Calcium for Calcium Deficiency

If your diet is short of calcium, more calcium is the fix, ideally from food first. The best trial evidence comes from children with rickets caused by very low calcium intakes: in a double-blind trial in 123 Nigerian children, 24 weeks of 1,000 mg of calcium a day healed rickets in 61%, against 19% with vitamin D injections alone (Nigerian rickets trial, 1999). A Cochrane review rated this low-certainty evidence, but in the same direction (Cochrane rickets review, 2020).

Calcium deficiency covers two different problems, and calcium does different jobs in each. Low intake slowly drains bone; extra calcium refills the gap, but in adults over 50 it raises bone density by only 0.7-1.8% and does little to prevent fractures (BMD meta-analysis, 2015; fracture meta-analysis, 2026). Low blood calcium (hypocalcemia) usually comes from missing parathyroid hormone, low vitamin D or low magnesium, so calcium is given alongside treatment of the cause, and severe cases need intravenous calcium in hospital (NIH ODS fact sheet; Society for Endocrinology guidance, 2016).

About 98% of the body's calcium sits in bone, which acts as a reservoir: when intake falls short, the body draws on it to keep blood calcium steady (NIH ODS fact sheet). That is why a long-term low intake rarely shows up on a blood test; blood calcium is not a good measure of calcium status, and the cost appears later as thinner bone or, in growing children, rickets (NIH ODS fact sheet).

Raising intake lets the body stop drawing on bone. Absorption adjusts to need: about 45% of food calcium is absorbed at intakes of 200 mg a day, against 15% at intakes above 2,000 mg (NIH ODS fact sheet). The gut needs vitamin D for active calcium absorption, and the lower someone's vitamin D, the more calcium they need to avoid rickets and osteomalacia (NIH ODS fact sheet). In one rickets trial, children whose bones healed tended to have higher vitamin D levels at the end of treatment, an association rather than proof (Nigerian vitamin D add-on trial, 2014).

Low blood calcium is mostly a hormone problem, not a diet problem. Its most frequent cause is damage to the parathyroid glands during neck surgery, and other causes include vitamin D deficiency, low magnesium, critical illness and some medicines (hypocalcemia review, 2020; NIH ODS fact sheet). Vitamin D is needed both to absorb calcium and to keep blood levels up (NIH ODS fact sheet), so without it, or without parathyroid hormone, tablets alone often fall short. Low blood calcium typically shows as tingling around the mouth and in the hands and feet and muscle spasms (NIH ODS fact sheet); severe cases bring carpopedal spasm, laryngospasm (spasm of the voice box), heart rhythm changes and seizures (Society for Endocrinology guidance, 2016). The work-up checks calcium adjusted for albumin, phosphate, parathyroid hormone, kidney function, vitamin D and magnesium (Society for Endocrinology guidance, 2016).

That is why treatment pairs calcium with the missing piece. For chronic hypoparathyroidism, first-line therapy is calcium plus active vitamin D, with vitamin D and magnesium corrected and kidney function monitored (hypoparathyroidism recommendations, 2025). UK emergency guidance adds magnesium when it is low and a vitamin D loading course when vitamin D deficiency is the cause (Society for Endocrinology guidance, 2016).

Low intakes are most likely in people who avoid dairy (lactose intolerance, milk allergy, vegan diets) and in postmenopausal women, whose falling estrogen reduces calcium absorption (NIH ODS fact sheet). For a low intake, the target is the recommended total from food and supplements together: 1,000 mg a day for adults aged 19-50 and men up to 70, 1,200 mg for women over 50 and everyone over 70, 1,300 mg at ages 9-18, and 700 mg at ages 1-3 or 1,000 mg at 4-8 (NIH ODS fact sheet). Count what your diet already provides and supplement only the gap; our guide to calcium-rich foods helps with the counting. Milk, yogurt and cheese are rich sources, and calcium-set tofu, fortified juices and plant drinks, and canned sardines and salmon with bones also provide it (NIH ODS fact sheet).

