Calcium

What it is

Calcium is the most abundant mineral in the body, present in foods, added to others, used in medicines such as antacids and sold as a supplement, usually as calcium carbonate or calcium citrate. Most of it is structural, in bone and teeth. A small ionised pool in blood and tissue is involved in blood vessel contraction and dilation, muscle function, blood clotting, nerve transmission and hormone secretion.

What the research says

Supplements raise bone density slightly and, in pooled trials, cut total fractures by about a tenth, but the effect shrinks to nothing in the trials at lowest risk of bias, and dietary calcium intake is not associated with fracture risk in cohort studies. The Women's Health Initiative, the largest trial, found no significant reduction in hip fracture and a higher risk of kidney stones. Whether calcium supplements raise heart attack risk is genuinely disputed: two meta-analyses say yes, the Women's Health Initiative's own cardiovascular analysis says neither up nor down.

Evidence grade: Disputed.

What goes wrong

Calcium with vitamin D increased the risk of kidney stones in the Women's Health Initiative. (Source 1)

• Randomized trial, Moderate certainty.
• Size: 36,282 postmenopausal women.
• Who: Healthy postmenopausal women aged 50 to 79.
• How long: Average 7.0 years.
• Result: Renal calculi HR 1.17 (95% CI 1.02 to 1.34)
• Funding: not stated in the abstract.

The risk of renal calculi increased with calcium plus vitamin D (hazard ratio, 1.17; 95 percent confidence interval, 1.02 to 1.34).

A patient-level and trial-level meta-analysis found calcium supplements without vitamin D were associated with more myocardial infarctions. (Source 2)

• Meta-analysis, Low certainty.
• Size: Five studies with patient-level data (8,151 participants) and eleven trials at trial level.
• Who: Participants of mean age over 40 in placebo-controlled trials of at least 500 mg/day for over a year.
• How long: More than one year per trial.
• Result: Patient-level myocardial infarction HR 1.31 (95% CI 1.02 to 1.67, P=0.035); trial-level pooled RR 1.27 (1.01 to 1.59, P=0.038)
• Funding: not stated in the abstract.

Calcium supplements (without coadministered vitamin D) are associated with an increased risk of myocardial infarction.

A later meta-analysis of double-blind placebo-controlled trials also found an increase in cardiovascular disease and coronary heart disease with calcium supplements. (Source 3)

• Meta-analysis, Low certainty.
• Size: 13 double-blind placebo-controlled RCTs.
• Who: Mostly healthy postmenopausal women.
• How long: Varied by trial.
• Result: CVD RR 1.15 (95% CI 1.06-1.25, I2 = 0.0%); CHD RR 1.16 (95% CI 1.05-1.28, I2 = 0.0%)
• Funding: This research received no external funding.

Calcium supplements significantly increased the risk of CVD (RR 1.15, 95% CI 1.06–1.25, I2 = 0.0%, n = 14) and CHD (RR 1.16, 95% CI 1.05–1.28, I2 = 0.0%, n = 9) in double-blind, placebo-controlled RCTs, specifically in healthy postmenopausal women.

Cochrane found small but statistically significant increases in gastrointestinal symptoms and renal disease with calcium plus vitamin D. (Source 4)

• Systematic review, High certainty.
• Size: 15 trials, 47,761 participants (gastrointestinal); 11 trials, 46,548 participants (renal)
• Who: Post-menopausal women and older men.
• How long: Varied.
• Result: Gastrointestinal symptoms RR 1.04 (95% CI 1.00 to 1.08), calcium plus vitamin D RR 1.05 (1.01 to 1.09); renal disease RR 1.16 (1.02 to 1.33)
• Funding: not stated in the abstract.

There was also a small increased risk of gastrointestinal symptoms (15 trials, 47,761 participants, RR 1.04, 95% CI 1.00 to 1.08), especially for calcium plus vitamin D (four trials, 40,524 participants, RR 1.05, 95% CI 1.01 to 1.09), and a significant increase in renal disease (11 trials, 46,548 participants, RR 1.16, 95% CI 1.02 to 1.33).

