Calcium: Benefits, Dosage, and Safety

What calcium is

Calcium is an essential dietary mineral and the most abundant mineral in the body. About 98 to 99 percent is stored in the bones and teeth as calcium hydroxyapatite, which the body uses as a reservoir to keep blood calcium tightly controlled. It is found naturally in some foods, added to others, present in some medicines such as antacids, and available as a dietary supplement (commonly calcium carbonate or calcium citrate). Vitamin D is required for calcium to be absorbed in the gut by active transport.

What it does

• Makes up much of the structure of bones and teeth and keeps tissue rigid, strong, and flexible
• Mediates blood vessel contraction and dilation
• Supports normal muscle function
• Is required for normal blood clotting
• Supports nerve transmission
• Supports hormonal secretion

How much calcium per day

These are population reference values from the NIH and the Dietary Reference Intakes. They are targets for healthy people, not personal prescriptions. The last column is the Tolerable Upper Intake Level, the most you should get from all sources combined.

Deficiency and who is at risk

Chronic inadequate intake reduces bone strength and can contribute to osteoporosis (fragile bones and a higher risk of falls and fractures). Long-term deficiency can cause rickets in children and osteomalacia (defective bone mineralization and softening) in children and adults, though these are more commonly caused by vitamin D deficiency. Low blood calcium (hypocalcemia, serum below 8.5 mg/dL or ionized below 4.61 mg/dL) is usually driven by vitamin D or magnesium deficiency, parathyroid problems, or certain medications rather than by low dietary intake; when symptomatic it most often causes increased neuromuscular irritability such as perioral numbness, tingling in the hands and feet, and muscle spasms.

Who may need more

• Postmenopausal women, whose RDA rises to 1,200 mg to lessen bone loss
• People with lactose intolerance or milk allergy who limit dairy
• Vegans and others who avoid dairy products
• People whose usual intake falls below the recommended amount from food alone

Food sources

Milk, yogurt, and cheese are the richest sources (about 72 percent of US calcium intake comes from dairy). Examples: plain low-fat yogurt (415 mg per 8 oz), calcium-fortified orange juice (349 mg per cup), part-skim mozzarella (333 mg per 1.5 oz), canned sardines with bones (325 mg per 3 oz), nonfat milk and calcium-fortified soymilk (299 mg per cup), firm tofu made with calcium sulfate (253 mg per half cup), and canned pink salmon with bones (181 mg per 3 oz). Nondairy sources include kale, broccoli, and Chinese cabbage (bok choi); absorption from these low-oxalate greens is similar to milk though the amount per serving is lower. Absorption from spinach and other high-oxalate foods is low (about 5 percent).

Forms and absorption

The two most common supplement forms are calcium carbonate (40 percent elemental calcium by weight, best absorbed when taken with a meal, more likely to cause gas or constipation, and dependent on stomach acid) and calcium citrate (21 percent elemental calcium by weight, absorbed with or without food and less dependent on stomach acid, better for people with low stomach acid). Other forms include calcium sulfate, ascorbate, microcrystalline hydroxyapatite, gluconate, lactate, and phosphate; calcium citrate malate is well absorbed and used in some fortified juices. Absorption is highest at single doses of 500 mg or less (about 36 percent of a 300 mg dose versus 28 percent of a 1,000 mg dose). Supplement labels list elemental calcium.

Safety, excess and interactions

Too much

Very high intake can raise the risk of kidney stones (linked in the Women's Health Initiative to 1,000 mg/day of supplemental calcium over 7 years, though later reviews did not confirm this). Some research suggests high supplemental calcium may increase the risk of cardiovascular disease and, from high dairy intake, prostate cancer, but not all studies agree. Hypercalcemia (serum above 10.5 mg/dL) and hypercalciuria are rare in healthy people and usually result from cancer or primary hyperparathyroidism rather than diet; they can cause poor muscle tone, renal insufficiency, hypophosphatemia, constipation, nausea, weight loss, fatigue, polyuria, and heart arrhythmias. Calcium supplements may also cause gas, bloating, and constipation, more so with calcium carbonate than calcium citrate.

Interactions to know

• Dolutegravir (HIV integrase inhibitor): calcium supplements can substantially reduce blood levels through chelation; take dolutegravir 2 hours before or 6 hours after calcium
• Levothyroxine (thyroid hormone): calcium carbonate can interfere with absorption; separate doses by at least 4 hours
• Lithium (bipolar disorder): long-term use can cause hypercalcemia, and combining with calcium supplements may increase that risk
• Quinolone antibiotics (ciprofloxacin, gemifloxacin, moxifloxacin): calcium reduces antibiotic absorption; take the antibiotic 2 hours before or 2 hours after calcium supplements

How status is measured

Total serum or plasma calcium (normal about 8.8 to 10.4 mg/dL) and ionized (free) calcium (normal about 4.6 to 5.3 mg/dL) can be measured, but because of tight homeostatic control these blood levels do not reflect nutritional status. Cumulative lifetime calcium status is best assessed indirectly by dual x-ray absorptiometry (DXA) measurement of bone mineral density.

Regulated as a dietary supplement ingredient under DSHEA; not FDA-approved to treat disease. The Daily Value (DV) for calcium on Nutrition and Supplement Facts labels is 1,300 mg for adults and children age 4 and older. The FDA has approved a health claim that adequate calcium and vitamin D may reduce the risk of osteoporosis.

What the evidence supports

We grade the strength of evidence behind each common reason people take calcium, mirroring how the NIH describes it.