Calcium (pronEng|ˈkælsiəm) is the
chemical elementwith the symbol Ca and atomic number20. It has an atomic mass of 40.078. Calcium is a soft grey alkaline earth metal, and is the fifth most abundant element by mass in the Earth's crust. Calcium is also the fifth most abundant dissolved ion in seawaterby both molarity and mass, after sodium, chloride, magnesium, and sulfate. [ [http://cdiac.esd.ornl.gov/ftp/cdiac74/chapter5.pdf chapter5 ] ]
Calcium is essential for living
organisms, particularly in cell physiology, where movement of the calcium ion Ca2+ into and out of the cytoplasmfunctions as a signal for many cellular processes. As a major material used in mineralization of bones and shells, calcium is the most abundant metalby mass in many animals.
Chemically calcium is reactive and soft for a metal (though harder than lead, it can be cut with a knife with difficulty). It is a silvery metallic element that must be extracted by
electrolysisfrom a fused salt like calcium chloride.Pauling, Linus "General Chemistry" p. 627, 1970 ed. Dover Publications] Once produced, it rapidly forms a grey-white oxideand nitridecoating when exposed to air. It is somewhat difficult to ignite, unlike magnesium, but when lit, the metal burns in air with a brilliant high-intensity red light. Calcium metal reacts with water, evolving hydrogen gas at a rate rapid enough to be noticeable, but not fast enough at room temperature to generate much heat. In powdered form, however, the reaction with water is extremely rapid, as the increased surface area of the powder accelerates the reaction with the water. Part of the slowness of the calcium-water reaction results from the metal being partly protected by insoluble white calcium hydroxide. In water solutions of acids where the salt is water soluble, calcium reacts vigorously.
Calcium, though it has a higher resistivity than
copperor aluminium, weight for weight, allowing for its much lower density calcium is a rather better conductor than either. However, its use in terrestrial applications is usually limited by its high reactivity with air. In vacuum use, calcium tends to sublime unless plated.
Calcium salts are colorless from any contribution of the calcium, and ionic solutions of calcium (Ca2+) are colorless as well. Many calcium salts are not soluble in water. When in solution, the calcium ion to the human taste varies remarkably, being reported as mildly salty, sour, "mineral like" or even "soothing." It is apparent that many animals can taste, or develop a taste, for calcium, and use this sense to detect the mineral in
salt licks or other sources. [ [http://physrev.physiology.org/cgi/content/full/81/4/1567 Calcium: Taste, Intake, and Appetite - Tordoff 81 (4): 1567 - Physiological Reviews ] ] In human nutrition, soluble calcium salts may be added to tart juices without much effect to the average palate.
Calcium is the fifth most abundant element by mass in the human body, where it is a common cellular ionic messenger with many functions, and serves also as a structural element in bone. It is the relatively high atomic-numbered calcium in the skeleton which causes bone to be radio-opaque. Of the human body's solid components after drying (as for example, after
cremation), about a third of the total mass is the approximately one kilogram of calcium which composes the average skeleton (the remainder being mostly phosphorus and oxygen).
Calcium is not naturally found in its elemental state. Calcium occurs most commonly in
sedimentary rocksin the minerals calcite, dolomiteand gypsum. It also occurs in igneous and metamorphic rockschiefly in the silicate minerals: plagioclase, amphiboles, pyroxenes and garnets.
"See also ."
Some uses are:
* as a
reducing agentin the extraction of other metals, such as uranium, zirconium, and thorium.
* as a deoxidizer, desulfurizer, or decarbonizer for various ferrous and nonferrous
* as an alloying agent used in the production of
aluminium, beryllium, copper, lead, and magnesiumalloys.
* in the making of
cements and mortars to be used in construction.
* in the making of
cheese, where calcium ionsinfluence the activity of renninin bringing about the coagulationof milk.
Calcium carbonate(CaCO3) used in manufacturing cementand mortar, lime, limestone(usually used in the steel industry); aids in production in the glass industry, also has chemical and optical uses as mineral specimens in toothpastes, for example.
Calcium hydroxidesolution (Ca(OH)2) (also known as limewater) is used to detect the presence of carbon dioxide by being bubbled through a solution. It turns cloudy where CO2 is present.