Absorption is highest from doses of 500 mg or less: the body absorbs about 36% of a 300 mg dose and 28% of a 1,000 mg dose (NIH ODS fact sheet; absorption study, 1999). Calcium carbonate is 40% calcium and is best taken with meals, especially with low stomach acid; calcium citrate is 21% calcium and can be taken without food (NIH ODS fact sheet). Taken with food, the two are absorbed about equally well (absorption study, 1999).

In children with calcium-deficiency rickets, 1,000 mg a day healed the bones faster than 500 mg, and 2,000 mg was no better than 1,000 mg (rickets dose trial, 2016). After thyroid surgery, and in hospital cases of low blood calcium, doses are set by the medical team (Society for Endocrinology guidance, 2016).

Most of the treatment evidence comes from one research group in Nigeria, where children with rickets ate a median of about 200 mg of calcium a day (Nigerian rickets trial, 1999), typical of African children (Nigerian prevention study, 2012). In the first double-blind trial, 123 children with rickets received calcium (1,000 mg a day), vitamin D injections (600,000 units at enrollment and 12 weeks) or both. After 24 weeks, 61% on calcium and 58% on the combination reached the combined goal of alkaline phosphatase (a bone-turnover marker) at 350 U/L or less and near-complete healing on X-ray, against 19% on vitamin D alone (P below 0.001) (Nigerian rickets trial, 1999).

Later trials refined the dose and source. In 65 children, 1,000 mg and 2,000 mg a day healed faster than 500 mg, with no extra gain from 2,000 mg, and some children needed more than 24 weeks (rickets dose trial, 2016). Calcium as ground fish (952 mg) or limestone (920 mg) healed most children, with no significant difference between them (66% vs 55%; P = .39) (limestone vs fish trial, 2015). Adding monthly vitamin D to calcium raised the healing rate from 44% to 67%, but the difference was not statistically significant (p = 0.06) (Nigerian vitamin D add-on trial, 2014).

The Cochrane review pooled 4 trials (286 children, in India and Nigeria) and found calcium alone improved healing compared with vitamin D alone (RR 3.26, 95% CI 1.59-6.69; 1 trial), with low-certainty evidence; it could not say whether calcium plus vitamin D beats calcium alone (Cochrane rickets review, 2020). These results apply to calcium-deficiency rickets. Vitamin D and calcium deficiencies both cause rickets and osteomalacia worldwide (global rickets consensus, 2016), and the most common cause of rickets is vitamin D deficiency (Cochrane rickets review, 2020).

Damage to the parathyroid glands during total thyroidectomy is a common cause of low blood calcium, and routine supplements after surgery have been tested in many trials. Across 15 trials (3,037 patients), routine calcium plus vitamin D3 lowered symptomatic hypocalcemia compared with treating only when blood calcium fell (risk difference -0.25; number needed to treat 4) (thyroidectomy meta-analysis, 2019).

Calcium alone does less. In one meta-analysis of 10 trials (1,620 patients), calcium alone cut transient hypocalcemia (OR 0.48) but not the need for intravenous calcium, while calcium plus vitamin D did both and beat calcium alone (OR 0.39) (thyroidectomy meta-analysis, Medicine 2019). A 2026 network meta-analysis of 20 trials (3,669 adults) found calcium alone showed no significant benefit, while the combination reduced clinical hypocalcemia (OR 0.31) and intravenous calcium needs (OR 0.15) compared with no supplementation (thyroidectomy network meta-analysis, 2026). Neither approach prevented permanent hypocalcemia (thyroidectomy meta-analysis, Medicine 2019).

In children used to a low-calcium diet, extra calcium builds bone. In 160 rural Gambian children aged 8-11 eating about 342 mg a day, 1,000 mg of calcium 5 days a week for a year raised forearm bone density by 4.5-7.0% compared with placebo, without changing height or weight (Gambian children trial, 2000). Low intake is common: national averages range from 175 to 1,233 mg a day, and many Asian countries average under 500 mg (global intake review, 2017).

