Calcium (carbonate vs citrate)
Summary
Calcium supplements are mostly carbonate (40% calcium) or citrate (21%). Citrate was absorbed about 22-27% better in pooled absorption studies. Supplements raised bone density slightly in adults over 50, but fracture trials disagree. Calcium alone was linked to more heart events in one meta-analysis, and calcium with vitamin D to more urinary stone reports in one large trial.
Key facts
- Carbonate and citrate (most common); also gluconate, lactate, phosphate, citrate malate and others[1]
- Carbonate is 40% calcium and is absorbed better with food; citrate is 21% calcium and is less dependent on stomach acid. Citrate absorbed about 22–27% better in a pooled analysis (grade C)[1][2]
- Bone mineral density ↑ small (A); fractures with calcium + vitamin D: mixed (no-effect in community-dwelling adults; ↓ in a broader meta-analysis, B); blood pressure ↓ slightly (A)[3][4][5][6]
- Heart events ↑ with calcium alone in one meta-analysis (C); urinary stones ↑ with calcium + vitamin D in the WHI trial (B)[7][8]
- RDA 1,000 mg (ages 19–50; men to 70), 1,200 mg (women 51+, everyone 71+). UL 2,500 mg (19–50), 2,000 mg (51+)[1]
- About 36% of a 300 mg dose vs 28% of a 1,000 mg dose; absorption is highest at 500 mg or less at a time[1]
- Dolutegravir, levothyroxine, quinolone antibiotics, lithium[1]
- 7 graded human outcomes on this page
- October 8, 2026
How it works
EstablishedAbout 98% of body calcium is stored in bone, mostly as hydroxyapatite; bone is constantly remodeled and acts as the reservoir that keeps blood calcium in a tight range. The small circulating pool controls blood-vessel contraction, muscle function, blood clotting, nerve signaling and hormone release. Vitamin D is required for active calcium absorption in the gut (established).[1]
What the human research says
Human trials only, one row per outcome. The form column shows what each trial actually tested. Grades follow our evidence-grading policy.
At a glance: strongest evidence per outcome
| Outcome | Grade | Effect | Strongest evidence | Rows |
|---|---|---|---|---|
| Bones & joints | Grade A | Improved | Meta-analysis · n = 12,257 · 2015 | 5 rows |
| Heart | Grade A | Worsened | Systematic review · n = 3,048 · 2015 | 2 rows |
Every graded row
| Outcome | Effect | Grade | Best evidence | PMID |
|---|---|---|---|---|
| Bone mineral density in adults over 50 | Improved | Grade A | Meta-analysis · n = 12,257 · 2015 Adults over 50 in 51 randomized trials of calcium supplements (with or without vitamin D) Form: calcium supplements, form not specified in abstract · Dose: Effects were similar for doses of ≥1000 vs <1000 mg/day and ≤500 vs >500 mg/day · Duration: 1 to more than 2 years Supplements increased bone mineral density by 0.7-1.8% at all five skeletal sites, but the gain did not grow after the first year. The authors judged the increases unlikely to meaningfully lower fracture risk. | 26420598 (opens PubMed)Source [3] |
| Blood pressure in people with normal blood pressure | Worsened | Grade A | Systematic review · n = 3,048 · 2015 Normotensive people in 16 randomized trials Form: calcium supplementation or fortification · Dose: Below 1000 to above 1500 mg/day; larger effects above 1500 mg/day · Duration: Varied across trials Systolic blood pressure fell by 1.43 mmHg (95% CI -2.15 to -0.72) and diastolic by 0.98 mmHg, with larger effects in people younger than 35. The authors rated the evidence high quality. | 26126003 (opens PubMed)Source [6] |
| Urinary stone reports with calcium plus vitamin D | Improved | Grade B | RCT · n = 36,282 · 2011 Postmenopausal women aged 50-79 in the Women's Health Initiative Form: calcium carbonate plus vitamin D3 · Dose: 1000 mg calcium plus 400 IU vitamin D3 daily (500 mg + 200 IU twice daily) · Duration: Average 7.0 years Self-reported urinary tract stones were 17% more common with supplements (HR 1.17; 95% CI 1.02 to 1.34): 449 vs 381 women. | 21525191 (opens PubMed)Source [8] |
