Calcium

Calcium is the most abundant mineral in your body, says the NIH (the US National Institutes of Health). About 99% of it sits in bones and teeth as calcium hydroxyapatite, a hard crystal of calcium and phosphate. An adult woman carries about 1,200 g and a man about 1,400 g.

The remaining 1% is dissolved in blood and cells. There it helps muscles and blood vessels work, nerves signal, blood clot and hormones release.

Your skeleton is also a calcium bank. The body keeps blood calcium in a narrow range, usually 8.8 to 10.4 mg/dL (milligrams per deciliter of blood). It does this by drawing on bone when intake falls, so a blood test does not show a shortfall. The cost builds up over years as lower bone mass.

In US survey data from 2007 to 2010, 39% of people aged 4 and older ate less than the estimated average requirement.

The surprises are about plants and pills. A boiled half cup of spinach lists 123 mg of calcium. Yet in one study only 5.1% of spinach calcium was absorbed, against 27.6% from milk. And calcium pills add only 0.7% to 1.8% to bone density, with no clear drop in fractures in healthy older adults living at home.

Health benefits of calcium

• ★★★Slightly higher bone density after age 50Extra calcium, from food or pills, raised bone density by only 0.7% to 1.8% in 59 trials of adults. The gain did not grow over time, so the authors say it is unlikely to cut fractures in a meaningful way.
• ★★★Slightly lower blood pressure from calcium pillsIn people with normal blood pressure, calcium pills lowered the top number a little (1.37 mm Hg systolic across 18 trials). Below 1,000 mg a day there was no clear drop.
• ★★No clear fracture drop in healthy older adultsIn adults over 50 living at home, calcium, vitamin D or both did not clearly cut hip fractures. In women averaging 84, calcium plus vitamin D cut hip fractures 43%.
• ★★Calcium plus vitamin D pills raised urinary stonesIn a large trial of women, 500 mg of calcium carbonate with 200 IU of vitamin D3 twice a day raised urinary stones by 17%. This was the combination, not calcium alone.
• ★★Mixed results on calcium pills and heart attacksA pooled analysis of calcium pills without vitamin D found 31% more heart attacks. A US expert panel found no link between calcium, with or without vitamin D, and heart disease in healthy adults.

Foods high in calcium

Per serving, canned sardines lead our USDA ranking with 351 mg in a 92 g can, 27% of the Daily Value (the FDA label amount). Next come semi-skimmed milk (295 mg per cup) and chia seeds (179 mg per ounce), then soybeans and navy beans.

Per 100 g, sesame seeds lead with 975 mg, but a tablespoon weighs only 9 g and holds 87.8 mg. Chia seeds, thyme, sardines, rosemary and almonds follow. Per 100 kcal, greens lead: bok choy (808 mg), basil, kale and arugula. The ranking below has the full list.

Dairy is the main source in the US: about 72% of calcium comes from dairy products and foods made with them, the NIH says. It also lists foods that are not in our ranking. Plain low-fat yogurt gives 415 mg per 8 oz, part-skim mozzarella 333 mg in 1.5 oz and calcium-fortified orange juice 349 mg per cup. Fortified soymilk gives 299 mg, and firm tofu made with calcium sulfate 253 mg per half cup.

Dairy and fortified foods are absorbed at about 30%. So check absorption, not just the number. Boiled spinach lists 123 mg per half cup, but only 5.1% of it was absorbed in one study.

Oxalate and phytate, plant compounds that bind calcium, explain this. The NIH names spinach, collard greens, sweet potatoes, rhubarb and beans as high-oxalate foods. The rhubarb in our ranking (105 mg per cup) is one of them, which its number does not show. Low-oxalate greens such as kale, broccoli and bok choy are absorbed about as well as milk, but a serving holds much less calcium.

Hard water can also make a significant contribution, EFSA says.

How calcium works

Most of this is established physiology rather than trials of extra calcium. The details come from reviews, cell work and small absorption studies. Four ideas cover it.

Calcium is the structure of bone and teeth, and a small dissolved pool does the signaling. The body holds blood calcium steady with hormones and bone. And absorption depends on dose, age, vitamin D and what else is in the food. Here is each one, and what has been seen in people.

It builds bones and teeth, and bone is the bank

About 99% of body calcium is calcium hydroxyapatite in bones and teeth, according to EFSA. Teeth keep their calcium, but bone is rebuilt all the time, with calcium constantly leaving and entering, as the NIH describes. That remodeling lets the skeleton grow, repair damage and act as a reservoir for blood calcium.

