8.3A: Calcium

Calcium is an essential trace element in living organisms. It is the most abundant metal by mass in many animals, and it is an important constituent of bone, teeth, and shells. In cell biology, the movement of the calcium ion into and out of the cytoplasm functions as a signal for many cellular processes. Calcium carbonate and calcium citrate are often taken as dietary supplements. Calcium is on the World Health Organization's List of Essential Medicines.

Cell physiology

In solution, the calcium ion varies remarkably to the human taste, 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. In human nutrition, soluble calcium salts may be added to tart juices without much effect to the average palate.

Recommended Dietary Allowance by the IOM for calcium:

Age	Calcium (mg/day)
0–6 months	200
7–12 months	260
1–3 years	700
4–8 years	1000
9–18 years	1300
19–50 years	1000
51–70 years (male)	1000
51–70 years (female)	1200
71+ years	1200
14–18 years (pregnant)	1300
19–50 years (pregnant)	1000
14–18 years (lact.)	1300
19–50 years (lact.)	1000

Calcium is an important component of a healthy diet and a mineral necessary for life. The National Osteoporosis Foundation states, "Calcium plays an important role in building stronger, denser bones early in life and keeping bones strong and healthy later in life." Approximately 99% of the calcium in the human body is in the bones and teeth. Intracellular calcium overload can cause oxidative stress and apoptosis in some cells, sometimes leading to several diseases. In the electrical conduction system of the heart, calcium replaces sodium as the mineral that depolarizes the cell, proliferating the action potential. In cardiac muscle, sodium influx commences an action potential, but during potassium efflux, the cardiac myocyte experiences calcium influx, prolonging the action potential and creating a plateau phase of dynamic equilibrium. Long-term calcium deficiency can lead to rickets and poor blood clotting; in menopausal women, deficiency can lead to osteoporosis, a condition in which the bone deteriorates and fractures more readily. While a lifelong deficit of calcium 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. Vitamin D is needed to absorb calcium.

Cell physiology

The release of calcium ions from the sarcoplasmic reticulum into the cytoplasm is an essential intracellular signal, important in many cellular functions and processes, including muscle contraction, neuronal transmission as in an excitatory synapse, cellular motility (including the movement of flagella and cilia), fertilization, cell growth or proliferation, learning, memory (as with synaptic plasticity), and secretion of saliva.

Dietary supplement

Calcium supplements are used to prevent and to treat calcium deficiencies. However, it has been found that the taking of calcium supplements by people with a history of stroke or with white matter lesions greatly increased their chances of developing dementia. The Office of Dietary Supplements (National Institutes of Health) recommends that no more than 600 mg of supplement should be taken at a time because the percent of calcium absorbed decreases as the amount of calcium in the supplement increases. It is therefore recommended to spread doses throughout the day. Recommended daily calcium intake for adults ranges from 1000 to 1300 mg. Calcium supplements may have side effects such as bloating and constipation in some people. It is suggested that taking the supplements with food may aid in nullifying these side effects.

500 milligram calcium supplements made from calcium carbonate

For U.S. dietary supplement and food labeling purposes the amount in a serving is expressed in milligrams and as a percent of Daily Value (%DV). The weight is for the calcium part of the compound - for example, calcium carbonate - in the supplement. For calcium labeling purposes 100% of the Daily Value was 1000 mg, but as of May 2016 it has been revised to 1300 mg. A table of the pre-change adult Daily Values is provided at Reference Daily Intake. Food and supplement companies have until July 2018 to comply with the labeling change.

Toxicity

Compared with other metals, the calcium ion and most calcium compounds have low toxicity. This is not surprising, given the very high natural abundance of calcium compounds in the environment and in organisms. As for oral consumption safety, the Food and Nutrition Board of the U.S. Institute of Medicine sets Tolerable Upper Intake Levels (known as ULs) for vitamins and minerals when evidence is sufficient. In the case of calcium the UL is set at 2500 mg/day for adults ages 19 to 50 and 2000 mg/day for ages 51 and up. This is not toxicity per se. From the FNB "Excessively high levels of calcium in the blood known as hypercalcemia can cause renal insufficiency, vascular and soft tissue calcification, hypercalciuria (high levels of calcium in the urine) and kidney stones." The European Food Safety Authority reviewed the same safety question and set its UL at 2500 mg/day.

Calcium poses few serious environmental problems

Calcium metal is hazardous because of its sometimes-violent reactions with water and acids. Calcium metal is found in some drain cleaners, where it functions to generate heat and calcium hydroxide that saponifies the fats and liquefies the proteins (e.g., hair) that block drains. When swallowed, calcium metal has the same effect on the mouth, esophagus, and stomach, and can be fatal.