Calcium

What does calcium do?

The NIH describes calcium as the most abundant mineral in the body. It forms most of the structure of bones and teeth; about 98% of calcium is in bone, and bone is used like a reservoir to maintain the level in the blood. The small, ionized circulating pool plays a role in the contraction and relaxation of blood vessels, muscle function, blood clotting, nerve conduction and hormone secretion. ^[1]

Absorption from the intestine occurs by both active transport and passive diffusion. When intake is low, most absorption is by active transport, and as intake rises it increasingly occurs by passive diffusion. Vitamin D is needed for active transport; if vitamin D is low, calcium absorption also falls. ^[1]

That a mineral is needed in the body does not mean that taking it above the requirement will perform the same function better. Inadequate intake can lead to weak bones, rickets in children and osteomalacia in children and adults; however, the NIH also states that rickets and osteomalacia are mostly caused by vitamin D deficiency. ^[1]

Detailed guideVitamin D: the vitamin that regulates calcium absorption→

How much calcium is needed per day?

The organizations that give reference values do not reach the same conclusion. The US Food and Nutrition Board values (RDA) reported by the NIH are based on the criterion of bone maintenance and neutral calcium balance in adults:

According to the NIH, 200 mg for ages 0–6 months and 260 mg for ages 7–12 months are given as adequate intake (AI); in pregnancy and breastfeeding it is 1,300 mg at ages 14–18 and 1,000 mg at ages 19–50. The values in the table cover total calcium from foods and supplements. ^[1]

EFSA, for its part, calculated an average requirement (750 mg/day) from calcium balance data and set the population reference intake (PRI) for adults over 25 at 950 mg/day; the PRI for ages 18–24 is 1,000 mg, and for pregnant and breastfeeding women the same value applies as for non-pregnant women of the same age group. EFSA decided that bone health measures (bone density, fracture) could not be used to derive a calcium reference, and that cardiovascular and cancer outcomes were also not useful for this purpose. ^[2]

Both organizations use calcium balance data; the NIH also cites bone maintenance as a criterion, whereas EFSA did not use bone outcomes to derive a reference. Both values are planning references for healthy people; they are not a personal treatment target or the treatment amount for diagnosed osteoporosis.

Detailed guideWhich foods contain calcium? Serving table and absorption→

Should calcium come from food or supplements?

Citing the Dietary Guidelines for Americans, the NIH reports that nutrient requirements should be met primarily through food, including fortified foods, and that supplements can be useful where the requirement cannot be met by food. ^[1]

Absorption varies with the food source. According to the NIH, absorption from dairy products and fortified foods is about 30%; in plants containing oxalic acid, such as spinach, calcium absorption falls to 5%, versus 27% in milk. In vegetables that do not contain oxalate or phytate, such as broccoli, kale (curly-leaf cabbage) and cabbage, bioavailability is similar to milk, but the amount of calcium per serving is lower. In a pattern in which a variety of foods is eaten, these interactions are expected to have very little nutritional consequence. ^[1]

Among groups more likely to have inadequate intake, the NIH lists postmenopausal women (because of accelerated bone loss after menopause) and those who avoid dairy products. In people with lactose intolerance, lactose-free or lactose-reduced dairy products contain the same amount of calcium as regular products. ^[1]

If a supplement is taken, which form and how much is absorbed?

The two most common forms in supplements are calcium carbonate and calcium citrate. Carbonate contains 40% and citrate 21% elemental calcium by weight; because the “Supplement Facts” section on the label reports elemental calcium, there is no need to calculate the weight of the compound. ^[1]

In people with low stomach acid, the solubility of carbonate falls and its absorption may be lower when taken without a meal; citrate is less dependent on stomach acid. The NIH states that, in general, supplement absorption is better when taken with food, whether or not stomach acid is low. ^[1]

The percentage absorbed falls as the amount of elemental calcium taken at one time rises; the NIH reports that absorption is highest at doses of 500 mg and below, and as an example that about 36% of a 300 mg dose and about 28% of a 1,000 mg dose are absorbed. Digestive complaints such as gas, bloating and constipation are reported more often with carbonate than with citrate. ^[1]

Some antacid chewable tablets contain calcium carbonate and provide about 270–400 mg of calcium per tablet; when used together with a separate supplement, total intake can therefore rise without being noticed. ^[1]

Does calcium supplementation prevent fractures?

This question is answered differently depending on who is examined.

