Calcium carbonate

Limits on what this shows

• Why no grade: Dual condition met: no Tier B or C source for oral calcium carbonate was opened to support a yellow, and no circulating consumer claim of antacid mitochondrial damage was found. A disclosed gap, not a finding of safety. Re-check when PubMed access is restored.

What was searched

Web search "calcium carbonate antacid oxidative stress OR mitochondrial OR ATP effect study gastric mucosa antioxidant": results were general calcium and mitochondria physiology papers and nanoparticle studies, none on oral antacid use. PubMed listing "calcium carbonate antacid mitochondrial OR ATP OR respiration": 10 results, none concerned oral calcium carbonate antacid use (cell-metabolism and mitochondrial methods papers). Crossref bibliographic query "calcium carbonate antacid oxidative stress mitochondrial": mitochondrial calcium in neurodegeneration, nano-cage cancer therapy and an emergency-medicine item, none relevant.

Channel outcomes

PubMed listing: opened once, later queries 429. Web search: budget exhausted partway through. AJP Cell physiology review: 403. PMC page: bot-verification. Intracellular calcium overload is a recognized mitochondrial concept, but no source opened links oral antacid calcium intake to it, and serum calcium is homeostatically regulated outside hypercalcemia. Mixed result, a clean zero on the channels reached plus a tooling limit. Repair pass 2026-10-04: web search returned a mix of nano-calcium carbonate and generic calcium physiology (nanoparticle and cell-line papers excluded as not antacid exposure); a Europe PMC listing for "calcium carbonate antacid mitochondrial" returned 20 results, none on oral calcium carbonate exposure and mitochondrial structure or function (topics were magnesium, aluminium, alkaline water, nanomedicine); a Europe PMC human-restricted mitochondria query returned 400, and further Europe PMC, Crossref and OpenAlex queries returned 429 and were not retried. Calcium carbonate and mitochondrial calcium uptake was treated as normal physiology, not a drug effect, and not graded.

Limits on what this shows

• Null findings: Total antioxidant capacity unchanged [29] and total glutathione unchanged [31] in the respective trials. Hahn and colleagues 2008, Clin Biochem [32] (75 postmenopausal women grouped by bone density and calcium supplementation status; abstract-level): catalase and SOD activity did not differ among groups, and the authors state calcium supplementation had no effect on the antioxidant enzyme parameters measured; the calcium form, dose and duration were not captured, malondialdehyde was not reported, and this is Tier C for calcium carbonate.
• At-risk subgroups: None identified. Both trials were in pregnancy; no antacid-pattern or non-pregnant data were found.

Regulatory status

No redox or antioxidant statement in the Drug Facts labels [1][2].

Population evidence

Asemi, Karamali and Esmaillzadeh 2014, Diabetologia [29] (randomized placebo-controlled trial, 56 women with gestational diabetes at 24 to 28 weeks, 6 weeks; calcium as carbonate 1,000 mg per day plus two 50,000 U vitamin D3 doses; abstract-level, read via a summarizing fetch tool): glutathione rose (+51.14 versus -47.27 micromol/L, p=0.03) and malondialdehyde was lower than placebo (+0.06 versus +0.93 micromol/L, p=0.03); total antioxidant capacity did not change significantly. Asemi, Samimi and colleagues 2016, Int J Prev Med [31] (randomized double-blind placebo-controlled trial, 42 healthy pregnant women completing, 9 weeks; calcium carbonate 500 mg calcium plus vitamin D3 200 IU per day; abstract-level, via a fetch tool, authors beyond the first two and DOI not captured): plasma total antioxidant capacity rose (89.3 +/- 118.0 versus -9.4 +/- 164.9 mmol/L, p=0.03); total glutathione did not differ (p=0.10); hs-CRP fell. Calcium carbonate is in both cohorts, as one of two co-administered agents. The 2016 trial in women at risk for pre-eclampsia that the earlier version of this dossier flagged (Samimi and colleagues, J Hum Nutr Diet) carries an Expression of Concern on the Wiley site and was not opened (403), so no figures from it are used [35].