Calcium arsenate(Ca3(AsO4)2) is used in insecticides.
Calcium carbide(CaC2) is used: to make acetylenegas (for use in acetylene torches for welding) and in the manufacturing of plastics.
Calcium chloride(CaCl2) is used: in iceremoval and dustcontrol on dirt roads, in conditioner for concrete, as an additive in canned tomatoes, and to provide body for automobile tires.
* Calcium cyclamate (Ca(C6H11NHSO3)2) was used as a sweetening agent but is no longer permitted for use because of suspected cancer-causing properties.
* Calcium gluconate (Ca(C6H11O7)2) is used as a
food additiveand in vitaminpills.
Calcium hypochlorite(Ca(OCl)2) is used: as a swimming pool disinfectant, as a bleaching agent, as an ingredient in deodorant, and in algaecideand fungicide.
* Calcium permanganate (Ca(MnO4)2) is used in liquid rocket propellant,
textileproduction, as a water sterilizing agent and in dental procedures.
Calcium phosphate(Ca3(PO4)2) is used as a supplement for animalfeed, fertilizer, in commercial production for doughand yeastproducts, in the manufacture of glass, and in dental products.
Calcium phosphide(Ca3P2) is used in fireworks, rodenticide, torpedoes and flares.
Calcium stearate(Ca(C18H35O2)2 is used in the manufacture of wax crayons, cements, certain kinds of plastics and cosmetics, as a food additive, in the production of water resistant materials and in the production of paints.
Calcium sulfate(CaSO4·2H2O) is used as common blackboard chalk, as well as, in its hemihydrate form being more well known as Plaster of Paris.
Calcium tungstate(CaWO4) is used in luminous paints, fluorescent lights and in X-raystudies.
Hydroxylapatite(Ca5(PO4)3(OH), but is usually written Ca10(PO4)6(OH)2) makes up seventy percent of bone. Also carbonated-calcium deficient hydroxylapatite is the main mineral of which dental enameland dentinare comprised.
H and K lines
In the visible portion of the spectrum of many stars, including the
Sun, strong absorption lines of singly-ionized calcium are shown. Prominent among these are the H-line at 3968.5 Å and the K line at 3933.7 Å of singly-ionized calcium, or Ca II. For the Sun and stars with low temperatures, the prominence of the H and K lines can be an indication of strong magnetic activity in the chromosphere. Measurement of periodic variations of these active regions can also be used to deduce the rotation periods of these stars. [cite web
title =H-K Project
publisher =Mount Wilson Observatory
accessdate = 2006-08-10 ]
Latin"calx," meaning "limestone") was known as early as the first century when the Ancient Romans prepared lime as calcium oxide. It was not isolated until 1808 in Englandwhen Sir Humphry Davy electrolyzed a mixture of lime and mercuric oxide. Davy was trying to isolate calcium; when he heard that Swedish chemist Jöns Jakob Berzeliusand Pontin prepared calcium amalgam by electrolyzing lime in mercury, he tried it himself. He worked with electrolysis throughout his life and also discovered/isolated sodium, potassium, magnesium, boronand barium.
Calcium, combined with
phosphateto form hydroxylapatite, is the mineral portion of human and animal bones and teeth. The mineral portion of some corals can also be transformed into hydroxylapatite. Calcium oxide(lime) is used in many chemical refinery processes and is made by heating and carefully adding water to limestone. When lime is mixed with sand, it hardens into a mortar and is turned into plasterby carbon dioxideuptake. Mixed with other compounds, lime forms an important part of Portland cement. Calcium carbonate(CaCO3) is one of the common compounds of calcium. It is heated to form quicklime (CaO), which is then added to water (H2O). This forms another material known as slaked lime (Ca(OH)2), which is an inexpensive base material used throughout the chemical industry. Chalk, marble, and limestone are all forms of calcium carbonate.
When water percolates through
limestoneor other soluble carbonaterocks, it partially dissolves the rock and causes cave formation and characteristic stalactites and stalagmites and also forms hard water. Other important calcium compounds are calcium nitrate, calcium sulfide, calcium chloride, calcium carbide, calcium cyanamideand calcium hypochlorite.