| Fractures with calcium plus vitamin D (including institutionalized adults) | Worsened | Grade B | Meta-analysis · n = 30,970 · 2016 Community-dwelling and institutionalized middle-aged to older adults in 8 randomized trials Form: calcium plus vitamin D · Dose: Calcium plus vitamin D; doses varied across trials · Duration: Varied across trials Total fractures were 15% lower (SRRE 0.85; 95% CI 0.73-0.98) and hip fractures 30% lower (SRRE 0.70; 95% CI 0.56-0.87). | 26510847 (opens PubMed)Source [5] |
| Heart and blood-vessel events with calcium alone | Improved | Grade C | Meta-analysis · n = 8,151 · 2010 Adults with mean age over 40 in placebo-controlled trials lasting over a year (patient-level data from 5 trials) Form: calcium supplements without vitamin D · Dose: 500 mg/day or more · Duration: Median follow-up 3.6 years Myocardial infarction was more common with calcium (HR 1.31; 95% CI 1.02 to 1.67); stroke, the composite end point and death were not significantly increased. Trial-level data (n=11 921) showed a similar result (RR 1.27). | 20671013 (opens PubMed)Source [7] |
| Calcium absorption: citrate vs carbonate | Improved | Grade C | Meta-analysis · n = 184 · 1999 15 absorption studies, fasting or with meals Form: calcium citrate vs calcium carbonate · Dose: Single test doses; amounts varied by study · Duration: Single-dose absorption studies Absorption from citrate was higher than from carbonate by 20.0% overall, 27.2% on an empty stomach and 21.6% with meals. | 11329115 (opens PubMed)Source [2] |
| Fractures in community-dwelling older adults (calcium, vitamin D or both) | No effect | No effect | Meta-analysis · n = 51,145 · 2017 Community-dwelling adults older than 50 in 33 randomized trials Form: calcium, vitamin D, or combined supplements · Dose: Varied across trials · Duration: Varied across trials No significant link with hip fracture for calcium (RR 1.53; 95% CI 0.97 to 2.42) or calcium plus vitamin D (RR 1.09; 95% CI 0.85 to 1.39), and none for nonvertebral, vertebral or total fractures, regardless of dose, sex or baseline intake. | 29279934 (opens PubMed)Source [4] |
Forms
| Form | Elemental % | Absorption (human data) | GI tolerance | Studied for |
|---|---|---|---|---|
| Calcium carbonate | 40%[1] | About 20–27% lower absorption than citrate in pooled absorption studies; depends on stomach acid, so absorbed better with a meal.[2][1] | Appears to cause more gas, bloating and constipation than citrate, especially in older adults with low stomach acid.[1] | Women's Health Initiative trial (with vitamin D3); most bone trials; also in antacids |
| Calcium citrate | 21%[1] | About 22–27% better absorbed than carbonate, fasting or with meals; less dependent on stomach acid.[2][1] | Fewer GI side effects than carbonate reported by ODS.[1] | Absorption comparisons |
| Other forms (gluconate, lactate, phosphate, citrate malate, hydroxyapatite) | Varies[1] | No head-to-head human data in our sources (ODS calls citrate malate, used in fortified juices, well absorbed).[1] | No form-specific tolerance data in our sources.[1] | Fortified foods and some supplements |
- Calcium carbonate400 mg
- Calcium citrate210 mg
Compound weight → elemental calcium
500 mg calcium carbonate ≈ 200 mg elemental calcium
Studied doses
Doses studied in human trials ranged from 500 to about 2,000 mg/day of elemental calcium, for weeks to about 7 years. Absorption per serving falls as the amount rises, and the adult UL is 2,000–2,500 mg/day from food and supplements combined.[3][6][7][8][1]
Intake reference: Adult RDA: 1,000 mg/day (ages 19–50, and men 51–70); 1,200 mg/day for women 51+ and everyone over 70; 1,000 mg in pregnancy and breastfeeding (19+). UL: 2,500 mg/day (19–50), 2,000 mg/day (51+).[1] UL is 2,500 mg/day for ages 19–50 and 2,000 mg/day from age 51; ulMg shows the lower adult value. It was based on kidney-stone findings from the WHI trial. NIH ODS fact sheet (updated June 22, 2026).