After menopause, women lose about 1% of their bone mineral density a year on average, the NIH says. That is why the US target rises to 1,200 mg for women over 50.

A small dissolved pool does the signaling

The ionized calcium in blood and cells is a messenger. The NIH lists blood vessel contraction and dilation, muscle function, blood clotting, nerve transmission and hormone release. EFSA calls it an essential messenger inside cells. Cells sense it with a receptor on their surface.

Parathyroid cells carry one, which was first cloned from cattle tissue in 1993. This is the pool the body protects first, even at the cost of bone.

The body holds blood calcium steady

When blood calcium dips, the parathyroid glands sense it and release parathyroid hormone (PTH). PTH draws calcium out of bone and makes the kidneys keep calcium and turn vitamin D into its active form. That active vitamin D makes the gut absorb more calcium, according to the Linus Pauling Institute. When blood calcium is high, the thyroid gland releases calcitonin, which pushes the other way.

Low magnesium can also blunt PTH release, and magnesium is covered on its own page. The result is that blood calcium barely reflects what you eat. EFSA says serum calcium is held in a narrow range whatever the intake, so it is not a useful marker of status.

Absorption depends on dose, age, vitamin D and food compounds

Calcium crosses the gut by active transport, which dominates at low intakes, and by passive diffusion, which takes over as intake rises, the NIH says. So the share absorbed falls as the amount rises: from about 45% at 200 mg a day to 15% above 2,000 mg. It is about 60% in infants and young children and about 25% in adults, and it keeps declining with age. A caffeine habit and phosphorus lower net absorption a little, and phosphorus is covered on its own page.

Vitamin D is needed for active absorption, but how much it adds is argued. In two experiments a year apart, absorption was 65% higher at a blood 25-hydroxyvitamin D of 86.5 nmol/L than at 50 nmol/L.

A randomized trial of 163 older women with low vitamin D tested a 100 mg dose. Its absorption rose only from 52% to 58% across a 25-hydroxyvitamin D range of 20 to 66 ng/ml. The authors say vitamin D matters most when levels are very low (under 10 ng/ml).

Plant compounds matter too. Oxalate and phytate bind calcium into salts the gut cannot take up. Spinach is the classic case, and kale, with little oxalate, is absorbed as well as milk. See oxalic acid.

What has been seen in people

The strongest human findings are modest. Raising calcium intake lifts bone density by 0.7% to 1.8% in adults over 50. That has not translated into clear fracture protection in healthy people at home. Calcium with vitamin D did cut hip fractures in one trial of women aged about 84.

Blood pressure falls by about 1.4 mmHg in people with normal pressure. For pre-eclampsia, the newest Cochrane update finds little to no difference. For colorectal adenomas, one trial found a drop and a later one did not. Calcium from food goes with fewer kidney stones in cohorts, while supplements have gone with more.

All of these are among the study cards above.

How much calcium do you need?

Adults need 1,000 mg of calcium a day in the US. The target rises to 1,200 mg for women over 50 and for everyone over 70. Children and teenagers aged 9 to 18 need 1,300 mg. EFSA’s population reference intake, which covers nearly everyone, is 950 mg from age 25 and 1,000 mg for ages 18 to 24.

Pregnancy and breastfeeding do not raise the target. The US keeps the age-based values: 1,300 mg for ages 14 to 18 and 1,000 mg for ages 19 to 50. EFSA says the value for non-pregnant women applies to pregnant and breastfeeding women of the same age, because calcium metabolism adapts.

Many people fall short. The NIH reports average intakes from food and drink of 1,083 mg for men and 842 mg for women aged 20 and older. In 2007 to 2010, 39% of people aged 4 and older ate below the estimated average requirement.

Labels use one value for everyone, as shown in the key facts above. That is a Daily Value of 1,300 mg in the US and an EU Reference Intake of 800 mg.

Calcium deficiency: signs and who is at risk

Low calcium intake rarely causes symptoms at first, because the body draws on bone to keep blood calcium steady. Over years, it leads to lower bone mass, osteopenia, osteoporosis and a higher fracture risk, according to EFSA. Rickets in children and osteomalacia (soft bones) in adults can also occur, though the NIH says vitamin D deficiency is the more common cause.

Low blood calcium (hypocalcemia, below 8.5 mg/dL) usually has another cause. Examples are vitamin D or magnesium deficiency, an underactive parathyroid, critical illness, or medicines such as bisphosphonates, cisplatin and proton pump inhibitors. It brings numbness around the mouth, tingling in the hands and feet and muscle spasms. In severe cases it brings seizures, so it needs a doctor.