The Cochrane review (53 studies, 91,791 participants; older adults living in the community, in care homes or in hospital) reported that vitamin D alone is unlikely to prevent hip fracture (RR 1.12; 95% CI 0.98–1.29) or any fracture (RR 1.03). In nine studies (49,853 participants) in which vitamin D and calcium were used together, a small reduction in hip fracture risk was seen (RR 0.84; 95% CI 0.74–0.96); for any fracture RR was 0.95 (0.90–0.99). The authors calculated the absolute difference separately for the two settings: about one fewer hip fracture per 1,000 older adults per year among community dwellers, and nine fewer per 1,000 people per year in institutionalized (high-risk) older adults. ^[4]

The 2018 USPSTF recommendation examines a very different group: adults who live in the community (not in care homes), with no known bone metabolism disease, osteoporosis or vitamin D deficiency, and no history of fracture. In this group, adequate evidence was found that calcium at 1,000 mg or less and vitamin D at 400 IU or less has no benefit for primary fracture prevention in postmenopausal women; evidence was considered insufficient for higher doses and for men and premenopausal women. The recommendation does not apply to people with a history of fracture, fall risk, osteoporosis or vitamin D deficiency. ^[5]

Bone density is a separate intermediate outcome. Several meta-analyses reported by the NIH have shown that calcium, or calcium with vitamin D, increased bone density in older adults; however, in the subgroup results of one analysis in postmenopausal women, calcium alone had no effect on femoral neck density, and the NIH states that it is not clear whether these increases are clinically meaningful. ^[1]

Do supplements increase cardiovascular risk?

The 2010 BMJ meta-analysis by Bolland and colleagues pooled randomized trials in which calcium supplementation of at least 500 mg/day was compared with placebo, with a mean age above 40, 100 or more participants and a duration longer than a year. This review covered 15 trials: patient-level data came from five trials (8,151 participants, median follow-up 3.6 years), and heart attack risk was higher in the calcium group than with placebo (HR 1.31; 95% CI 1.02–1.67). The increases in stroke, the composite outcome of heart attack-stroke-sudden death, and death were not statistically significant. A similar finding was obtained with trial-level data (11 trials, 11,921 participants) (RR 1.27; 1.01–1.59). The result was reported for calcium supplements given without vitamin D. ^[3]

In later years the picture did not become clear. The NIH reports that some cohort studies and randomized trials showed increased cardiovascular risk with supplements, while some found no association. For example, a meta-analysis of 14 studies in 28,935 healthy postmenopausal women reported a 15% increase in cardiovascular risk and a 16% increase in coronary heart disease; in the 22.3-year long-term follow-up of the Women’s Health Initiative, no effect was found on cardiovascular disease risk, but cardiovascular death was 6% higher in the calcium and vitamin D group. However, an expert panel of the National Osteoporosis Foundation and the American Society for Preventive Cardiology concluded, based on moderate-quality evidence, that in healthy adults calcium from food and supplements (provided the upper limit is not exceeded) neither increases nor decreases cardiovascular disease risk. ^[1]

To say that “supplements harm the heart” or that they are “definitely harmless” would therefore be more certain than the sources support. Calcium from food is a separate question: the NIH reports that in large observational studies, low calcium intake was associated with higher risk of hypertension, stroke and atherosclerosis. ^[1]

Does calcium cause kidney stones?

Calcium from food and supplements need to be evaluated separately. In the Nurses’ Health Study (91,731 women aged 34–59 with no history of stones, 12 years of follow-up, 864 stone cases), the risk was lower in the fifth with the highest dietary calcium intake than in those with the lowest (RR 0.65; 95% CI 0.50–0.83). In supplement users, the risk was higher than in nonusers (RR 1.20; 1.02–1.41). In 67% of the women using supplements, calcium was either not taken with meals or was taken with low-oxalate meals; the authors state that dietary calcium reduces oxalate absorption and that timing may explain this difference, and that other factors in dairy products may also play a role. This is an observational study and does not show causality. ^[6]

The USPSTF reports that it found adequate evidence that vitamin D and calcium supplementation increase the frequency of kidney stones. ^[5] The NIH reports that the upper limit set in the US was based on the increase in stones in the Women’s Health Initiative, but that two later systematic reviews (10 studies in adults with osteoporosis and 11 RCTs in adults aged 50 and older) found no such relationship. ^[1] The decision on supplementation in people with a history of stones therefore needs to be made together with the physician.

What else has calcium been studied for?

Premenstrual syndrome. A double-blind, placebo-controlled study conducted at 12 centers in the US enrolled 497 women aged 18–45 with moderate-to-severe cyclic symptoms (466 in the efficacy analysis); participants received 1,200 mg per day of elemental calcium (carbonate) or placebo for 3 cycles. In the third cycle, the total symptom score fell 48% from baseline in the calcium group and 30% in the placebo group. ^[7] This is a single study; larger studies with different designs are needed.