Interactions
Published interactions only: each row cites a source. No row means none was found in our sources, not that a combination is safe.
Medicines
- DolutegravirDrug labelCalcium can substantially lower dolutegravir blood levels (chelation). The FDA labels advise dolutegravir 2 hours before or 6 hours after calcium supplements.[1]
- LevothyroxineDrug labelCalcium carbonate can reduce levothyroxine absorption; the FDA label says to avoid taking them within 4 hours of each other.[1]
- Quinolone antibioticsNIH fact sheetTaken together, calcium reduces quinolone absorption; ODS reports separating them by 2 hours prevents this.[1]
- LithiumNIH fact sheetLong-term lithium can raise blood calcium, and adding calcium supplements could increase this risk.[1]
Who should be careful
- High intakes
- The adult UL is 2,500 mg/day (ages 19–50) and 2,000 mg/day (51+) from food and supplements combined. Very high blood calcium is rare in healthy people but can cause constipation, nausea, fatigue, kidney problems and heart-rhythm problems.[1]
- Kidney stones and heart
- Calcium plus vitamin D raised urinary stone reports in a large trial, and one meta-analysis linked calcium alone to more heart attacks; other reviews found no such effects. People with a history of kidney stones or heart disease should discuss calcium supplements with a clinician.[8][7][1]
- Medicines
- Dolutegravir, levothyroxine, quinolone antibiotics and lithium interact with calcium supplements.[1]
- Before you start
- Talk to your pharmacist or clinician before adding this supplement, especially if you take prescription medicines, are pregnant or breastfeeding, or have a chronic health condition.[1]
Talk to your pharmacist or clinician before starting a supplement, especially if you take prescription medicines.
Articles about Calcium (carbonate vs citrate)
Plain-English answers to common questions, built on the evidence above.
Frequently asked questions
Is calcium citrate or calcium carbonate better?
Carbonate packs more calcium per tablet (40% vs 21%) but needs stomach acid, so it is absorbed better with food. Citrate was absorbed about 22–27% better in a pooled analysis of small studies and causes fewer GI complaints. Supplement labels list elemental calcium.[1][2]
Does it matter how much calcium is in one serving?
Yes. The share absorbed falls as the amount rises: about 36% of a 300 mg dose vs 28% of a 1,000 mg dose. ODS notes absorption is highest at 500 mg or less at a time.[1]
Do calcium supplements lower fracture risk?
The evidence is mixed. Supplements raise bone density slightly. A 2017 meta-analysis of community-dwelling older adults found no fracture benefit from calcium, vitamin D or both, while a 2016 meta-analysis that included institutionalized adults found 15% fewer total fractures with calcium plus vitamin D.[3][4][5]
Sources
- [1]Calcium: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements, 2026. fact-sheet
- [2]Meta-analysis of calcium bioavailability: a comparison of calcium citrate with calcium carbonate.. Am J Ther, 1999. meta-analysis
- [3]Calcium intake and bone mineral density: systematic review and meta-analysis.. BMJ, 2015. meta-analysis
- [4]Association Between Calcium or Vitamin D Supplementation and Fracture Incidence in Community-Dwelling Older Adults: A Systematic Review and Meta-analysis.. JAMA, 2017. meta-analysis
- [5]Calcium plus vitamin D supplementation and risk of fractures: an updated meta-analysis from the National Osteoporosis Foundation.. Osteoporos Int, 2016. meta-analysis
- [6]Calcium supplementation for prevention of primary hypertension.. Cochrane Database Syst Rev, 2015. systematic-review
- [7]Effect of calcium supplements on risk of myocardial infarction and cardiovascular events: meta-analysis.. BMJ, 2010. meta-analysis
- [8]Urinary tract stone occurrence in the Women's Health Initiative (WHI) randomized clinical trial of calcium and vitamin D supplements.. Am J Clin Nutr, 2011. RCT
- [9]Vitamin D, Calcium, or Combined Supplementation for the Primary Prevention of Fractures in Community-Dwelling Adults: Evidence Report and Systematic Review for the US Preventive Services Task Force.. JAMA, 2018. systematic-review
What changed
- First draft. Every evidence-row PMID checked with NCBI E-utilities (esummary + efetch abstract) and re-read in an independent adversarial pass.