• Who is at risk. Postmenopausal women, who lose about 1% of bone mineral density a year, are at risk. So are people who avoid dairy, including vegans and people with lactose intolerance or milk allergy. They typically need fortified foods or supplements to reach the target, the NIH says. In US data from 2009 to 2012, 47% to 48% of non-Hispanic Black and Asian people ate below the estimated average requirement. Among non-Hispanic White people it was 24%.
• Testing. A blood test does not reflect calcium intake or status, because blood levels are so tightly controlled. A DXA scan (dual x-ray) of bone density shows lifetime calcium status, the NIH says. Talk to a doctor about bone density testing rather than guessing from diet.

Forms of calcium: absorption and effects

Calcium supplements and fortified foods use different calcium salts. They differ in how much actual (elemental) calcium each tablet holds, whether they need stomach acid to dissolve, and side effects. The NIH lists calcium carbonate as 40% calcium by weight and calcium citrate as 21%. So a tablet with 1,000 mg of carbonate holds about 400 mg of calcium.

The Supplement Facts panel lists the elemental amount, which is the number to use.

The honest bottom line is that nearly all the head-to-head work compares how much calcium reaches the blood, not bone density or fractures. With a meal, carbonate and citrate were absorbed about equally in the best-controlled trial. Citrate does better on an empty stomach, with little stomach acid and after weight-loss surgery.

Absorption also falls as the dose rises, which is why supplements are split into doses of 500 mg or less. We found no trial that tested whether one form protects bone better, so such claims have nothing behind them.

Form	Share of calcium by weight	What studies show	Source
Calcium carbonate	40% (NIH)	Needs stomach acid to dissolve, so it is best with food. Equal to citrate with a light breakfast in 37 adults, and 4.2% absorbed against 45.2% for citrate in 11 fasting people with no stomach acid. It is the form used in the Women’s Health Initiative and both adenoma trials.	1, 4, 5, 15, 16, 17
Calcium citrate	21% (NIH)	Depends less on stomach acid and can be taken without food. A meta-analysis found 21.6% better absorption than carbonate with meals, but a head-to-head trial with a meal found no difference. Better absorbed after weight-loss surgery (150 people).	1, 3, 4, 6, 7
Calcium citrate malate	Not stated	Used in some fortified juices (NIH). In orange juice it gave 148 mg of absorbed calcium from 500 mg, against 100 mg for tricalcium phosphate with calcium lactate, in 25 women.	1, 8
Tricalcium phosphate	Not stated	Used with vitamin D3 in the French hip fracture trial (1.2 g of elemental calcium). In soymilk it was absorbed less (0.181) than carbonate (0.211) in 20 women.	14, 9
Calcium lactate and calcium gluconate	Not stated	Listed by the NIH as supplement forms. A review calls them less concentrated and not practical as oral supplements. We found no human trial comparing their absorption with carbonate.	1, 2
Calcium sulfate and hydroxyapatite	Not stated	Listed by the NIH. Calcium sulfate is a salt used to set some tofu: half a cup of firm tofu made with it holds 253 mg. A review says research on hydroxyapatite is limited and does not recommend it.	1, 2
Milk and dairy	Not applicable	About 30% absorbed (NIH). In 13 adults 27.6% of 200 mg of calcium was absorbed from milk, and in 11 women 32.1% of 300 mg.	1, 10, 11
Kale and other low-oxalate greens	Not applicable	40.9% of the calcium in kale was absorbed in 11 women. The NIH says broccoli, kale and cabbage are absorbed about as well as milk.	1, 11
Spinach and other high-oxalate foods	Not applicable	5.1% of the calcium in spinach was absorbed in 13 adults.	10
Calcium in mineral water	Not applicable	Absorbed as well as milk in two crossover trials (9 and 21 people), including a water with 467 mg of calcium per liter.	12, 13

The outcome trials above used these forms. Calcium carbonate was used in the Women’s Health Initiative and in both adenoma trials, and tricalcium phosphate in the French hip fracture trial. So most of what we know about fractures, stones and heart events comes from carbonate, often with vitamin D. We found no trial that compared forms for those outcomes.

The NIH says calcium carbonate appears to cause more gas, bloating and constipation than citrate, especially in older adults with low stomach acid. It suggests switching form, taking smaller doses more often, or taking it with meals. We found no trial that compared constipation between forms, so this point rests on the NIH summary. Milk and mineral water give calcium without tablets.

