Calcium

How much you need

The official Recommended Dietary Allowance for adults, per day:

People aged 18 fall in the official 14–18 group, which has higher values — 1,300 mg a day, including in pregnancy and breastfeeding, with an upper limit of 3,000 mg. Pregnancy and breastfeeding change what you need — confirm yours with your midwife, obstetrician or doctor.

Upper limit

The upper limit is the most a day that is unlikely to cause harm, not a target. For adults it falls from 2,500 mg to 2,000 mg at age 51.

What it does

Calcium is the most abundant mineral in the body. Almost all of it — 98% — is stored in the bones, which act as a reservoir the body draws on to keep blood levels steady ^[1]. It makes up much of the structure of bones and teeth, and the small amount in the blood and tissues is needed for blood vessels to contract and relax, for muscles and nerves to work, for blood to clot and for hormones to be released ^[1]. Vitamin D is required for the gut to absorb calcium by active transport and to keep blood levels adequate ^[1].

Forms

Supplements usually contain calcium carbonate or calcium citrate ^[1]. Calcium carbonate is 40% calcium by weight and calcium citrate 21%, but the label lists the amount of calcium itself, so there is nothing to calculate ^[1]. Calcium citrate depends less on stomach acid and can be taken without food; calcium carbonate is absorbed less well by people with low stomach acid unless it is taken with a meal, and it causes more gas, bloating and constipation, especially in older adults ^[1]. Calcium carbonate is also the calcium in some antacids, such as Tums and Rolaids ^[1].

Where it comes from

1. Yogurt, plain, low fat8 ounces415 mg
2. Orange juice, calcium fortifiedfortified1 cup349 mg
3. Yogurt, fruit, low fat8 ounces344 mg
4. Mozzarella, part skim1.5 ounces333 mg
5. Sardines, canned in oil, with bones3 ounces325 mg

How these five were chosen: The five foods with the most calcium per serving in the NIH fact sheet's food table. Fortified foods are marked.

Getting the most from it

The more calcium taken in at once, the smaller the share absorbed: about 45% of an intake of 200 mg a day from food, but only 15% above 2,000 mg a day ^[1]. From supplements, absorption is highest at 500 mg or less at a time — the body absorbs about 36% of a 300 mg dose and 28% of a 1,000 mg dose — and is greater when they are taken with food ^[1]. Absorption also falls with age, to about 25% in adulthood and lower after that ^[1]. Some plants contain compounds that bind calcium: only 5% of the calcium in spinach is absorbed, against 27% from milk, while broccoli, kale and cabbage are absorbed about as well as milk, though they contain less ^[1]. Caffeine and phosphorus reduce absorption slightly, and low vitamin D status reduces it more ^[1].

What about vitamin D?

Calcium is often sold together with vitamin D ^[1]. The reason is real — vitamin D is needed for the gut to absorb calcium by active transport — and the Food and Drug Administration allows supplements that contain both to claim they reduce the risk of osteoporosis ^[1]. Whether the combination prevents fractures is less clear. A meta-analysis of eight trials in 30,970 adults over 50, who took 500 to 1,200 mg of calcium and 10 to 20 mcg (400 to 800 IU) of vitamin D a day, found 15% fewer fractures overall and 30% fewer hip fractures; two other reviews, one of 11 trials in 51,419 adults, found no effect on hip or total fractures ^[1]. The U.S. Preventive Services Task Force concluded with moderate certainty that daily amounts below 1,000 mg of calcium and 10 mcg (400 IU) of vitamin D do not prevent fractures in postmenopausal women, and that the evidence on larger amounts is inadequate ^[4]. The combination also brings a question of risk. In the Women's Health Initiative, 36,282 women aged 50 to 79 took 1,000 mg of calcium and 10 mcg of vitamin D a day or a placebo. Its link between supplemental calcium and kidney stones is the basis for the upper limit, although two later systematic reviews found no such link ^[1]. Over a median 22.3 years of follow-up, women given the supplements had 7% lower cancer mortality but a 6% higher risk of death from cardiovascular disease ^[3].

In the United States

Many Americans get less calcium than recommended: in national survey data from 2007–2010, 39% of everyone aged 4 and older took in less than the estimated average requirement from food and supplements ^[1]. Men aged 20 and older average 1,083 mg a day from food and drink and women 842 mg; about 22% of men and 32% of women take a supplement containing calcium, bringing average totals to 1,156 mg and 1,009 mg ^[1]. About 72% of the calcium Americans eat comes from dairy products and foods with dairy ingredients ^[1]. Inadequate intake is more common among non-Hispanic Black and non-Hispanic Asian Americans (47% to 48%) than among Hispanic (30%) and non-Hispanic White Americans (24%), and among adults 50 and older in low-income households ^[1].

