Form Comparison

Calcium Carbonate vs Calcium Citrate

How calcium carbonate and calcium citrate differ in elemental content, absorption, tolerance, and cost.

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Quick verdict. Taken with a meal, carbonate absorbs about as well as citrate and delivers the most calcium per tablet (40% vs 21%) at the lowest cost. Citrate dissolves without stomach acid, so it is the better choice on PPIs, after gastric bypass, or between meals, and it causes less constipation. Choose carbonate with food if your stomach acid is normal and citrate otherwise, keeping each dose at 500-600 mg elemental or less.

Side by Side

Calcium Carbonate Calcium Citrate
Elemental content 40% elemental calcium 21.1% elemental calcium
Absorption Moderate Moderate
How it is absorbed Needs stomach acid to dissolve. Taken with a meal it is absorbed about as well as citrate; fasting or on acid suppression, absorption falls. Dissolves without stomach acid, so absorption holds up when fasting, on PPIs, and after gastric bypass.
Typical dose 500-600 mg elemental per dose, with food 200-500 mg elemental per dose
Common uses general supplementation, antacid low stomach acid, after bariatric surgery
Side effects Constipation, gas, bloating. Less constipation than carbonate.
Cost Low Moderate
Also called calcium from oyster shell, calcified seaweed calcium citrate malate (related)
Labels in index 1,075 1,061

When to Choose Calcium Carbonate

Choose calcium carbonate when you take it with meals and do not use acid-suppressing drugs. The most calcium per tablet and the lowest cost. Oyster shell and algae sources are still mostly carbonate.

Calcium Carbonate

The most calcium per tablet at the lowest cost. Works well with meals in people with normal stomach acid.

What to look for: 500-600 mg elemental calcium per serving, taken with food. Expect some constipation.

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When to Choose Calcium Citrate

Choose calcium citrate when you take PPIs, have had gastric bypass, or take calcium between meals. About 21% calcium, so tablets are larger or more numerous for the same dose.

Calcium Citrate

The form that does not depend on stomach acid. Bigger tablets, gentler on the gut.

What to look for: Elemental calcium per serving stated; 200-500 mg per dose. The choice with PPIs or after bariatric surgery.

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Products on Titrate

In Titrate's index of NIH label data, 1,075 labels named as calcium products list calcium carbonate and 1,061 list calcium citrate.

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Calcium does not have its own search category yet. Each product above links to its full label in the NIH database.

Related

References

NIH Office of Dietary Supplements. Calcium: Fact Sheet for Health Professionals.

Recker RR. Calcium absorption and achlorhydria. N Engl J Med. 1985;313(2):70-73.

Heaney RP, Dowell MS, Bierman J, Hale CA, Bendich A. Absorbability and cost effectiveness in calcium supplementation. J Am Coll Nutr. 2001;20(3):239-246.

Jackson RD, LaCroix AZ, Gass M, et al. Calcium plus vitamin D supplementation and the risk of fractures. N Engl J Med. 2006;354(7):669-683.

Bolland MJ, Avenell A, Baron JA, et al. Effect of calcium supplements on risk of myocardial infarction and cardiovascular events: meta-analysis. BMJ. 2010;341:c3691.

Bolland MJ, Grey A, Avenell A, Gamble GD, Reid IR. Calcium supplements with or without vitamin D and risk of cardiovascular events: reanalysis of the Women's Health Initiative limited access dataset and meta-analysis. BMJ. 2011;342:d2040.

Kopecky SL, Bauer DC, Gulati M, et al. Lack of evidence linking calcium with or without vitamin D supplementation to cardiovascular disease in generally healthy adults: a clinical guideline from the National Osteoporosis Foundation and the American Society for Preventive Cardiology. Ann Intern Med. 2016;165(12):867-868.