Moderate evidenceVitamins & minerals

Calcium Carbonate

The cheapest and most concentrated calcium salt (40% elemental calcium) and a fast antacid. It is absorbed and demonstrably acts on bone turnover, but supplementing calcium does very little for fracture risk.

Evidence last reviewed: August 2026

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What the science says

Absorbed and biologically active on bone turnover Moderate evidence
In a randomized trial in 100 postmenopausal women, 1 g/day of calcium as carbonate or citrate raised serum calcium for up to 8 hours after each dose and reduced bone resorption markers (CTX) both acutely and over 3 months.
Little to no fracture prevention Strong evidence
A 2026 systematic review of 69 randomized trials (153,902 participants) found little to no effect of calcium supplements on any fracture (RR 0.91, 95% CI 0.81–1.01) or on falls, with moderate-to-high certainty. Combined calcium plus vitamin D reached at most a marginal reduction.

Dosage & safety

Studied dose500–600 mg elemental calcium per dose, taken with food because absorption depends on stomach acid. Total intake from food plus supplements of about 1000–1200 mg/day; more than ~500 mg at once is poorly absorbed.

SafetyConstipation, bloating and acid rebound are common. It repeatedly raises serum calcium through the day, which is the basis of the ongoing debate about cardiovascular risk from calcium supplements. It blocks absorption of levothyroxine, iron, and tetracycline and quinolone antibiotics — separate by at least four hours. People with kidney stones, hypercalcemia or on proton-pump inhibitors (which reduce carbonate absorption) should ask a doctor; getting calcium from food is preferable where possible.

Scientific references

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 — Br J Nutr, 2014
Calcium, vitamin D, or combined supplementation to prevent fractures and falls: systematic review and meta-analysis
Massé O, Mercurio CM, Dupuis S — BMJ, 2026

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