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[calci- + -ium]
SYMB: Ca A silver-white metallic chemical element, atomic number 20, atomic weight (mass) 40.08. Lime (calcium oxide), CaO, is its oxide. Calcium is a major component of limestone. Hydroxylapatite, a calcium phosphate, makes up about 75% of body ash and about 85% of mineral matter in bones.
Calcium is important for blood clotting, enzyme activation, and acid-base balance. It gives firmness and rigidity to bones and teeth. It is essential for lactation, the function of nerves and muscles (including heart muscle), and maintenance of membrane permeability. Most absorption of calcium occurs in the duodenum and is dependent on the presence of calcitriol. Dietary factors affecting calcium absorption include phytic acid, consumption of too much phosphorus, and polyphenols found in tea. Approximately 40% of the calcium consumed is absorbed. Blood levels of calcium are regulated by parathyroid hormone; deficiency of this hormone produces hypocalcemia. The serum level of calcium is normally about 8.5 to 10.5 mg/dL. Low blood calcium causes tetany. Blood deprived of its calcium will not clot. Calcium is deposited in the bones but can be mobilized from them to keep the blood level constant when dietary intake is inadequate. At any given time, the body of an adult contains about 700 g of calcium phosphate; of this, 120 g is the element calcium. Adults should consume at least 1 g of calcium daily. Pregnant, lactating, and postmenopausal women should consume 1.2-1.5 g of calcium per day.
Excellent sources of calcium include milk and milk products (but not cottage cheese), and calcium-fortified orange juice. Good sources include canned salmon and sardines, broccoli, tofu, rhubarb, almonds, figs, and turnip greens.
SEE: Dietary Reference Intakes: Recommended Intakes for Individuals
1. Laboratory error and variation may sometimes cause inaccurate or inconsistent values in evaluating calcium levels.
2. Excessive calcium supplementation has been associated with a small increased risk of vascular calcification and heart attack.
SEE: hypercalcemia; SEE: hypocalcemia; SEE: osteoporosis