Is calcium safe? Supplements, dose and interactions

Calcium from food is not a concern for most people. The US tolerable upper intake level (UL), the most a day unlikely to cause harm, counts food and supplements together. It is 2,500 mg a day for ages 19 to 50, 2,000 mg from age 51, and 3,000 mg for ages 9 to 18.

The NIH says these limits rest on Women’s Health Initiative evidence. It showed more kidney stones at 1,000 mg a day of supplemental calcium for 7 years, though two later reviews did not find that link.

EFSA keeps an upper level of 2,500 mg a day from all sources for adults, including pregnant and breastfeeding women. It says there were not enough data to set a level for infants, children or adolescents. EFSA also notes that high blood calcium (hypercalcemia) is unlikely from food alone, but can follow high-dose supplements, especially with vitamin D.

• Doses and side effects. Supplements typically hold 500 or 600 mg, and multivitamins 200 to 300 mg. Gas, bloating and constipation can occur, and the NIH says carbonate appears to cause more than citrate. High blood calcium is rare in healthy people and usually comes from cancer or an overactive parathyroid. It can cause constipation, nausea, tiredness and abnormal heart rhythm.
• Kidney stones. Calcium from food went with fewer stones in large cohorts and a diet trial, while supplements went with more. Calcium with vitamin D gave 17% more urinary stones in the Women’s Health Initiative, and supplements went with 20% more in the Nurses’ Health Study. The USPSTF also found adequate evidence that calcium with vitamin D increases stones. Ask a doctor first if you have had kidney stones.
• Heart attacks. It is unsettled. Two analyses from Auckland linked calcium pills to 24% to 31% more heart attacks. A US expert panel found no relationship, and cohorts split by sex. The NIH calls the findings mixed and says more research is needed. Calcium from food has not been linked to harm. Some cohorts link high calcium from dairy to prostate cancer, but not supplements, the NIH says.
• Milk-alkali syndrome. Very large amounts of calcium with absorbable alkali, such as antacids, can cause high blood calcium, kidney failure and metabolic alkalosis. A review says it has returned with the wide use of calcium carbonate for osteoporosis. A case report describes it in a woman taking calcium carbonate, vitamin D and a thiazide diuretic (a water pill). So people on thiazides should ask a doctor.
• Thyroid, antibiotic and HIV medicines. In 20 people on levothyroxine, 1,200 mg a day of calcium carbonate taken with it lowered free T4. It also raised thyroid-stimulating hormone from 1.6 to 2.7 mIU/L. The FDA label says to avoid levothyroxine within 4 hours of calcium carbonate, the NIH reports. Taking the quinolone antibiotic gemifloxacin with calcium carbonate cut its exposure by 21% in 16 volunteers. The NIH advises taking quinolones 2 hours before or after calcium. Calcium carbonate cut exposure to the HIV drug dolutegravir by 39% when fasting, but not with a meal. The NIH also warns that lithium plus calcium can raise blood calcium.
• Bisphosphonates, tetracyclines and iron. Alendronate is barely absorbed even when taken correctly (0.76% on an empty stomach). Taking it with breakfast cut absorption by more than 85%, so it is taken alone with water. We found no trial of calcium with alendronate or with tetracycline antibiotics, so ask a pharmacist about spacing. In single-meal tests with 126 people, 300 to 600 mg of calcium cut iron absorption by 50% to 60%, heme iron included.

How to get more calcium

• Food first. A cup of milk gives 276 to 299 mg and 3 oz of canned sardines with bones 325 mg, the NIH says. Half a cup of firm tofu made with calcium sulfate gives 253 mg. Dairy and fortified foods are absorbed at about 30%. Kale and bok choy are absorbed as well as milk, while spinach is a poor source.
• Split the dose and match the form. Absorption is highest with doses of 500 mg or less, and 28.4% of a 1,000 mg dose was absorbed against 36.0% of 300 mg. Take carbonate with a meal. Citrate works any time and is the usual choice with acid-lowering medicines or low stomach acid, a review says.
• Add up the gap before buying a pill. US adults average 1,083 mg (men) and 842 mg (women) from food and drink. So count your own intake and fill only the shortfall. Vitamin D helps most when levels are very low. In 163 older women, raising it from 20 to 66 ng/ml added only 6 mg to the calcium absorbed from a 100 mg dose.

History of calcium

The word calcium comes from the Latin calx, meaning lime, the material made by heating limestone. Humphry Davy first isolated the metal in 1808. He ran an electric current through a moist mixture of lime and mercury oxide, which produced an amalgam, and distilled off the mercury.

