Calcium

Calcium is the most abundant mineral in the human body, with approximately 99% stored in bones and teeth as hydroxyapatite, providing structural rigidity and serving as a reservoir for maintaining serum calcium concentrations. The remaining 1% circulates in the blood and soft tissues, where it plays essential roles in muscle contraction (including cardiac muscle), nerve signal transmission, blood clotting, hormone secretion, and intracellular signaling.¹

The body tightly regulates serum calcium within a narrow range through the actions of parathyroid hormone (PTH), calcitriol (active vitamin D), and calcitonin. When dietary calcium intake is insufficient to maintain serum concentrations, PTH stimulates the release of calcium from bone, reducing bone mineral density over time — this is the mechanism by which chronic calcium inadequacy contributes to osteoporosis.

Calcium supplements are available in multiple salt forms. The two most common are calcium carbonate (the most concentrated form at approximately 40% elemental calcium; requires stomach acid for optimal absorption; should be taken with food) and calcium citrate (approximately 21% elemental calcium; does not require stomach acid; can be taken with or without food; generally better tolerated by individuals with reduced stomach acid, those on proton pump inhibitors, and older adults).¹

Calcium supplementation is used for prevention and treatment of calcium deficiency, prevention of osteoporosis and reduction of bone loss, and management of conditions associated with hypocalcemia. The NIH ODS notes that calcium is also used in combination with vitamin D for prevention of postmenopausal bone loss, and that adequate calcium intake throughout life is essential to maximizing peak bone mass and slowing age-related bone loss.¹

For cardiovascular disease and cancer prevention, large calcium supplementation trials have produced mixed or null results. The Women's Health Initiative (WHI) calcium/vitamin D trial found no significant effect of calcium supplementation (1,000 mg daily) plus vitamin D on all-cause mortality, total cancer, or overall cardiovascular outcomes, though a trend toward increased risk of kidney stones was observed. Importantly, the same trial found no increase in cardiovascular events with calcium supplementation in the primary analysis; however, some subsequent meta-analyses of observational and trial data have raised questions about potential cardiovascular risk from calcium supplementation above habitual dietary intake, particularly in older adults. This is an area of ongoing discussion in the clinical literature, and the NIH ODS does not endorse a definitive conclusion on this point.¹

Recommended Dietary Allowances (RDA) for calcium from all sources (diet plus supplements):¹

• Adults 19–50 years: 1,000 mg/day
• Males 51–70 years: 1,000 mg/day; Females 51–70 years: 1,200 mg/day
• Adults 71 years and older: 1,200 mg/day
• Pregnant and lactating individuals (18 years and older): 1,000 mg/day

The Tolerable Upper Intake Level (UL) for adults 19–50 is 2,500 mg/day; for adults 51 and older, 2,000 mg/day. Exceeding the UL increases risk of adverse effects including hypercalcemia, kidney stones, and potential interference with absorption of other minerals.

For supplementation, calcium is best absorbed when taken in doses of 500 mg or less at one time; larger doses should be split across multiple servings. Total calcium intake (diet plus supplements combined) should be used when assessing adequacy relative to the RDA and UL — many individuals with good dietary calcium intake do not need supplementation.

Oral, in tablets, capsules, chewables, and liquids.

Standard storage at room temperature, away from heat and moisture.

Several important considerations apply to calcium supplementation:¹

• Kidney stones: Calcium supplementation (particularly at higher doses) has been associated with increased risk of kidney stones in susceptible individuals. Notably, adequate dietary calcium actually reduces kidney stone risk by binding oxalate in the gut; it is supplemental calcium — especially taken apart from meals — that raises stone risk in predisposed individuals.
• Drug interactions: Calcium impairs absorption of several medications when taken concurrently, including levothyroxine (thyroid hormone), bisphosphonates (osteoporosis drugs), fluoroquinolone and tetracycline antibiotics, and iron supplements. These should be taken at least 2–4 hours apart from calcium.
• Cardiovascular risk uncertainty: Some meta-analyses suggest that calcium supplementation (particularly calcium-only, not food-form calcium) may modestly increase cardiovascular risk in older adults. This remains a subject of debate; individuals with cardiovascular risk factors should discuss calcium supplementation with their clinician.
• Gastrointestinal effects: Calcium carbonate commonly causes constipation and gas. Calcium citrate is generally better tolerated.
• Vitamin D: Calcium absorption depends on vitamin D; supplementing calcium in the presence of significant vitamin D deficiency may be less effective.