Mechanism

None named for calcium carbonate. Vitamin D3 is a co-intervention in every trial opened and is the more plausible driver of any redox change; the authors' interpretation was not captured in the retrieved text.

Why yellow

Calcium carbonate was in the cohort of two opened trials that measured redox markers, which keeps this out of the ungraded section, but the combination exposure, the pregnancy setting and the integrity notice [30] limit it to a reverse-direction context flag. No Tier A source isolates calcium carbonate. Nanoparticle and cell-line calcium carbonate papers seen in listings were excluded as not antacid exposure.

Counter-evidence held on record

Counter-evidence and confounding Vitamin D3 co-administration, small samples, surrogate markers with no clinical endpoint, a different dose and duration from antacid use, and journal notices on trial integrity [30] (the Diabetologia trial's dates and blinding were questioned; an ethics investigation found reporting errors and undisclosed company involvement but insufficient evidence of fabrication). The null in [32] points the other way. Framing caution Not a deficiency claim and not an antioxidant benefit claim. It does not suggest taking calcium carbonate for redox effects, and the exposure studied was a supplement taken daily with vitamin D, not antacid use.

Limits on what this shows

• Null findings: No change in iron status at 6 months [11]. Calcium citrate 450 mg, n=15 and n=12: non-heme iron reductions of 20 to 25% across meal types were not significant, while heme iron uptake fell by 20% (49% to 40%, P=0.02) [12]. The authors note, citing a different study, that citrate was less inhibitory than carbonate or phosphate in a high-bioavailability meal [12] (secondhand). Tier C for this drug.
• Dose and pattern context: The trial dose of 1200 mg per day equals four 300 mg chewables per day by simple arithmetic from the label value [1], given at every meal for months. Symptom-driven antacid use is usually intermittent, so exposure differs. No source read gave a validated separation interval for iron.
• At-risk subgroups: Not defined by the sources read. The studies were in iron-replete adults and do not address iron-deficient people, people with high iron needs, or people on diets very low in bioavailable iron.

Regulatory status

Drug Facts labels warn only of interaction with prescription drugs and do not name iron [1][2]. StatPearls lists iron among agents whose absorption calcium carbonate reduces [10].

Population evidence

Minihane and Fairweather-Tait 1998 [11] (read via a summarizing fetch tool; abstract-level): randomized controlled trial in two phases. Short-term phase, 14 volunteers: calcium carbonate supplements, 1200 mg calcium per day (400 mg per meal), cut non-heme iron absorption from 15.8% to 4.7% (71% reduction, p less than 0.001). Long-term phase, 11 supplemented and 13 controls over 6 months: no change in any hematologic index (hemoglobin, hematocrit, zinc protoporphyrin, plasma ferritin). Reviews: Lynch 2000 [13] reports that calcium salts reduce both heme and non-heme iron absorption in an amount-dependent way, that single-meal studies exaggerate the effect, and that supplementation has little effect on iron absorption over multiple meals unless habitual calcium intake is very low. Fairweather-Tait 1995 [14] reports dose-related inhibition (75 to 80% within 300 to 400 mg calcium) and no significant ferritin effect of 500 mg calcium daily for 12 weeks.

Mechanism

Present but not resolved. Lynch 2000 states the mechanism has not been elucidated [13]. A study using calcium citrate suggests inhibition of initial mucosal uptake of heme iron, possibly via DMT-1 [12]. That study did not use calcium carbonate, so the mechanism is not established for the carbonate salt.

Counter-evidence held on record

Counter-evidence and confounding Reduced absorption of a test dose is a functional effect, not a deficiency. The 6-month result is a direct null on iron status. Anemia is an outcome and iron deficiency is a nutrient state, and neither was shown here (Hard Rule 2). The shared study [11] is also the null source for the Oxygen delivery/utilization entry. Framing caution This is a co-ingestion absorption effect. It is not a finding that antacid use causes iron deficiency or anemia, and it does not call for iron supplementation, which has its own excess risks.

Limits on what this shows

• Null findings: No copper deficiency or low-copper measurement with calcium carbonate was found. Other copper-calcium hits in search results were Tier D pages (not opened).

