Is Calcium Carbonate Constipating?
Yes, calcium carbonate commonly causes constipation and is well-documented as a side effect in both clinical guidelines and FDA labeling. 1, 2
Evidence from Guidelines and FDA Labeling
The New England Journal of Medicine clinical practice guidelines explicitly state that constipation and bloating are common side effects of calcium supplements, particularly calcium carbonate. 1 The FDA drug label for calcium carbonate directly warns that "constipation may occur" with use of this product. 2
Mechanism and Clinical Significance
Calcium carbonate contains 40% elemental calcium, the highest concentration among calcium supplements, and must be taken with meals for optimal absorption due to its dependence on gastric acidity. 1 This formulation is specifically noted to "cause constipation" compared to other calcium preparations. 1
The American College of Gastroenterology and American Gastroenterological Association both recognize that calcium carbonate is more likely to cause constipation, bloating, and gastrointestinal symptoms compared to alternative calcium supplements like calcium citrate. 3
Contradictory Research Evidence
One small pilot study (n=27 healthy women) found that 500 mg/day of calcium carbonate did not affect stool frequency or form over 8 weeks. 4 However, this study used a relatively low dose and short duration in a highly selected population of healthy women, limiting its generalizability.
Interestingly, one study in intestinal bypass patients with diarrhea found that calcium carbonate (2400-3600 mg Ca²⁺ daily) actually reduced bowel frequency by 49% and had a "constipating effect." 5 This paradoxical finding supports the constipating properties of calcium carbonate, even though it was studied in a different clinical context.
Clinical Implications
- Constipation risk is dose-dependent: Higher doses of calcium carbonate are more likely to cause constipation. 2, 6
- Individual variation exists: While constipation is common, not all patients experience it. In one large trial, 13.4% of calcium-treated patients reported constipation versus 9.1% in the placebo group. 6
- Alternative formulations available: Calcium citrate provides less elemental calcium (21%) but has a lower risk of gastrointestinal side effects and may be preferred for patients prone to constipation. 1, 3
Practical Recommendations
For patients who develop constipation on calcium carbonate, consider switching to calcium citrate, which is less dependent on gastric acidity and associated with fewer gastrointestinal symptoms. 3 Calcium citrate can be taken with or without food, providing greater dosing flexibility. 7
If cost is a barrier (calcium carbonate is the least expensive option), advise patients to take it with meals, ensure adequate hydration, maintain dietary fiber intake, and monitor for constipation symptoms. 1, 3