The cheapest and most concentrated calcium salt, supplying about 40% elemental calcium by weight. It is the calcium in chalk, in limestone and in most antacid tablets.
Calcium supplements are where the "correct a deficiency, do not add to sufficiency" rule has its sharpest edge, because both halves are well documented. In older adults with low calcium and vitamin D intake — the classic trial population was French nursing home residents — calcium with vitamin D reduced hip fractures. In community-dwelling adults with adequate intake, supplementation has not reduced fractures, and the US Preventive Services Task Force recommends against low-dose supplementation for that purpose. Against that thin benefit sit real harms: a 17% increase in kidney stones in the Women's Health Initiative, and a contested but persistent signal of increased myocardial infarction in meta-analyses of calcium supplements. Dietary calcium carries none of these findings.
Sold as
Tablet, Chewable tablet, Antacid tablet, Powder, Combination with vitamin D
Regulation: The RDA is 1,000 mg/day for most adults and 1,200 mg/day for women over 50 and everyone over 70, with a Tolerable Upper Intake Level of 2,500 mg/day (2,000 mg/day above age 50) from all sources combined. Widely sold over the counter as both a supplement and an antacid, which makes double-counting easy.
What the research used
The amounts, forms and durations published trials of Calcium carbonate actually administered.
These are the amounts published trials administered — not a recommendation, and not a dose to copy. Trial participants were screened, supervised and given a characterised preparation. What is in a retail bottle is frequently not what was studied, and the amount that suits one person is not general advice.
Studied for Recommended Dietary Allowance, adults
1000–1200 mg
Strong
From all sources — food included. Most people eating dairy already meet a substantial part of this, which is why supplementing the full RDA on top of a normal diet is a common way to overshoot.
Studied for Tolerable Upper Intake Level, adults
2000–2500 mg
Strong
2,500 mg/day for adults up to 50 and 2,000 mg/day above that. Set on kidney stones and hypercalcaemia.
Studied for Fracture trials in older adults with low intake
1200 mg
Strong
With 800 IU vitamin D3, daily · 18 months–3 years
The regimen used in institutionalised older women with low baseline calcium and vitamin D, where hip fractures fell. Trials in replete community-dwelling adults using similar doses have not reproduced this.
Studied for Absorption per dose
500 mg
Moderate
Fractional absorption falls as the dose rises, so amounts above roughly 500 mg of elemental calcium at once are absorbed inefficiently. Splitting is standard practice on that basis.
This is general information about food, not medical advice. It cannot diagnose, treat or replace guidance from a clinician who knows your history.
When it was taken
Each window says whether a trial compared it against an alternative, or whether it is convention and pharmacology. Most timing advice is the second kind.
Each timing below was actually compared against an alternative in a trial, which is unusual.
🍽️With food
Strong
Calcium carbonate needs stomach acid to dissolve, and absorption is substantially lower when taken fasted — measurably so in people with reduced gastric acid, where it approaches negligible. Taking it with a meal is one of the few timing rules in this catalogue with direct experimental support.
Timing was tested. A trial compared this window against another and reported a difference.
🔀Split through the day
Moderate
Absorption efficiency drops above about 500 mg elemental calcium per dose, so daily amounts above that are conventionally split. This follows from absorption studies rather than an outcome trial.
Timing was tested. A trial compared this window against another and reported a difference.
The evidence in depth
Best supported: Moderate
What each association actually rests on, and the papers behind it. A rating describes the state of the research, not how promising the supplement sounds. How we rate evidence.
Calcium with vitamin D reduced hip and non-vertebral fractures in older adults with low baseline intake, reproduced across trials and meta-analyses. In community-dwelling adults with adequate calcium the same regimens have not reduced fractures, and calcium alone has a much weaker effect than calcium with vitamin D. Both findings are well established and they apply to different people.
What may be going on: Calcium is the mineral substrate of bone. When dietary intake is insufficient, parathyroid hormone rises and mobilises calcium from the skeleton to maintain blood levels.
Moderate evidence. Several human studies point the same way, with real caveats.
Meta-analyses find a small reduction in blood pressure with calcium supplementation, on the order of a couple of millimetres of mercury, largest in people with low calcium intake. The clinical relevance of a change that size in isolation is modest.
Limited evidence. Early human work, or mostly lab and animal research.
Calcium supplements increased kidney stone incidence by around 17% in the Women's Health Initiative, while dietary calcium is associated with fewer stones because it binds oxalate in the gut. The direction of effect depends on whether the calcium arrives with food or as a separate tablet.
Not enough evidence. Not enough human research to say anything useful yet.
Deep research mode adds each paper’s own conclusion and stated limitations.
Things to know
Interactions, contraindications and dose limits, most serious first. Concentrated extracts interact with medication in ways the whole plant does not, so this section carries more weight here than it does on a food page.
Too much
Kidney stones
The Women's Health Initiative found a significant increase in kidney stones among women taking calcium with vitamin D. Anyone with a history of calcium-oxalate stones is generally advised to get calcium from food, where it binds oxalate in the gut rather than raising urinary calcium.
Too much
A contested but persistent cardiovascular signal
Meta-analyses of calcium supplement trials have reported roughly a 20–30% relative increase in myocardial infarction, a finding disputed by other analyses and by expert panels that reviewed the same data and concluded there was no clear risk. It remains unresolved, applies to supplements rather than dietary calcium, and is a reason to supplement only the gap between diet and requirement.
Interaction
Blocks absorption of several important medicines
Interacts withLevothyroxine, tetracyclines, fluoroquinolones, bisphosphonates, iron supplements
Calcium substantially reduces absorption of levothyroxine, tetracycline and fluoroquinolone antibiotics, bisphosphonates and iron. Standard practice is to separate them by at least four hours, and this is a common cause of unexplained treatment failure — particularly with thyroid replacement.
Too much
Milk-alkali syndrome
Large intakes of calcium carbonate, particularly as antacids alongside supplements, can cause hypercalcaemia, alkalosis and kidney injury. Cases have re-emerged with the popularity of calcium supplements and over-the-counter antacid use together.
Worth knowing
Needs stomach acid, which many people do not have
Absorption of the carbonate salt is poor in people on proton pump inhibitors or with achlorhydria, which is common in older adults — the group most often advised to take calcium. Calcium citrate does not have this problem.
Worth knowing
Constipation and bloating
Constipation, gas and bloating are the most common reasons people stop taking calcium carbonate, and are more pronounced than with citrate.
In the food catalogue
The whole-food form, where one exists. The evidence for a concentrated extract rarely transfers to the food, or the other way round — these are cross-links, not equivalents.