Calcium ions (Ca²⁺) contribute to the physiology and biochemistry of organisms cell. They play an important role in signal transduction pathways,^{【1】【2】} where they act as a second messenger, in neurotransmitter release from neurons, in contraction of all muscle cell types, and in fertilization. Many enzymes require calcium ions as a cofactor, including several of the coagulation factors. Extracellular calcium is also important for maintaining the potential difference across excitable cell membranes, as well as proper bone formation.

Plasma calcium levels in mammals are tightly regulated,^{【1】【2】} with bone acting as the major mineral storage site. Calcium ions, Ca²⁺, are released from bone into the bloodstream under controlled conditions. Calcium is transported through the bloodstream as dissolved ions or bound to proteins such as serum albumin. Parathyroid hormone secreted by the parathyroid gland regulates the resorption of Ca²⁺ from bone, reabsorption in the kidney back into circulation, and increases in the activation of vitamin D3 to calcitriol. Calcitriol, the active form of vitamin D3, promotes absorption of calcium from the intestines and bones. Calcitonin secreted from the parafollicular cells of the thyroid gland also affects calcium levels by opposing parathyroid hormone; however, its physiological significance in humans is dubious.

Dietary recommendations

The U.S. Institute of Medicine (IOM) established Recommended Dietary Allowances (RDAs) for calcium in 1997 and updated those values in 2011.^【3】See table. The European Food Safety Authority (EFSA) uses the term Population Reference Intake (PRIs) instead of RDAs and sets slightly different numbers: ages 4–10 800 mg, ages 11–17 1150 mg, ages 18–24 1000 mg, and >25 years 950 mg.^【4】

Global dietary calcium intake among adults (mg/day)^【5】

Because of concerns of long-term adverse side effects such as calcification of arteries and kidney stones, the IOM and EFSA both set Tolerable Upper Intake Levels (ULs) for the combination of dietary and supplemental calcium. From the IOM, people ages 9–18 years are not supposed to exceed 3,000 mg/day; for ages 19–50 not to exceed 2,500 mg/day; for ages 51 and older, not to exceed 2,000 mg/day.^【6】The EFSA set UL at 2,500 mg/day for adults but decided the information for children and adolescents was not sufficient to determine ULs.^【7】

For U.S. food and dietary supplement labeling purposes the amount in a serving is expressed as a percent of Daily Value (%DV). For calcium labeling purposes 100% of the Daily Value was 1000 mg, but as of May 27, 2016 it was revised to 1300 mg to bring it into agreement with the RDA.^{【8】【9】}Compliance with the updated labeling regulations was required by 1 January 2020, for manufacturers with $10 million or more in annual food sales, and by 1 January 2021, for manufacturers with less than $10 million in annual food sales.^{【10】【11】【12】} During the first six months following the 1 January 2020 compliance date, the FDA plans to work cooperatively with manufacturers to meet the new Nutrition Facts label requirements and will not focus on enforcement actions regarding these requirements during that time.^【10】A table of the old and new adult Daily Values is provided at Reference Daily Intake.

Age-adjusted daily calcium recommendations (from U.S. Institute of Medicine RDAs)^【3】

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

Food sources

The United States Department of Agriculture (USDA) web site has a very complete searchable table of calcium content (in milligrams) in foods, per common measures such as per 100 grams or per a normal serving.^{【13】【14】}

