Editing Calcium (section)

==Biological and pathological role==
{{main|Calcium in biology}}
{| class="wikitable" style="float:right;"
|+ Age-adjusted daily calcium recommendations (from U.S. Institute of Medicine RDAs)<ref>{{cite book | title = Dietary Reference Intakes for Calcium and Vitamin D|chapter =ch. 5. Dietary Reference Intakes |pages =345–402 | publisher = National Academies Press | location = Washington, D.C. | year = 2011 | isbn = 978-0-309-16394-1 | url = https://www.nap.edu/read/13050/chapter/7| doi = 10.17226/13050 | pmid = 21796828 | author1 = Institute of Medicine (US) Committee to Review Dietary Reference Intakes for Vitamin D Calcium | last2 = Ross | first2 = A. C. | last3 = Taylor | first3 = C. L. | last4 = Yaktine | first4 = A. L. | last5 = Del Valle | first5 = H. B. |s2cid =58721779 }}</ref>
|-
! Age
! Calcium (mg/day)
|-
| 1–3 years
| 700
|-
| 4–8 years
| 1000
|-
| 9–18 years
| 1300
|-
| 19–50 years
| 1000
|-
| >51 years
| 1000
|-
| Pregnancy
| 1000
|-
| Lactation
| 1000
|}

[[File:Calcium_intake_world_map.svg|thumb|upright=1.4|Global dietary calcium intake among adults (mg/day).<ref>{{cite journal | vauthors = Balk EM, Adam GP, Langberg VN, Earley A, Clark P, Ebeling PR, Mithal A, Rizzoli R, Zerbini CA, Pierroz DD, Dawson-Hughes B | title = Global dietary calcium intake among adults: a systematic review | journal = Osteoporosis International | volume = 28 | issue = 12 | pages = 3315–24 | date = December 2017 | pmid = 29026938 | pmc = 5684325 | doi = 10.1007/s00198-017-4230-x }}</ref>
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===Function===
Calcium is an [[essential element]] needed in large quantities.<ref name="lpi" /><ref name="ods" /> The Ca<sup>2+</sup> ion acts as an [[electrolyte]] and is vital to the health of the muscular, circulatory, and digestive systems; is indispensable to the building of bone in the form of [[Hydroxyapatite#Biological function|hydroxyapatite]]; and supports synthesis and function of blood cells. For example, it regulates the [[Muscle contraction|contraction of muscles]], nerve conduction, and the clotting of blood. As a result, intra- and extracellular calcium levels are tightly regulated by the body. Calcium can play this role because the Ca<sup>2+</sup> ion forms stable [[coordination complex]]es with many organic compounds, especially [[protein]]s; it also forms compounds with a wide range of solubilities, enabling the formation of the [[skeleton]].<ref name="lpi" />
<ref>
Sosa Torres, Martha; Kroneck, Peter M.H; "Introduction: From Rocks to Living Cells" pp. 1–32 in "Metals, Microbes and Minerals: The Biogeochemical Side of Life" (2021) pp. xiv + 341. Walter de Gruyter, Berlin. Editors Kroneck, Peter M.H. and Sosa Torres, Martha. {{doi|10.1515/9783110589771-001}}</ref>

===Binding===
Calcium ions may be complexed by proteins through binding the [[carboxyl group]]s of [[glutamic acid]] or [[aspartic acid]] residues; through interacting with [[phosphorylation|phosphorylated]] [[serine]], [[tyrosine]], or [[threonine]] residues; or by being [[chelation|chelated]] by γ-carboxylated amino acid residues. [[Trypsin]], a digestive enzyme, uses the first method; [[osteocalcin]], a bone matrix protein, uses the third.{{sfn|Hluchan|Pomerantz|2005|pp = 489–94}}

Some other bone matrix proteins such as [[osteopontin]] and [[bone sialoprotein]] use both the first and the second. Direct activation of enzymes by binding calcium is common; some other enzymes are activated by noncovalent association with direct calcium-binding enzymes. Calcium also binds to the [[phospholipid]] layer of the [[cell membrane]], anchoring proteins associated with the cell surface.{{sfn|Hluchan|Pomerantz|2005|pp = 489–94}}