In adults over 50, the gains are small. Across 59 trials, calcium from food or supplements raised bone density by 0.7-1.8% over one to two years; with supplements there was no further rise after the first year, and the effect was similar whether baseline intake was under or over 800 mg a day (BMD meta-analysis, 2015). Calcium supplements reduced total fractures by 11% in pooled trials (RR 0.89), but not in the trials at lowest risk of bias (fracture meta-analysis, 2015). The most recent review (69 trials in all) found little to no effect of calcium supplements alone on fractures (11 trials, 9,067 people; RR 0.91, 95% CI 0.81-1.01; moderate certainty) (fracture meta-analysis, 2026). For bone loss itself, see calcium for osteoporosis.

Low intake matters in pregnancy too. In a Cochrane review of 27 trials, at least 1 g of calcium a day reduced pre-eclampsia (13 trials, 15,730 women; RR 0.45, 95% CI 0.31-0.65), and the effect was clear in women with low-calcium diets (RR 0.36; 8 trials, 10,678 women) but not in those with adequate diets (Cochrane pregnancy review, 2018). The evidence was rated low quality, and the largest single trial found only an 8% reduction (Cochrane pregnancy review, 2018). WHO recommends 1,500-2,000 mg a day for pregnant women with low intakes (NIH ODS fact sheet).

Nearly all trials of calcium for calcium-deficiency rickets come from one research group in Nigeria, with arms of roughly 20-50 children, so independent replication is lacking (rickets dose trial, 2016; Cochrane rickets review, 2020). Oral calcium for chronic hypocalcemia rests mainly on systematic reviews and expert consensus rather than placebo-controlled trials (hypoparathyroidism recommendations, 2025). And adult fracture trials rarely selected people with a low intake: in one review, the only trial with a clear fracture reduction was in frail elderly women in residential care with low calcium intakes and low vitamin D (fracture meta-analysis, 2015).

Calcium supplements often cause gas, bloating and constipation, more with carbonate than citrate; switching form, taking smaller doses more often, or taking them with meals helps (NIH ODS fact sheet). They also raise kidney stone risk: in the Women's Health Initiative trial of 36,282 postmenopausal women, 1,000 mg of calcium plus 400 IU of vitamin D a day raised kidney stones by 17% (HR 1.17, 95% CI 1.02-1.34) (WHI trial, 2006), although two later systematic reviews found no such link (NIH ODS fact sheet). If you have had stones, see calcium and kidney stones.

Heart risk is debated. A meta-analysis of trials of calcium without vitamin D found more heart attacks with supplements (HR 1.31, 95% CI 1.02-1.67) (cardiovascular meta-analysis, 2010), and a 2025 review put the increase at 10-20%, with no such risk from food calcium (calcium safety review, 2025). An expert panel found no effect on cardiovascular disease at intakes within the upper limit (NIH ODS fact sheet). The upper limit is 2,500 mg a day to age 50 and 2,000 mg after, from all sources (NIH ODS fact sheet).

With active vitamin D (calcitriol or alfacalcidol), which is potent at raising blood calcium, calcium can tip into hypercalcemia, so these combinations need frequent blood tests (Society for Endocrinology guidance, 2016). High blood calcium can itself cause kidney damage (NIH ODS fact sheet), and guidance for chronic hypoparathyroidism calls for monitoring kidney function and urine calcium (hypoparathyroidism recommendations, 2025), so people with kidney disease should have their calcium dose set by their doctor.

Calcium binds several medicines in the gut. Calcium carbonate reduces levothyroxine absorption, and the label advises a 4-hour gap; calcium also lowers levels of the HIV drug dolutegravir and of quinolone antibiotics (NIH ODS fact sheet). For ciprofloxacin, the FDA label is stricter than a general 2-hour rule: take it at least 2 hours before or 6 hours after products containing calcium (ciprofloxacin label). Calcium supplements also interfere with absorption of the osteoporosis drug alendronate, whose label says to wait at least 30 minutes after a dose before any other oral medicine (alendronate label), and they impair absorption of tetracycline antibiotics such as doxycycline (doxycycline label).

Some medicines push calcium up. Thiazide diuretics such as hydrochlorothiazide reduce calcium excretion, so calcium salts can raise blood calcium (hydrochlorothiazide label), and long-term lithium can cause high blood calcium that calcium supplements may add to (NIH ODS fact sheet). Our guide to taking calcium and magnesium together covers mineral timing.