NutriStack
Calcium Carbonate and Iron, timing-sensitive.
Calcium carbonate can reduce iron absorption when taken at the same time. Human studies show calcium can inhibit both heme and non-heme iron absorption in single-meal settings, and calcium carbonate antacid reduced the plasma iron rise in an iron absorption study. This matters most when treating iron deficiency or in people with high iron needs.
One pair, every claim cited. The two substances, the type, the mechanism, the recommendation, and the primary literature.
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At a glance
- Substances
- Calcium Carbonate and Iron
- Pair type
- Timing Sensitive
- Evidence (highest tier)
- Moderate
- Source citations
- 3 sources
- Stack Score effect
- −5 to your Stack Score (per scored timing-sensitive row).
- Scope
- Supplement × Prescription
- Last verified
- May 30, 2026
Timing Sensitive · Moderate evidence
Timing Sensitive
What is happening. Calcium carbonate can reduce iron absorption when taken at the same time. Human studies show calcium can inhibit both heme and non-heme iron absorption in single-meal settings, and calcium carbonate antacid reduced the plasma iron rise in an iron absorption study. This matters most when treating iron deficiency or in people with high iron needs.
Mechanism. Calcium competes with iron uptake and may interfere with mucosal transfer during absorption. Calcium carbonate also raises gastric pH, reducing solubilization of non-heme iron salts needed for duodenal uptake.
Recommendation. Separate iron supplements from calcium carbonate by at least 2 to 4 hours. Take iron with vitamin C or a vitamin C-containing drink if tolerated, and recheck ferritin or CBC as directed. If iron levels are not improving, review calcium carbonate timing with your clinician.
Minimum separation. 120
Sources (3)
- O'Neil-Cutting MA, Crosby WH. The effect of antacids on the absorption of simultaneously ingested iron. JAMA. 1986;255(11):1468-1470. PMID 3005669
- Hallberg L, Brune M, Erlandsson M, Sandberg AS, Rossander-Hulten L. Calcium: effect of different amounts on nonheme- and heme-iron absorption in humans. Am J Clin Nutr. 1991;53(1):112-119. PMID 1984335
- Lynch SR. The effect of calcium on iron absorption. Nutr Res Rev. 2000;13(2):141-158. PMID 19087437
Stack Score
How this pair moves the number.
Effect on the composite score
If both Calcium Carbonate and Iron are in the same stack, this pair applies −5 to your Stack Score (per scored timing-sensitive row).
The full algorithm, the clamping rules, and four worked stacks are documented at /methodology/stack-score.
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