Food, calcium per 100 grams

parmesan (cheese) = 1140 mg

milk powder = 909 mg

goat hard cheese = 895 mg

Cheddar cheese = 720 mg

tahini paste = 427 mg

molasses = 273 mg

almonds = 234 mg

collard greens = 232 mg

kale = 150 mg

goat milk = 134 mg

sesame seeds (unhulled) = 125 mg

nonfat cow milk = 122 mg

plain whole-milk yogurt = 121 mg

Food, calcium per 100 grams

hazelnuts = 114 mg

tofu, soft = 114 mg

beet greens = 114 mg

spinach = 99 mg

ricotta (skimmed milk cheese) = 90 mg

lentils = 79 mg

chickpeas = 53 mg

eggs, boiled = 50 mg

orange = 40 mg

human milk = 33 mg

rice, white, long-grain = 19 mg

beef = 12 mg

cod = 11 mg

1. Brini, Marisa; Ottolini, Denis; Calì, Tito; Carafoli, Ernesto (2013). "Chapter 4. Calcium in Health and Disease". In Astrid Sigel, Helmut Sigel and Roland K. O. Sigel ed.). Interrelations between Essential Metal Ions and Human Diseases. Metal Ions in Life Sciences. 13. Springer. pp. 81–137. doi:10.1007/978-94-007-7500-8_4. ISBN 978-94-007-7499-5. PMID 24470090.
2. Jump up to:a b Brini, Marisa; Call, Tito; Ottolini, Denis; Carafoli, Ernesto (2013). "Chapter 5 Intracellular Calcium Homeostasis and Signaling". In Banci, Lucia (ed.). Metallomics and the Cell. Metal Ions in Life Sciences. 12. Springer. pp.  119–68.  doi:10.1007/978-94-007-5561-1_5. ISBN 978-94-007-5560-4. PMID 23595672. electronic-book ISBN 978-94-007-5561-1ISSN 1559-0836 electronic-ISSN 1868-0402
3. Institute of Medicine (US) Committee to Review Dietary Reference Intakes for Vitamin D Calcium; Ross, A. C.; Taylor, C. L.; Yaktine, A. L.; Del Valle, H. B. (2011). Dietary Reference Intakes for Calcium and Vitamin D, Chapter 5 Dietary Reference Intakes pages 345-402. Washington, D.C: National Academies Press. doi:10.17226/13050. ISBN 978-0-309-16394-1. PMID 21796828.
4. "Overview on Dietary Reference Values for the EU population as derived by the EFSA Panel on Dietetic Products, Nutrition and Allergies" (PDF). 2017.
5. Balk EM, Adam GP, Langberg VN, Earley A, Clark P, Ebeling PR, Mithal A, Rizzoli R, Zerbini CA, Pierroz DD, Dawson-Hughes B (December 2017). "Global dietary calcium intake among adults: a systematic review". Osteoporosis International. 28 (12): 3315–3324. doi:10.1007/s00198-017-4230-x. PMC 5684325. PMID 29026938.
6. Institute of Medicine (US) Committee to Review Dietary Reference Intakes for Vitamin D Calcium; Ross, A. C.; Taylor, C. L.; Yaktine, A. L.; Del Valle, H. B. (2011). Dietary Reference Intakes for Calcium and Vitamin D, Chapter 6 Tolerable Upper Intake Levels pages 403-456. Washington, D.C: National Academies Press. doi:10.17226/13050. ISBN 978-0-309-16394-1. PMID 21796828.
7. Tolerable Upper Intake Levels For Vitamins And Mine
8. "Federal Register May 27, 2016 Food Labeling: Revision of the Nutrition and Supplement Facts Labels. FR page 33982" (PDF).
9. "Daily Value Reference of the Dietary Supplement Label Database (DSLD)". Dietary Supplement Label Database (DSLD). Retrieved 16 May2020.
10. Jump up to:a b "FDA provides information about dual columns on Nutrition Facts label". U.S. Food and Drug Administration (FDA). 30 December 2019. Retrieved 16 May 2020.  This article incorporates text from this source, which is in the public domain.
11. "Changes to the Nutrition Facts Label". U.S. Food and Drug Administration (FDA). 27 May 2016. Retrieved 16 May 2020.  This article incorporates text from this source, which is in the public domain.
12. "Industry Resources on the Changes to the Nutrition Facts Label". U.S. Food and Drug Administration (FDA). 21 December 2018. Retrieved 16 May 2020.  This article incorporates text from this source, which is in the public domain.
13. "Food Composition Databases Show Nutrients List". USDA Food Composition Databases. United States Department of Agriculture: Agricultural Research Service. Retrieved November 29, 2017.
14. "SR Legacy Nutrient Search". usda.gov. Retrieved April 7, 2020.