Calcium has four stable
isotopes (40Ca and 42Ca through 44Ca), plus two more isotopes (46Ca and 48Ca) that have such long half-lives that for all practical purposes they can be considered stable. It also has a cosmogenic isotope, radioactive41Ca, which has a half-lifeof 103,000 years. Unlike cosmogenic isotopes that are produced in the atmosphere, 41Ca is produced by neutronactivation of 40Ca. Most of its production is in the upper metre or so of the soil column, where the cosmogenic neutron flux is still sufficiently strong. 41Ca has received much attention in stellar studies because it decays to 41K, a critical indicator of solar-system anomalies.
97% of naturally occurring calcium is in the form of 40Ca. 40Ca is one of the daughter products of 40K decay, along with 40Ar. While
K-Ar datinghas been used extensively in the geological sciences, the prevalence of 40Ca in nature has impeded its use in dating. Techniques using mass spectrometryand a double spike isotope dilution have been used for K-Ca age dating.
The most abundant isotope, 40Ca, has a nucleus of 20
protons and 20 neutrons. This is the heaviest stable isotope of any element which has equal numbers of protons and neutrons. In supernova explosions, calcium is formed from the reaction of carbon with various numbers of alpha particles (helium nuclei), until the most common calcium isotope (containing 10 helium nuclei) has been synthesized.Fact|date=April 2008
Calcium is an important component of a
healthy diet. Calcium is essential for the normal growth and maintenance of bones and teeth, and calcium requirements must be met throughout life. Long-term calcium deficiency can lead to rickets and poor blood clotting and in case of a menopausal woman, it can lead to osteoporosis, in which the bone deteriorates and there is an increased risk of fractures. While a lifelong deficit can affect bone and tooth formation, over-retention can cause hypercalcemia (elevated levels of calcium in the blood), impaired kidney function and decreased absorption of other minerals.Standing Committee on the Scientific Evaluation of Dietary Reference Intakes, Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D and fluoride. Washington DC: The National Academies Press, 1997] High calcium intakes or high calcium absorption were previously thought to contribute to the development of kidney stones. However, more recent studies show that high dietary calcium intakes actually decrease the risk for kidney stones.Curhan G, Willett WC, Rimm E, Stampher MJ. A prospective study of dietary calcium and other nutrients and the risk of symptomatic kidney stones. N Engl J Med 1993;328:833-8] Vitamin Dis needed to absorb calcium. Dairy products, such as milk and cheese, are a well-known source of calcium. However, some individuals are allergic to dairy products and even more people, particularly those of non Indo-European descent, are lactose-intolerant, leaving them unable to consume non-fermented dairy products in quantities larger than about half a liter per serving. Others, such as vegans, avoid dairy products for ethical and health reasons. Fortunately, many good sources of calcium exist. These include seaweedssuch as kelp, wakameand hijiki; nuts and seeds (like almonds and sesame); blackstrap molasses; beans; oranges; figs; quinoa; amaranth; collard greens; okra; rutabaga; broccoli; dandelionleaves; kale; and fortified products such as orange juice and soy milk. (However, calcium fortified orange juice often contains vitamin D3derived from lanolin, and is thus unacceptable for vegans. [cite web|url=http://findarticles.com/p/articles/mi_m0FDE/is_3_23/ai_n6138556|title=Sources of vitamin D in orange juice] ) An overlooked source of calcium is eggshell, which can be ground into a powder and mixed into food or a glass of water. [cite journal |author=Anne Schaafsma, Gerard M Beelen |title=Eggshell powder, a comparable or better source of calcium than purified calcium carbonate: piglet studies |journal=Journal of the Science of Food and Agriculture |volume=79 |issue=12 |pages=1596–1600 |year=1999 |url=http://www3.interscience.wiley.com/cgi-bin/abstract/63003036/ABSTRACT |doi=10.1002/(SICI)1097-0010(199909)79:12<1596::AID-JSFA406>3.0.CO;2-A |format=abstract] cite journal |author=Schaafsma A, van Doormaal JJ, Muskiet FA, Hofstede