Who is most likely to run low

• Postmenopausal women — falling estrogen reduces calcium absorption and increases losses from urine and bone, and women lose about 1% of their bone density a year after menopause; the recommended amount rises to 1,200 mg after 50 to lessen that loss ^[1]
• People who avoid dairy, including those with lactose intolerance or a milk allergy, and vegans; lactose-free dairy has the same calcium, but those who avoid dairy entirely typically need fortified foods or supplements to reach recommended amounts ^[1]

What deficiency looks like

Calcium deficiency reduces bone strength and can lead to osteoporosis, with fragile bones and a higher risk of falling ^[1]. Chronic deficiency can also cause osteomalacia — poorly mineralised, softened bone — although that is more often caused by lack of vitamin D ^[1]. Low blood calcium (hypocalcemia) usually results not from diet but from vitamin D or magnesium deficiency, parathyroid problems, critical illness or certain medicines; when it causes symptoms, the most common are numbness around the mouth, tingling in the hands and feet, and muscle spasms ^[1]. A blood test does not show calcium status, because blood levels are tightly controlled; a bone density scan reflects status over a lifetime ^[1].

What the evidence supports, and what it doesn't

Established. Bone maintenance. For adults, the official values were set mainly at the amount needed to maintain bone and keep calcium balance neutral ^[2], and the recommended amount for women rises from 1,000 mg to 1,200 mg after age 50 to lessen bone loss after menopause ^[1].

Not supported by trials. Weight loss: a meta-analysis of 41 trials in 4,802 adults found that calcium supplements had no effect on body weight or body fat, and that more calcium from dairy had none either except when combined with an energy-restricted diet ^[5]. Cancer prevention: most trials do not support it — the Women's Health Initiative found no effect on cancer incidence during the 7-year trial or over 22.3 years of follow-up, and a meta-analysis of 10 trials in 10,496 people who took at least 500 mg of calcium a day found no change in total cancer risk ^[1].

Mixed. Bone density and fractures in older adults: some trials and meta-analyses find that calcium, usually with vitamin D, increases bone density, but whether the increases matter clinically is not clear, and results for fractures conflict — see "What about vitamin D?" above ^[1]. The U.S. Preventive Services Task Force found the evidence inadequate to judge calcium for preventing fractures in men and premenopausal women ^[4]. Heart disease: some trials and cohort studies suggest calcium supplements raise cardiovascular risk and others find no link; an expert panel of the National Osteoporosis Foundation and the American Society for Preventive Cardiology judged that calcium from food or supplements neither increases nor decreases it, and that intakes within the upper limit are safe for the heart ^[6]. Pre-eclampsia: supplements in pregnancy might reduce the risk, but possibly only for women with low calcium intakes, and much of the evidence comes from studies with methodological weaknesses ^[1].

Where official guidance disagrees

Several bodies recommend calcium supplements in pregnancy, to reduce the risk of pre-eclampsia, for women whose intake is low. The World Health Organization recommends 1,500 to 2,000 mg a day for pregnant women with low dietary intakes, and the American College of Obstetricians and Gynecologists says that amount might reduce the severity of pre-eclampsia in women who take in less than 600 mg a day ^[1]. Canadian, international, and Australian and New Zealand groups make similar recommendations ^[1].

Too much

High blood and urine calcium are rare in healthy people and usually result from cancer, overactive parathyroid glands or other conditions, not diet ^[1]. They can cause poor muscle tone, kidney problems, constipation, nausea, weight loss, fatigue, frequent urination and heart rhythm problems ^[1]. Some studies suggest that high intakes may raise the risk of cardiovascular disease and of prostate cancer — for prostate cancer, possibly only calcium from dairy — but not all studies agree ^[1]. The evidence behind the upper limit itself, a link with kidney stones, is described under "What about vitamin D?" above ^[1].

Medicines that interact

Calcium supplements can substantially lower blood levels of the HIV drug dolutegravir (Tivicay, Dovato), and the drug's labels advise taking it 2 hours before or 6 hours after calcium. Calcium carbonate can interfere with the absorption of levothyroxine, a thyroid hormone, and its label advises not taking the two within 4 hours of each other. Calcium supplements reduce the absorption of quinolone antibiotics such as ciprofloxacin, which taking the antibiotic 2 hours before or after the calcium prevents. Long-term use of lithium can raise blood calcium, and calcium supplements could add to that risk ^[1]. If you take any of these regularly, talk to your doctor or pharmacist before taking a calcium supplement.