Colorectal adenoma. The NIH reports that most observational studies found an association between high calcium intake and lower colorectal cancer risk, but that randomized trials gave mixed results. In a meta-analysis of four trials, 1,200–2,000 mg per day of elemental calcium for 36–60 months reduced the likelihood of recurrent adenoma by 11%, but there was no effect on advanced adenoma; in a follow-up study of 1,121 participants, no effect on recurrence was seen. ^[1]

Preeclampsia in pregnancy. The NIH reports that WHO recommends 1,500–2,000 mg per day to reduce the risk of preeclampsia in pregnant women with low calcium intake. In contrast, the 2025 Cochrane update included 10 randomized trials (37,504 participants). In 6 trials comparing calcium with placebo (15,364 women), the difference in preeclampsia may be little or none (RR 0.83; 95% CI 0.67–1.04; low certainty); when trials with fewer than 500 participants were removed, there was also no difference (4 trials, 14,730 women; RR 0.92; 0.79–1.05; high certainty). According to the authors, baseline calcium intake level and preeclampsia risk did not change these results. The update excluded 20 trials included in the previous version (11 because eligibility criteria changed, 9 as awaiting-classification studies because of trustworthiness concerns). ^[8] The earlier Cochrane version reported by the NIH (27 trials, 18,064 women) had reported a 64% reduction in preeclampsia in high-dose supplementation trials in women with low dietary calcium intake (8 trials, 10,678 women), but rated the quality of this evidence as low; the 2025 update left out some of the trials in that version for the reasons above. ^[1] The decision on supplementation in pregnancy is made together with a health professional.

Which drugs does it interact with?

The NIH states that calcium supplements can interact with some drugs and that some drugs can affect calcium levels. The information it reports (time intervals from FDA-approved labels) is as follows:

• Dolutegravir (HIV drug): When taken together with a calcium supplement, blood levels fall markedly; the label provides for dolutegravir to be taken 2 hours before or 6 hours after the supplement.
• Levothyroxine: Calcium carbonate can impair absorption; the label says that levothyroxine should not be taken within 4 hours of calcium carbonate.
• Quinolone antibiotics (such as ciprofloxacin, moxifloxacin): Concurrent use reduces absorption; the NIH states that taking the antibiotic 2 hours before or 2 hours after the supplement prevents this effect.
• Lithium: Long-term use can cause hypercalcemia; a calcium supplement can increase this risk.

^[1]

The ciprofloxacin drug information itself (MedlinePlus) gives a longer interval: it states that antacids and multivitamins containing calcium, magnesium, zinc or iron should be taken at least 6 hours before or 2 hours after ciprofloxacin. ^[11] For calcium and iron supplements, the NIH iron fact sheet reports that, although it has not been conclusively shown that calcium reduces iron absorption, experts recommend taking the two supplements at different times of the day. ^[10]

Detailed guideWhen to take iron? Calcium, tea and medication intervals→

Oral bisphosphonates used in osteoporosis treatment are also among the interacting drugs. The MedlinePlus alendronate information lists calcium supplements and antacids among the products that can interact and states that no food, drink or other medicines (including vitamins or antacids) should be taken for at least 30 minutes after alendronate. ^[9] The fact that thiazide-group diuretics can increase the risk of hypercalcemia together with calcium or vitamin D supplements is covered, with sources, in the guides on high calcium and vitamin D excess. The intervals above are label information; a person’s medication plan is arranged together with a physician or pharmacist.

What do excess and deficiency mean?

In healthy people, high blood calcium (hypercalcemia) is rare and is mostly due to diseases such as cancer and primary hyperparathyroidism. The NIH defines serum above 10.5 mg/dL (2.63 mmol/L) as hypercalcemia. Low blood calcium (hypocalcemia) is mostly associated with vitamin D or magnesium deficiency, parathyroid hormone insufficiency and certain drugs; the NIH counts serum below 8.5 mg/dL (2.12 mmol/L) as hypocalcemia. Because both can be an indicator of a disease rather than of food intake, they require physician assessment. The NIH states that hypercalcemia and hypercalciuria can also lead to kidney failure, reduced muscle tone and heart rhythm disorders; milk-alkali syndrome due to large amounts of calcium carbonate also runs with kidney failure (high calcium guide). ^[1]

Detailed guideHigh calcium (hypercalcemia): causes, symptoms and tests→ Detailed guideCalcium deficiency symptoms: are hypocalcemia and low intake the same thing?→

Evidence table: claim, population and limits

This guide does not recommend a personal dose. If a supplement is being considered, total intake (including food, antacids and multivitamins), history of kidney stones, medications used and, if needed, blood calcium level are evaluated together with a physician or pharmacist.