GJ, Pakan I, van der Veer E |title=Positive effects of a chicken eggshell powder-enriched vitamin-mineral supplement on femoral neck bone mineral density in healthy late post-menopausal Dutch women |journal=Br. J. Nutr. |volume=87 |issue=3 |pages=267–75 |year=2002 |month=March |pmid=12064336 |doi=10.1079/BJNBJN2001515 |url=] cite journal |author=Rovenský J, Stancíková M, Masaryk P, Svík K, Istok R |title=Eggshell calcium in the prevention and treatment of osteoporosis |journal=Int J Clin Pharmacol Res |volume=23 |issue=2-3 |pages=83–92 |year=2003 |pmid=15018022 |doi= |url=] Cultivated vegetables generally have less calcium than wild plants. [ [http://www.beyondveg.com/nieft-k/instincto-guide/instincto-guide1e.shtml Original Wild Foods vs. Available Foods Today for Instinctos ] ]
The calcium content of most foods can be found in the USDA National Nutrient Database. [ [http://www.nal.usda.gov/fnic/foodcomp/search USDA National Nutrient Database] ]
Dietary calcium supplements
Calcium supplements are used to prevent and to treat calcium deficiencies. Most experts recommend that supplements be taken with food and that no more than 600 mg should be taken at a time because the percent of calcium absorbed decreases as the amount of calcium in the supplement increases. It is recommended to spread doses throughout the day. Recommended daily calcium intake for adults ranges from 1000 to 1500 mg. It is recommended to take supplements with food to aid in absorption.
Vitamin Dis added to some calcium supplements. Vitamin D is not necessary, but it might be beneficial if the person has low vitamin D status. Proper vitamin D status is important because vitamin D is converted to a hormone in the body which then induces the synthesis of intestinal proteins responsible for calcium absorption. [Combs, G: "The Vitamins", page 161. Academic Press, 2008]
* The absorption of calcium from most food and commonly-used dietary supplements is very similar. [Weaver, CM: "Calcium" in Present Knowledge in Nutrition, 9th Ed., Vol I, page 377. ILSI Press, 2006.] This is contrary to what many calcium supplement manufacturers claim in their promotional materials.
Milkis an excellent source of dietary calcium because it has a high concentration of calcium and the calcium in milk is excellently absorbed. [Weaver, CM: "Calcium" in Present Knowledge in Nutrition, 9th Ed., Vol I, page 377. ILSI Press, 2006]
Calcium carbonateis the most common and least expensive calcium supplement. It should be taken with food. The absorption of calcium from calcium carbonate is similar to the absorption of calcium from milk. [Zhao, Y. et al: "Calcium bioavailability of calcium carbonate fortified soy milk is equivalent to cow's milk in young women", "J. Nutr.", 135(10):2379] While most people digest calcium carbonate very well, some might develop gastrointestinal discomfort or gas. Taking magnesium with it can help to avoid constipation. Calcium carbonate is 40% elemental calcium. 1000 mg will provide 400 mg of calcium. However, supplement labels will usually indicate how much calcium is present in each serving, not how much calcium carbonate is present.
Antacids, such as Tums, frequently contain calcium carbonate, and are a very commonly-used, inexpensive calcium supplement.
Coral Calciumis a salt of calcium derived from fossilized coral reefs. Coral calcium is composed of calcium carbonate and trace minerals.
Calcium citratecan be taken without food and is the supplement of choice for individuals with achlorhydria or who are taking histamine-2 blockers or proton-pump inhibitors.Straub, DA: "Calcium supplementation in clinical practice: a review of forms, doses, and indications", "Nutr. Clin. Pract.", 22(3):286, 2007.] It is more easily digested and absorbed than calcium carbonate if taken on empty stomach and less likely to cause constipation and gas than calcium carbonate. It also has a lower risk of contributing to the formation of kidney stones. Calcium citrate is about 21% elemental calcium. 1000 mg will provide 210 mg of calcium. It is more expensive than calcium carbonate and more of it must be taken to get the same amount of calcium.
Calcium phosphatecosts more than calcium carbonate, but less than calcium citrate. It is easily absorbed and is less likely to cause constipation and gas than either.