===Solubility===
As an example of the wide range of solubility of calcium compounds, [[monocalcium phosphate]] is very soluble in water, 85% of extracellular calcium is as [[dicalcium phosphate]] with a solubility of 2.00&nbsp;[[Molar concentration|mM]], and the [[hydroxyapatite]] of bones in an organic matrix is [[tricalcium phosphate]] with a solubility of 1000&nbsp;μM.{{sfn|Hluchan|Pomerantz|2005|pp = 489–94}}

=== Nutrition ===
Calcium is a common constituent of [[multivitamin]] [[dietary supplement]]s,<ref name="lpi" /> but the composition of calcium complexes in supplements may affect its [[bioavailability]] which varies by solubility of the salt involved: [[calcium citrate]], [[Calcium malate|malate]], and [[Calcium lactate|lactate]] are highly bioavailable, while the [[Calcium oxalate|oxalate]] is less. Other calcium preparations include [[calcium carbonate]], [[calcium citrate malate]], and [[calcium gluconate]].<ref name="lpi" /> The intestine absorbs about one-third of calcium eaten as the [[Radical ion|free ion]], and plasma calcium level is then regulated by the [[kidney]]s.<ref name="lpi" />

===Hormonal regulation of bone formation and serum levels===
[[Parathyroid hormone]] and [[vitamin D]] promote the formation of bone by allowing and enhancing the deposition of calcium ions there, allowing rapid bone turnover without affecting bone mass or mineral content.<ref name="lpi" /> When plasma calcium levels fall, cell surface receptors are activated and the secretion of parathyroid hormone occurs; it then proceeds to stimulate the entry of calcium into the plasma pool by taking it from targeted kidney, gut, and bone cells, with the bone-forming action of parathyroid hormone being antagonized by [[calcitonin]], whose secretion increases with increasing plasma calcium levels.{{sfn|Hluchan|Pomerantz|2005|pp = 489–94}}

===Abnormal serum levels===
Excess intake of calcium may cause [[hypercalcemia]]. However, because calcium is absorbed rather inefficiently by the intestines, high serum calcium is more likely caused by excessive secretion of parathyroid hormone (PTH) or possibly by excessive intake of vitamin D, both of which facilitate calcium absorption. All these conditions result in excess calcium salts being deposited in the heart, blood vessels, or kidneys. Symptoms include anorexia, nausea, vomiting, memory loss, confusion, muscle weakness, increased urination, dehydration, and metabolic bone disease.{{sfn|Hluchan|Pomerantz|2005|pp = 489–94}}

Chronic hypercalcaemia typically leads to [[calcification]] of soft tissue and its serious consequences: for example, calcification can cause loss of elasticity of [[vascular wall]]s and disruption of laminar blood flow—and thence to [[Vulnerable plaque|plaque rupture]] and [[thrombosis]]. Conversely, inadequate calcium or vitamin D intakes may result in [[hypocalcemia]], often caused also by inadequate secretion of parathyroid hormone or defective PTH receptors in cells. Symptoms include neuromuscular excitability, which potentially causes [[tetany]] and disruption of conductivity in cardiac tissue.{{sfn|Hluchan|Pomerantz|2005|pp = 489–94}}

===Bone disease===
As calcium is required for bone development, many bone diseases can be traced to the organic matrix or the [[hydroxyapatite]] in molecular structure or organization of bone. [[Osteoporosis]] is a reduction in mineral content of bone per unit volume, and can be treated by supplementation of calcium, vitamin D, and [[bisphosphonate]]s.<ref name="lpi" /><ref name="ods" /> Inadequate amounts of calcium, vitamin D, or phosphates can lead to softening of bones, called [[osteomalacia]].{{sfn|Hluchan|Pomerantz|2005|pp = 489–94}}