Calcium had a medical story before it was a supplement. In the early 20th century, doctors treated peptic ulcers with milk and alkali, and some patients developed high blood calcium, kidney failure and alkalosis. This became known as milk-alkali syndrome. It became rare after acid-lowering drugs arrived and has returned with the wide use of calcium carbonate for osteoporosis.

Advice has swung back and forth since. A 1992 French trial in women aged about 84 found 43% fewer hip fractures with calcium and vitamin D. The Institute of Medicine set the current US intakes and upper levels in 2011.

The 2006 Women’s Health Initiative trial found no clear drop in hip fractures in healthy postmenopausal women. Meta-analyses in 2010 and 2011 raised heart concerns, and in 2018 the USPSTF advised against low-dose supplements for healthy postmenopausal women. In 2025 the Cochrane review of pregnancy trials set aside trials it could not trust and found little to no difference in pre-eclampsia.

Frequently asked questions about calcium

Do calcium supplements prevent fractures?

Not clearly, in healthy adults living at home. A 2017 meta-analysis of 33 trials with 51,145 older adults found no clear drop in hip fractures with calcium, vitamin D or both. A 2015 review found total fractures 11% lower with calcium supplements, but not in the four trials at lowest risk of bias. The USPSTF recommends against 1,000 mg or less of calcium with 400 IU or less of vitamin D for healthy postmenopausal women. One trial in women aged about 84 did find 43% fewer hip fractures with calcium and vitamin D.

Do calcium supplements cause heart attacks?

It is unsettled: a 2010 meta-analysis found 31% more heart attacks with calcium supplements in patient-level data, and a 2011 reanalysis 24% more. In 2016 a US expert panel judged, from moderate-quality evidence, that calcium has no relationship to heart disease in healthy adults. The Nurses’ Health Study found no excess in women. In a large cohort, men taking over 1,000 mg of supplemental calcium had 20% more cardiovascular deaths. Calcium from food has not been linked to harm. If you need a supplement, ask your doctor and use the lowest dose that closes your gap.

How much calcium should women over 50 take?

The US target is 1,200 mg a day from food and supplements together, with an upper level of 2,000 mg for ages 51 and over. Men aged 51 to 70 stay at 1,000 mg, and everyone over 70 needs 1,200 mg. EFSA sets 950 mg for all adults aged 25 and older. Women lose about 1% of bone mineral density a year after menopause, the NIH says, which is why the target is higher.

Can you get enough calcium without dairy?

Yes, with planning. Firm tofu made with calcium sulfate gives 253 mg per half cup. Fortified soymilk (299 mg per cup) and fortified orange juice (349 mg per cup) help too. So do canned sardines or salmon with bones, kale and bok choy. But the NIH says people who avoid dairy typically need fortified foods or supplements to reach the target. Spinach is a poor source (5.1% absorbed), and in 20 women, soymilk fortified with calcium carbonate was absorbed as well as cow’s milk.

Which foods are highest in calcium?

In our USDA ranking, canned sardines lead per serving (351 mg in a 92 g can). Semi-skimmed milk (295 mg per cup) and chia seeds (179 mg per ounce) follow, then soybeans. Per 100 g, sesame seeds lead with 975 mg. Not in our ranking, the NIH adds plain low-fat yogurt (415 mg per 8 oz), part-skim mozzarella (333 mg in 1.5 oz) and calcium-set tofu.

Is calcium carbonate or calcium citrate better?

With a meal, about the same. In 37 adults eating a light breakfast, absorption did not differ, though a meta-analysis found citrate 21.6% better with meals and 27.2% better fasting. Citrate is the better choice with little stomach acid: in 11 fasting people without it, 45.2% was absorbed from citrate and 4.2% from carbonate. Carbonate is 40% calcium by weight and citrate 21%, so carbonate needs fewer pills. But the NIH says it seems to cause more gas and constipation.

What is the best time to take calcium?

Take carbonate with a meal, citrate at any time, and keep each dose to 500 mg or less. Space calcium away from levothyroxine: the label says 4 hours from calcium carbonate, the NIH reports. Space it from quinolone antibiotics too, taking the antibiotic 2 hours before or after. Calcium also cut iron absorption by 50% to 60% in single meals, so take iron at a different time of day.

What is a calcium score test?

A coronary artery calcium score is a CT scan of calcium deposits in the heart artery walls, not of how much calcium you eat. In a study of 5,448 adults, a higher total intake went with a lower risk of new deposits over 10 years. Calcium supplement use went with a higher risk. Ask a doctor whether a scan is right for you.