Calcium lactatehas similar absorption as calcium carbonate [Martin, BR: "Calcium absorption from three salts and CaSo4- fortified bread in premenopausal women", "J. Agric. Food Chem.", 50:3874, 2002.] , but is more expensive. Calcium lactate and calcium gluconate are less concentrated forms of calcium and are not practical oral supplements.
chelatesare synthetic calcium compounds, with calcium bound to an organic molecule, such as malate, aspartate, or fumarate. These forms of calcium may be better absorbed on an empty stomach. However, in general they are absorbed similarly to calcium carbonate and other common calcium supplements when taken with food. [Weaver, CM, et al: "Absorption of calcium fumarate salts is equivalent to other calcium salts when measured in the rat model.", "J. Agric. Food Chem.", 50(17):4974, 2002] The 'chelate' mimics the action that natural food performs by keeping the calcium soluble in the intestine. Thus, on an empty stomach, in some individuals, chelates might theoretically be absorbed better.
* Microcrystalline hydroxyapatite (MH) is marketed as a calcium supplement, and has in some randomized trials been found to be more effective than calcium carbonate.
Orange juicewith calcium added is a good dietary source for persons who have lactose intolerance.
The National Nutritional Food Association — NNFA (Newport Beach, Calif.) defines a chelate very specifically, and several criteria must be met in order for chelation to actually occur. Some of the claimed "chelates" on the market are the various Krebs (Citric Acid) Cycle chelates, such as citrate, malate, and aspartate. Dicalcium malate (chelated with malic acid) is a newer form of a true calcium chelate. It contains a high amount of elemental calcium (30%).
In July 2006, a report citing research from
Fred Hutchinson Cancer Research Centerin Seattle, Washington claimed that women in their 50s gained 5 pounds less in a period of 10 years by taking more than 500 mg of calcium supplements than those who did not. However, the doctor in charge of the study, Dr. Alejandro J. Gonzalez also noted it would be "going out on a limb" to suggest calcium supplements as a weight-limiting aid.cite web | title=Calcium May Help With Weight Loss | accessdate=2007-07-10 | url=http://www.rxalternativemedicine.com/headlines_news.php#headline77|author=Anne Harding ]
Prevention of fractures due to osteoporosis
Such studies often do not test calcium alone, but rather combinations of calcium and vitamin D.
Randomized controlled trialsfound both positivecite journal |author=Dawson-Hughes B, Harris SS, Krall EA, Dallal GE |title=Effect of calcium and vitamin D supplementation on bone density in men and women 65 years of age or older |journal=N. Engl. J. Med. |volume=337 |issue=10 |pages=670–6 |year=1997 |pmid=9278463 |doi=] and negativecite journal |author=Jackson RD, LaCroix AZ, Gass M, "et al" |title=Calcium plus vitamin D supplementation and the risk of fractures |journal=N. Engl. J. Med. |volume=354 |issue=7 |pages=669–83 |year=2006 |pmid=16481635 |doi=10.1056/NEJMoa055218] cite journal |author=Grant AM, Avenell A, Campbell MK, "et al" |title=Oral vitamin D3 and calcium for secondary prevention of low-trauma fractures in elderly people (Randomised Evaluation of Calcium Or vitamin D, RECORD): a randomised placebo-controlled trial |journal=Lancet |volume=365 |issue=9471 |pages=1621–8 |year=2005 |pmid=15885294 |doi=10.1016/S0140-6736(05)63013-9] cite journal |author=Porthouse J, Cockayne S, King C, "et al" |title=Randomised controlled trial of calcium and supplementation with cholecalciferol (vitamin D3) for prevention of fractures in primary care |journal=BMJ |volume=330 |issue=7498 |pages=1003 |year=2005 |pmid=15860827 |doi=10.1136/bmj.330.7498.1003] cite journal |author=Prince RL, Devine A, Dhaliwal SS, Dick IM |title=Effects of calcium supplementation on clinical fracture and bone structure: results of a 5-year, double-blind, placebo-controlled trial in elderly women |journal=Arch. Intern. Med. |volume=166 |issue=8 |pages=869–75 |year=2006 |pmid=16636212 |doi=10.1001/archinte.166.8.869] effects. The different results may be explained by doses of calcium and underlying rates of calcium supplementation in the control groups.cite journal |author=Fletcher RH |title=Calcium plus vitamin D did not prevent hip fracture or colorectal cancer in postmenopausal women |journal=ACP J. Club |volume=145 |issue=1 |pages=4–5 |year=2006 |pmid=16813354 |doi=|url=http://www.acpjc.org/Content/145/1/issue/ACPJC-2006-145-1-004.htm |format=subscription required] However, it is clear that increasing the intake of calcium promotes deposition of calcium in the bones, where it is of more benefit in preventing the compression fractures resulting from the osteoporotic thinning of the dendriticweb of the bodies of the vertebrae, than it is at preventing the more serious cortical bone fractures which happen at hip and wrist.
