, beans, almonds, and canned fish that contain the bones, such as sardines and salmon.
"Calcium." Medical Discoveries. 1997. Encyclopedia.com. (September 30, 2016). http://www.encyclopedia.com/doc/1G2-3498100066.html
"Calcium." Medical Discoveries. 1997. Retrieved September 30, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3498100066.html
calcium (kăl´sēəm) [Lat.,=lime], metallic chemical element; symbol Ca; at. no. 20; at. wt. 40.078; m.p. about 839°C; b.p. 1,484°C; sp. gr. 1.55 at 20°C; valence +2. Calcium is a malleable, ductile, silver-white, relatively soft metal with face-centered, cubic crystalline structure. Chemically it resembles strontium and barium; it is classed with them as an alkaline-earth metal in Group 2 of the periodic table. Calcium is chemically active; it tarnishes rapidly when exposed to air and burns with a bright yellow-red flame when heated, mainly forming the nitride. It reacts directly with water, forming the hydroxide. It combines with other elements, e.g., with oxygen, carbon, hydrogen, chlorine, fluorine, arsenic, phosphorus, and sulfur, forming many compounds.
Although lime (calcium oxide) has been known since ancient times, elemental calcium was first isolated by Sir Humphry Davy in 1808. Today, calcium metal is usually prepared by electrolysis of fused calcium chloride to which a little calcium fluoride has been added. It is used in alloys with other metals, such as aluminum, lead, or copper; in preparation of other metals, such as thorium and uranium, by reduction; and (like barium) in the manufacture of vacuum tubes to remove residual gases.
The metal is of little commercial importance compared to its compounds, which are widely and diversely used. The element is a constituent of lime (see calcium oxide), chloride of lime (bleaching powder), mortar, plaster, cement (see cement, concrete, whiting, putty, precipitated chalk, gypsum, and plaster of Paris. Tremolite, a form of asbestos, is a naturally occurring compound of calcium, magnesium, silicon, and oxygen. Calcium carbide reacts with water to form acetylene gas; it is also used to prepare calcium cyanamide, which is used as a fertilizer. The phosphate is a major constituent of bone ash. The arsenate and the cyanide are used as insecticides. Calcium bicarbonate causes temporary hardness in water; calcium sulfate causes permanent hardness. Generally, calcium compounds show an orange or yellow-red color when held in the Bunsen burner flame.
Although calcium is the fifth most abundant element in the earth's crust, of which it constitutes about 3.6%, it is not found uncombined. It is found widely distributed in its compounds, e.g., Iceland spar, marble, limestone, feldspar, apatite, calcite, dolomite, fluorite, garnet, and labradorite. It is a constituent of most plant and animal matter.
Calcium is essential to the formation and maintenance of strong bones and teeth; the recommended daily dietary allowance for all but young children ranges from 1,000 to 1,300 milligrams. In the human adult the bone calcium is chiefly in the form of the phosphate and carbonate salts. A sufficient store of vitamin D (see under vitamin) in the body is necessary for the proper utilization of calcium. Calcium also functions in the regulation of the heartbeat and in the conversion of prothrombin to thrombin, a necessary step in the clotting of blood.
"calcium." The Columbia Encyclopedia, 6th ed.. 2016. Encyclopedia.com. (September 30, 2016). http://www.encyclopedia.com/doc/1E1-calcium.html
"calcium." The Columbia Encyclopedia, 6th ed.. 2016. Retrieved September 30, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1E1-calcium.html
melting point: 839°C
boiling point: 1,484°C
density: 1.55 g/cm3
most common ions: Ca2+
Calcium is the fifth most abundant element in Earth's crust, with calcium oxide, CaO (lime), being among the most common of all terrestrial compounds. Calcium is very important from a biological standpoint, being critical to bones, teeth, and shells of various animals, most often appearing in the form of insoluble calcium phosphate, Ca3(PO4)2. Thus calcium is an important part of a healthy diet.
In elemental form, calcium is a relatively soft, silvery metal . Like other alkaline earths, it is too reactive to be found as a free element in nature. It was not until 1808 that Sir Humphry Davy isolated it by doing electrolysis on a mixture of lime (CaO) and mercuric oxide. Calcium's name comes from the Latin word calx, which means lime, a substance used since ancient Roman times in various ways, including as plasters for construction.
In addition to its biological role, calcium's presence is widespread in both nature and industry. As lime, it has many important commercial uses including in the treatment of drinking water and in the production and purification of iron and lead. Because of its usefulness, global consumption of lime exceeds 100 million tons annually. Other calcium compounds include calcium carbonate (CaCO3), which is better known as limestone and is the principal component of stalactites and stalagmites in underground caves. Because it is a weak base, calcium carbonate is also used as an antacid. Calcium silicates (Ca2SiO4 and Ca3SiO5) are major ingredients in Portland cement, named because it resembles natural calcium minerals found on the Isle of portland in England. Calcium chloride (CaCl2) is an excellent deicing and drying agent. In short, calcium is a very important and useful element.
see also Alkaline Earth Metals; Davy, Humphry.
David A. Dobberpuhl
Heiserman, David L. (1992). Exploring Chemical Elements and Their Compounds. Blue Ridge Summit, PA: Tab Books.
Krebs, Robert E. (1998). The History and Use of Our Earth's Chemical Elements: A Reference Guide. Westport, CT: Greenwood Press.
Swertka, Albert (2002). A Guide to the Elements. New York: Oxford University Press.
Dobberpuhl, David A.. "Calcium." Chemistry: Foundations and Applications. 2004. Encyclopedia.com. (September 30, 2016). http://www.encyclopedia.com/doc/1G2-3400900078.html
Dobberpuhl, David A.. "Calcium." Chemistry: Foundations and Applications. 2004. Retrieved September 30, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3400900078.html
The absorption of calcium from the intestinal tract requires vitamin D, and together with parathyroid hormone, vitamin D also controls the body's calcium balance, mobilizing it from the bones to maintain the plasma concentration within a very narrow range. An unacceptably high plasma concentration of calcium is hypercalcaemia.
Loss of calcium from bones occurs as a normal part of the ageing process, and may lead to osteoporosis.
The richest sources of calcium are milk and cheese; in some countries it is added to flour. Other rich sources include: haggis, canned pilchards and sardines, spinach, sprats, tripe.
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cal·ci·um / ˈkalsēəm/ • n. the chemical element of atomic number 20, a soft gray metal. (Symbol: Ca)
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