Calcium

Is calcium citrate better than calcium carbonate?

Updated 4 Nov 2026

Short answer

Neither is clearly better for most people. Carbonate has about twice as much calcium by weight, and two studies, one of them with a light breakfast, found it as well absorbed as citrate.[1][2][3] Citrate did better in other studies, can be taken without food and appears to cause fewer stomach problems.[4][5][1]

What each form contains

Calcium carbonate is 40% calcium by weight, and calcium citrate is 21%.[1] This means that 1,000 mg of calcium needs about 2.5 g of carbonate or 4.8 g of citrate. The Supplement Facts label in the US lists the amount of calcium itself, called elemental calcium, so no calculation is needed.[1]

What absorption studies found

The studies disagree:

  • A 1999 meta-analysis of 15 studies with 184 people found 20% to 27% better absorption from citrate, with or without meals.[4]
  • A 1999 study in 37 healthy adults, with a light breakfast, found the same absorption from both: 36% at a dose of 300 mg and 28% at 1,000 mg.[2]
  • A 2001 crossover study in 24 women after menopause found identical rises in blood calcium with the two forms, when both were commercial products.[3]

What bone studies found

Studies that measured bone are also mixed:

  • A 2-year trial gave 301 women after menopause a placebo or 500 mg of calcium a day. Among women at least 6 years past menopause with a low calcium intake, calcium citrate malate prevented bone loss at the hip, forearm and spine. Calcium carbonate held bone density at the hip and forearm, but not at the spine.[6]
  • In a crossover trial, 34 women took 1,000 mg a day of each form for 12 weeks. Markers of bone breakdown fell by up to 31% with citrate and did not change with carbonate.[5]
  • In 25 women, 500 mg of calcium as citrate lowered bone breakdown over 12 hours at least as well as 1,000 mg as carbonate.[7]
  • A 3-month trial randomized 100 women to 1 g of calcium a day in one of several forms or to a placebo. There was no difference between citrate and carbonate.[8]

Stomach acid and food

People with low stomach acid dissolve calcium carbonate more slowly, which could lower absorption unless they take it with a meal.[1] Citrate depends less on stomach acid, so it can be taken without food.[1] Absorption of any calcium supplement is better with food, even in people with low stomach acid.[1]

Side effects and interactions

Calcium carbonate appears to cause more gas, bloating and constipation than citrate, mainly in older adults with low stomach acid.[1] Changing the form, taking smaller doses more often or taking the supplement with meals can help.[1]

The label of levothyroxine asks people who take calcium carbonate to keep the two 4 hours apart.[1]

Other forms and lead

Other forms include calcium sulfate, gluconate, lactate, phosphate and microcrystalline hydroxyapatite.[1] In one trial, hydroxyapatite raised ionized blood calcium less than citrate or carbonate, and lowered bone turnover to a similar degree.[8]

Products from dolomite, bone meal and natural carbonate had more lead in the past than refined forms. The gap has closed considerably since 2000, but some products can still contain too much, and this is hard to know without independent testing.[9]

What is still unknown

Trials that compared the two forms measured bone density or markers, not fractures.[6][5][8] The largest fracture trial used calcium carbonate.[10]

Sources (10)

  1. Calcium: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements. ods.od.nih.gov
  2. Absorption of calcium as the carbonate and citrate salts, with some observations on method. Heaney RP, Dowell MS, Barger-Lux MJ Osteoporos Int, 1999. PMID 10367025
  3. Absorbability and cost effectiveness in calcium supplementation. Heaney RP, Dowell MS, Bierman J, Hale CA, Bendich A J Am Coll Nutr, 2001. PMID 11444420
  4. Meta-analysis of calcium bioavailability: a comparison of calcium citrate with calcium carbonate. Sakhaee K, Bhuket T, Adams-Huet B, Rao DS Am J Ther, 1999. PMID 11329115
  5. Comparison of the effects of calcium loading with calcium citrate or calcium carbonate on bone turnover in postmenopausal women. Kenny AM, Prestwood KM, Biskup B, Robbins B, Zayas E, Kleppinger A, et al. Osteoporos Int, 2004. PMID 14722627
  6. A controlled trial of the effect of calcium supplementation on bone density in postmenopausal women. Dawson-Hughes B, Dallal GE, Krall EA, Sadowski L, Sahyoun N, Tannenbaum S N Engl J Med, 1990. PMID 2203964
  7. Suppression of parathyroid hormone and bone resorption by calcium carbonate and calcium citrate in postmenopausal women. Thomas SD, Need AG, Tucker G, Slobodian P, O'Loughlin PD, Nordin BE Calcif Tissue Int, 2008. PMID 18553042
  8. Acute and 3-month effects of microcrystalline hydroxyapatite, calcium citrate and calcium carbonate on serum calcium and markers of bone turnover: a randomised controlled trial in postmenopausal women. Bristow SM, Gamble GD, Stewart A, Horne L, House ME, Aati O, et al. Br J Nutr, 2014. PMID 25274192
  9. Lead in Mineral or Multivitamin-Multimineral Products. White CM Ann Pharmacother, 2022. PMID 34096339
  10. Calcium plus vitamin D supplementation and the risk of fractures. Jackson RD, LaCroix AZ, Gass M, Wallace RB, Robbins J, Lewis CE, et al. N Engl J Med, 2006. PMID 16481635

More about calcium

What is calcium?Which foods contain calcium?Does calcium need vitamin D to work?How much calcium can the body absorb at one time?What is calcium used for?What happens if you do not get enough calcium?

For education; not medical advice. Talk to a doctor before you start, stop or change a supplement.