Prevention of cancer?
meta-analysiscite journal |author=Weingarten MA, Zalmanovici A, Yaphe J |title=Dietary calcium supplementation for preventing colorectal cancer and adenomatous polyps |journal=Cochrane database of systematic reviews (Online) |volume= |issue=3 |pages=CD003548 |year=2005 |pmid=16034903 |doi=10.1002/14651858.CD003548.pub3] by the international Cochrane Collaborationof two randomized controlled trialscite journal |author=Baron JA, Beach M, Mandel JS, "et al" |title=Calcium supplements for the prevention of colorectal adenomas. Calcium Polyp Prevention Study Group |journal=N. Engl. J. Med. |volume=340 |issue=2 |pages=101–7 |year=1999 |pmid=9887161 |doi=] cite journal |author=Bonithon-Kopp C, Kronborg O, Giacosa A, Räth U, Faivre J |title=Calcium and fibre supplementation in prevention of colorectal adenoma recurrence: a randomised intervention trial. European Cancer Prevention Organisation Study Group |journal=Lancet |volume=356 |issue=9238 |pages=1300–6 |year=2000 |pmid=11073017 |doi=] found that calcium "might contribute to a moderate degree to the prevention of adenomatous colonic polyps".
More recent studies were conflicting, and one which was positive for effect (Lappe, et al.) did control for a possible anti-carcinogenic effect of
vitamin D, which was found to be an independent positive influence from calcium-alone on cancer risk (see second study below) [for abstract see PMID 17556697] .
randomized controlled trialfound that 1000 mg of elemental calcium and 400 IU of vitamin D3 had no effect on colorectal cancercite journal |author=Wactawski-Wende J, Kotchen JM, Anderson GL, "et al" |title=Calcium plus vitamin D supplementation and the risk of colorectal cancer |journal=N. Engl. J. Med. |volume=354 |issue=7 |pages=684–96 |year=2006 |pmid=16481636 |doi=10.1056/NEJMoa055222]
randomized controlled trialfound that 1400–1500 mg supplemental calcium and 1100 IU vitamin D3 reduced aggregated cancers with a relative riskof 0.402.cite journal |author=Lappe JM, Travers-Gustafson D, Davies KM, Recker RR, Heaney RP |title=Vitamin D and calcium supplementation reduces cancer risk: results of a randomized trial |journal=Am. J. Clin. Nutr. |volume=85 |issue=6 |pages=1586–91 |year=2007 |pmid=17556697 |doi=]
* An observational
cohort studyfound that high calcium and vitamin D intake was associated with "lower risk of developing premenopausal breast cancer."cite journal |author=Lin J, Manson JE, Lee IM, Cook NR, Buring JE, Zhang SM |title=Intakes of calcium and vitamin d and breast cancer risk in women |journal=Arch. Intern. Med. |volume=167 |issue=10 |pages=1050–9 |year=2007 |pmid=17533208 |doi=10.1001/archinte.167.10.1050]
Exceeding the recommended daily calcium intake for an extended period of time can result in
Calcium in biology
Disorders of calcium metabolism
* Rebecca J. Donatelle. Health, The Basics. 6th ed. San Francisco: Pearson Education, Inc. 2005.
* [http://www.webelements.com/webelements/elements/text/Ca/index.html WebElements.com — Calcium]
* [http://www.nal.usda.gov/fnic/foodcomp/Data/SR17/wtrank/sr17a301.pdf USDA National Nutrient Database, Calcium content of selected foods]
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