What the evidence supports

Pooled randomised trials show calcium supplements reduce total and vertebral fracture modestly, with signs of publication bias. (Source 5)

• Systematic review, Low certainty.
• Size: 26 randomised controlled trials; 20 studies with 58,573 participants for total fracture.
• Who: Adults over 50.
• How long: Trial durations varied.
• Result: Total fracture RR 0.89 (95% CI 0.81 to 0.96); vertebral 0.86 (0.74 to 1.00); hip 0.95 (0.76 to 1.18); forearm 0.96 (0.85 to 1.09)
• Funding: not stated in the abstract.

In 26 randomised controlled trials, calcium supplements reduced the risk of total fracture (20 studies, n=58 573; relative risk 0.89, 95% confidence interval 0.81 to 0.96)

In the Women's Health Initiative, calcium with vitamin D produced a small increase in hip bone density. (Source 1)

• Randomized trial, Moderate certainty.
• Size: 36,282 postmenopausal women (density measured at three centres)
• Who: Postmenopausal women aged 50 to 79.
• How long: Average 7.0 years.
• Result: Hip bone density 1.06 percent higher than placebo (P<0.01)
• Funding: not stated in the abstract.

Hip bone density was 1.06 percent higher in the calcium plus vitamin D group than in the placebo group (P<0.01).

A Cochrane review found vitamin D plus calcium gave a small reduction in hip fracture risk. (Source 4)

• Systematic review, High certainty.
• Size: Nine trials, 49,853 participants.
• Who: Post-menopausal women and older men in community and institutional settings.
• How long: Varied.
• Result: RR 0.84 (95% CI 0.74 to 0.96), P=0.01; about nine fewer hip fractures per 1000 per year in institutional populations (95% CI 2 to 14)
• Funding: not stated in the abstract.

There is high quality evidence that vitamin D plus calcium results in a small reduction in hip fracture risk (nine trials, 49,853 participants; RR 0.84, 95% confidence interval (CI) 0.74 to 0.96; P value 0.01).

What the evidence does not support

In the largest trial, calcium with vitamin D did not significantly reduce hip fracture. (Source 1)

• Randomized trial, Moderate certainty.
• Size: 36,282 postmenopausal women.
• Who: Healthy postmenopausal women aged 50 to 79.
• How long: Average 7.0 years.
• Result: Hip fracture HR 0.88 (95% CI 0.72 to 1.08); clinical spine 0.90 (0.74 to 1.10); total fractures 0.96 (0.91 to 1.02)
• Funding: not stated in the abstract.

Intention-to-treat analysis indicated that participants receiving calcium plus vitamin D supplementation had a hazard ratio of 0.88 for hip fracture (95 percent confidence interval, 0.72 to 1.08), 0.90 for clinical spine fracture (0.74 to 1.10), and 0.96 for total fractures (0.91 to 1.02).

When the fracture trials at lowest risk of bias were analysed on their own, calcium supplements had no effect at any site. (Source 5)

• Systematic review, Low certainty.
• Size: Four trials, 44,505 participants.
• Who: Adults over 50.
• How long: Varied.
• Result: No effect on risk of fracture at any site; funnel plot and Egger's regression suggested bias in the wider published data.
• Funding: not stated in the abstract.

In randomised controlled trials at lowest risk of bias (four studies, n=44 505), there was no effect on risk of fracture at any site.

Dietary calcium intake, including milk and dairy intake, was not associated with fracture risk in most cohort studies. (Source 5)

• Cohort study, Low certainty.
• Size: 50 reports from 44 cohort studies.
• Who: Adults over 50.
• How long: Varied.
• Result: No association in 14/22 studies for total fracture, 17/21 for hip, 7/8 for vertebral, 5/7 for forearm; milk 25/28 and dairy 11/13 reported no association.
• Funding: not stated in the abstract.

For dietary calcium, most studies reported no association between calcium intake and fracture (14/22 for total, 17/21 for hip, 7/8 for vertebral, and 5/7 for forearm fracture).

The Women's Health Initiative's own cardiovascular analysis found no change in coronary or cerebrovascular risk with calcium plus vitamin D. (Source 6)

• Randomized trial, Moderate certainty.
• Size: 36,282 postmenopausal women.
• Who: Generally healthy postmenopausal women aged 50 to 79.
• How long: 7 years.
• Result: Myocardial infarction or coronary death HR 1.04 (95% CI 0.92 to 1.18); stroke HR 0.95 (0.82 to 1.10)
• Funding: not stated in the abstract.

