Benefits
Slower bone loss with age
Calcium is a main structural mineral of bone. In a meta-analysis of 23 trials in people over 50, calcium with or without vitamin D slowed bone loss by about 0.5% at the hip and 1.2% at the spine. Fracture results are weak and inconsistent: pooled trials found about 12% fewer fractures, but the trials at lowest risk of bias, and a 33-trial review of adults living in the community, found no reduction.
Cardiovascular concerns: supplements vs dietary calcium
Several pooled analyses of trials found more heart attacks with calcium supplements: about 24% more in a 2011 analysis, and about 15% more cardiovascular events in a 2021 analysis of 13 double-blind trials, mostly in postmenopausal women. A 2016 review of 4 trials and 27 cohort studies found no link within the upper limit. Heart events were secondary outcomes in every trial, and food calcium has not shown the signal.
Calcium in pregnancy: WHO guidance and newer large trials
WHO has advised 1.5 to 2 g a day of calcium in pregnancy where diets are low in calcium. Two 2024 trials in 22,000 first-time mothers found 500 mg a day worked as well as 1,500 mg for pre-eclampsia. A 2025 Cochrane review that excluded untrustworthy trials found calcium versus placebo made little or no difference to pre-eclampsia in the large trials. Pregnant women should follow their clinician's advice.
Muscle function and nerve signaling
Calcium in blood and tissues is needed for muscle contraction and nerve transmission, and very low blood calcium causes tingling and muscle spasms. The body keeps blood calcium in a tight range by drawing on bone, so low blood calcium usually comes from medical causes such as low parathyroid hormone or vitamin D deficiency rather than diet, and a supplement rarely fixes muscle symptoms in healthy adults.
Blood pressure in people with normal readings
In a 2022 Cochrane review of 18 trials in 3,140 people with normal blood pressure, increasing calcium intake lowered systolic pressure by about 1.4 mmHg and diastolic by about 1.5 mmHg, a little more in people under 35. No trial reported whether this changes the chance of developing high blood pressure later.
Food first, supplements to fill the gap
The cardiovascular and kidney-stone signals come from supplement trials, not from calcium in dairy, leafy greens or fortified foods. The usual advice is to meet calcium needs from food first and use a supplement only if your diet falls well short, keeping single doses at 500 mg or less, since absorption drops at larger doses.
Mechanism of action
Bone mineralization and remodeling
More than 99% of body calcium is stored as hydroxyapatite in bones and teeth. Bone is constantly rebuilt, with osteoclasts removing old bone and osteoblasts laying down new bone. Vitamin D enables calcium absorption from the gut; when intake falls short, parathyroid hormone releases calcium from bone to keep blood levels steady.
Excitation-contraction coupling in muscle
A nerve signal triggers calcium release inside muscle cells. Calcium binds troponin, which lets myosin pull on actin and the muscle contract; pumping the calcium back out lets the muscle relax.
Nerve transmission
Calcium entering nerve endings through calcium channels triggers the release of neurotransmitters. Very low blood calcium makes nerves overexcitable, which shows up as tingling and muscle spasms.
Blood clotting cascade
Calcium is a cofactor in the clotting cascade, needed to activate several clotting factors and form fibrin. Lab tubes use calcium-binding chemicals such as EDTA to stop blood from clotting.
Cardiac action potential
Calcium entering heart muscle cells shapes the heart's electrical cycle and triggers each contraction. Very low or very high blood calcium can disturb heart rhythm.
Clinical trials
Post-hoc reanalysis of the Women's Health Initiative limited-access dataset combined with a meta-analysis of placebo-controlled calcium trials, updating a 2010 analysis (Bolland et al. 2011, BMJ)
28,072 participants from eight placebo-controlled calcium trials plus the WHI calcium and vitamin D participants who were not already taking their own calcium supplements. Mostly older women.
In the 16,718 WHI women not already taking their own calcium, calcium plus vitamin D raised cardiovascular event rates (hazard ratios 1.13 to 1.22). Pooled with eight calcium trials, calcium with or without vitamin D increased heart attack risk (RR 1.24, 95% CI 1.07 to 1.45) and heart attack or stroke (RR 1.15, 95% CI 1.03 to 1.27). The conclusions remain disputed.
Meta-analysis of double-blind placebo-controlled trials of calcium supplements and cardiovascular disease; it pools largely the same older trials as the Bolland analyses, so it is not independent confirmation (Myung et al. 2021, Nutrients)
13 double-blind placebo-controlled trials with 28,935 participants, mostly postmenopausal women.
Calcium supplements increased cardiovascular disease risk (RR 1.15, 95% CI 1.06 to 1.25) and coronary heart disease risk (RR 1.16, 95% CI 1.05 to 1.28) compared with placebo, mainly in healthy postmenopausal women, including subgroups taking 1,000 mg a day. The absolute increase is small.
Systematic review and meta-analysis of randomized trials in community-dwelling adults over 50; people in residential care were excluded (Zhao et al. 2017, JAMA)
51,145 community-dwelling older adults across 33 clinical trials.
Calcium, vitamin D, or both together were not associated with fewer hip, nonvertebral, vertebral or total fractures versus placebo or no treatment. For calcium alone, hip fracture risk was nonsignificantly higher (RR 1.53, 95% CI 0.97 to 2.42). Results were consistent regardless of dose, dietary calcium intake or baseline vitamin D level. The authors concluded the findings do not support routine use of these supplements in community-dwelling older people.
Randomized double-blind placebo-controlled trial of calcium plus vitamin D in postmenopausal women already enrolled in the Women's Health Initiative (Jackson et al. 2006, N Engl J Med)
36,282 postmenopausal women aged 50 to 79 randomized to 1,000 mg calcium carbonate plus 400 IU vitamin D3 daily or placebo; average follow-up 7.0 years.
Calcium plus vitamin D raised hip bone density by 1.06 percent versus placebo but did not significantly reduce hip fracture (HR 0.88, 95% CI 0.72 to 1.08) or total fractures (HR 0.96, 95% CI 0.91 to 1.02). Kidney stones increased (HR 1.17, 95% CI 1.02 to 1.34). Hip fracture was lower among women who kept taking the pills (HR 0.71), a non-randomized comparison. The 400 IU vitamin D dose is low by current standards.
Two randomized noninferiority trials testing whether 500 mg a day of calcium is as good as 1,500 mg a day for pre-eclampsia and preterm birth in populations with low calcium intake (Dwarkanath et al. 2024, N Engl J Med)
22,000 nulliparous pregnant women (11,000 in each trial) in India and Tanzania, populations with low dietary calcium intake.
Pre-eclampsia occurred in 3.0% (500 mg) vs 3.6% (1,500 mg) in India and 3.0% vs 2.7% in Tanzania, meeting noninferiority in both. For preterm birth the low dose was noninferior in India but not in Tanzania. Both groups took calcium, so these trials compare doses and cannot show whether calcium prevents pre-eclampsia compared with none. Funded by the Bill and Melinda Gates Foundation and others.