Calcium/vitamin D supplementation neither increased nor decreased coronary or cerebrovascular risk in generally healthy postmenopausal women over a 7-year use period.

Where the research disagrees

Whether calcium supplements increase heart attack risk

• Bolland and colleagues, BMJ 2010 meta-analysis, meta-analysis: Calcium supplements (without coadministered vitamin D) are associated with an increased risk of myocardial infarction. (Source 2)
• Hsia and colleagues, Women's Health Initiative cardiovascular analysis, rct: Calcium/vitamin D supplementation neither increased nor decreased coronary or cerebrovascular risk in generally healthy postmenopausal women over a 7-year use period. (Source 6)
• Myung and colleagues, 2021 meta-analysis of double-blind trials, meta-analysis: Calcium supplements significantly increased the risk of CVD (RR 1.15, 95% CI 1.06–1.25, I2 = 0.0%, n = 14) and CHD (RR 1.16, 95% CI 1.05–1.28, I2 = 0.0%, n = 9) in double-blind, placebo-controlled RCTs, specifically in healthy postmenopausal women. (Source 3)
• NIH Office of Dietary Supplements summary of the field, position: In contrast, several prospective cohort studies and RCTs have shown that calcium supplements increase the risk of CVD. (Source 7)

Whether more calcium, from food or pills, means fewer fractures

• Bolland and colleagues, BMJ 2015 systematic review, systematic-review: Dietary calcium intake is not associated with risk of fracture, and there is no clinical trial evidence that increasing calcium intake from dietary sources prevents fractures. Evidence that calcium supplements prevent fractures is weak and inconsistent. (Source 5)
• Cochrane review of vitamin D with calcium, systematic-review: There is high quality evidence that vitamin D plus calcium results in a small reduction in hip fracture risk (nine trials, 49,853 participants; RR 0.84, 95% confidence interval (CI) 0.74 to 0.96; P value 0.01). (Source 4)

How much

• Reference intake: The NIH Office of Dietary Supplements reports Food and Nutrition Board Recommended Dietary Allowances for adults of 1,000 mg/day at ages 19-50, 1,000 mg/day for men and 1,200 mg/day for women aged 51-70, and 1,200 mg/day from age 71. (Source 8)
• Upper limit: The Food and Nutrition Board's Tolerable Upper Intake Levels for adults are 2,500 mg/day at ages 19-50 and 2,000 mg/day from age 51. The NIH states these are based on observational evidence from the Women's Health Initiative linking 1,000 mg/day of supplemental calcium for 7 years to a greater risk of kidney stones, and that two later systematic reviews found no such association. (Source 9)
• Studied: The Women's Health Initiative gave 1000 mg of elemental calcium as calcium carbonate with 400 IU of vitamin D3 daily for an average of 7 years. (Source 1)
• Studied: The 2010 meta-analysis included placebo-controlled trials of calcium supplements of at least 500 mg/day lasting more than one year. (Source 2)

A common belief, and what the research shows

The belief: Taking a calcium supplement is a straightforward way to protect your bones.

What the research shows: In the trials at lowest risk of bias "there was no effect on risk of fracture at any site.", the largest trial found calcium with vitamin D "did not significantly reduce hip fracture, and increased the risk of kidney stones.", and dietary calcium intake shows no association with fracture in most cohort studies. A small benefit on hip fracture appears when calcium is combined with vitamin D, largest in frail institutionalised people.

Questions and answers

What is it?

Calcium is the most abundant mineral in the body. It is in food, added to other foods, present in medicines such as antacids, and sold as supplements, usually calcium carbonate or calcium citrate. (Source 8)

What does it do in the body?

Most calcium is structural, making up much of bone and teeth. A small ionised pool circulating in blood and tissues is involved in blood vessel contraction and dilation, muscle function, blood clotting, nerve transmission and hormone secretion. (Source 8)

Is it good or bad for you?

Adequate calcium matters for bone, but supplements are a different question from dietary calcium. In the largest trial, calcium with vitamin D slightly raised hip bone density, did not significantly cut hip fractures, and raised kidney stone risk. Two meta-analyses link supplements to more heart attacks, while the same trial's cardiovascular analysis found no change. (Source 1)

How do you get more of it?

Dairy foods are the main dietary route: milk, yogurt and cheese are rich natural sources and about 72% of US calcium intake comes from dairy products and foods containing dairy ingredients. Non-dairy sources include canned sardines and salmon with bones and some vegetables. Trials of supplements typically used 500 to 1000 mg a day of elemental calcium. (Source 10)

If it is harmful, what reduces it?

The problems attributed to calcium come from high supplemental intakes rather than food. The NIH states that the Food and Nutrition Board's upper limits are based on Women's Health Initiative evidence linking 1,000 mg/day of supplemental calcium for 7 years to a greater risk of kidney stones. The same fact sheet notes that two later systematic reviews, covering more than 8,000 adults with osteoporosis and 11 trials in 51,419 adults aged 50 and over, found no such association. (Source 9)

Why might someone be low in it or missing it?

Low intake usually comes from avoiding dairy, or from changes in absorption and losses. People with lactose intolerance, milk allergy or a vegan diet are listed as at higher risk of inadequate intake, and after menopause falling oestrogen reduces calcium absorption and increases urinary loss and resorption from bone. Plant compounds such as oxalate also cut absorption. (Source 11)

Which whole foods contain it or feed it?

Milk, yogurt and cheese; canned sardines and salmon eaten with their bones; and vegetables such as kale, broccoli and bok choy. Absorption differs by food: about 30% from dairy and fortified foods, but only about 5% from spinach because oxalate binds the calcium. (Source 10)

What happens if you do not have it?

Long-term shortfall weakens the skeleton: the NIH states that calcium deficiency can reduce bone strength and lead to osteoporosis, with fragile bones and an increased risk of falling. Short-term blood calcium is defended by hormones, so a low intake does not usually show up as low blood calcium. (Source 12)

How can you test for it?

There is no good blood test of calcium nutrition. Serum or plasma calcium can be measured, but the NIH states these levels are not a good reflection of an individual's calcium status, because they are under tight homeostatic control. Dual x-ray absorptiometry testing of bone mineral density is the measure the fact sheet says can be used to assess cumulative calcium status over the lifetime. (Source 13)

References

1. New England Journal of Medicine. Calcium plus vitamin D supplementation and the risk of fractures. 2006. PMID 16481635, DOI 10.1056/NEJMoa055218. Read the source
2. BMJ. Effect of calcium supplements on risk of myocardial infarction and cardiovascular events: meta-analysis. 2010. PMID 20671013, DOI 10.1136/bmj.c3691. Read the source
3. Nutrients. Calcium Supplements and Risk of Cardiovascular Disease: A Meta-Analysis of Clinical Trials. 2021. PMID 33530332, DOI 10.3390/nu13020368. Read the source
4. Cochrane Database of Systematic Reviews. Vitamin D and vitamin D analogues for preventing fractures in post-menopausal women and older men.. 2014. PMID 24729336, DOI 10.1002/14651858.CD000227.pub4. Read the source
5. BMJ. Calcium intake and risk of fracture: systematic review. 2015. PMID 26420387, DOI 10.1136/bmj.h4580. Read the source
6. Circulation. Calcium/vitamin D supplementation and cardiovascular events. 2007. PMID 17309935, DOI 10.1161/CIRCULATIONAHA.106.673491. Read the source
7. NIH Office of Dietary Supplements. Calcium - Health Professional Fact Sheet. 2024. Read the source
8. NIH Office of Dietary Supplements. Calcium - Health Professional Fact Sheet. 2024. Read the source
9. NIH Office of Dietary Supplements. Calcium - Health Professional Fact Sheet. 2024. Read the source
10. NIH Office of Dietary Supplements. Calcium - Health Professional Fact Sheet. 2024. Read the source
11. NIH Office of Dietary Supplements. Calcium - Health Professional Fact Sheet. 2024. Read the source
12. NIH Office of Dietary Supplements. Calcium - Health Professional Fact Sheet. 2024. Read the source
13. NIH Office of Dietary Supplements. Calcium - Health Professional Fact Sheet. 2024. Read the source