Regulatory status

Not mentioned in the Drug Facts labels [1][2].

Population evidence

Kies and Harms 1989 [19] (book chapter, abstract-level summary via a fetch tool; sample size and calcium form not captured): calcium supplements depressed fecal copper losses and improved body copper retention, whereas magnesium and selenium increased apparent fecal copper loss. The authors suggest calcium may neutralize the inhibitory effect of high dietary ascorbic acid on copper absorption. Tier C for this drug, since the calcium form is not stated and the study is not an antacid study.

Mechanism

No depleting mechanism named. A hypothesized interaction with ascorbic acid is described in the opposite direction [19].

Counter-evidence held on record

Counter-evidence The single study is old, its sample size was not captured, and it is not carbonate-specific. It argues against a depletion claim but does not establish a benefit. Framing caution Not a deficiency claim and not a benefit claim. Copper status should not be changed on the basis of this entry.

Limits on what this shows

• Null findings: The rodent changes were metabolically neutral or favorable in direction as summarized, not harmful. No human trial was found.

Regulatory status

No microbiome statement in the Drug Facts labels [1][2].

Population evidence

Animal studies (Tier B), read only as one-line summaries from a PubMed search listing, abstracts not opened [28]: Cha and colleagues 2020, mice on a high-fat diet given calcium carbonate or citrate at two levels; both forms increased propionate production and GLP-1 expression, with citrate showing greater metabolic benefit. Wang and colleagues 2021, 30 ovariectomized rats given calcium carbonate for 12 weeks; Firmicutes increased and Bacteroidetes decreased. He and colleagues 2022, 49 ovariectomized rats on dairy calcium or calcium carbonate; calcium increased intestinal short-chain fatty acids. In humans, a hemodialysis microbiome study recorded calcium carbonate use in 72 to 87% of every group (H2 blocker users, PPI users and controls) and did not adjust for it [25], so it cannot isolate calcium carbonate. A piglet study used calcium carbonate as the control [27].

Mechanism

Plausible but unstudied for antacid use. The drug raises gastric pH [10], but whether transient neutralization changes the microbiota has not been studied in any source opened. Dietary calcium effects on SCFA in rodents are a different exposure.

Counter-evidence held on record

Counter-evidence The large PPI cohort finding that is often called an "antacid" effect did not study calcium carbonate [26] and is not borrowed (see Fails verification). Framing caution Do not extend PPI microbiome findings to calcium carbonate. This entry is not a claim of dysbiosis or infection risk.

Limits on what this shows

• Why no grade: Dual condition met: no direct Tier B or C evidence of impaired oxygen delivery, and no circulating claim of calcium carbonate lowering oxygen-carrying capacity was found; the nulls above are not a signal. Any effect would run through the iron entry and is not double-counted (see Overlap map). Anemia is an outcome, not a nutrient (Hard Rule 2).

What was searched

Measured parameters: in the 6-month calcium carbonate trial, hemoglobin, hematocrit, zinc protoporphyrin and plasma ferritin did not change in 11 supplemented adults versus 13 controls [11] (iron-replete adults, supplement-level dose). PubMed listing "calcium carbonate antacid hemoglobin OR anemia OR methemoglobin": 429, not read. No methemoglobinemia or hemolysis signal appears in the Drug Facts labels [1][2].

Channel outcomes

PubMed hemoglobin and anemia query blocked by rate limiting; web search budget exhausted. A null from [11] is the only calcium carbonate-specific measured oxygen-carrying data. Repair pass 2026-10-04: a pregnancy trial pair (India and Tanzania; 500 versus 1,500 mg daily calcium, each about 11,000 participants, hemoglobin measured in 8,953 and 8,496) found no difference in hemoglobin at 32 weeks (mean difference 0.01 g/dL, 95% CI -0.03 to 0.04; and -0.02 g/dL, 95% CI -0.07 to 0.03) or in anemia (RR 1.01 and 1.00) [33]; the calcium form was not stated in the abstract opened, the comparison is dose against dose rather than against no calcium, and it is Tier C for this drug. A systematic review and dose-response meta-analysis of 30 randomized trials and crossover studies, whose calcium forms included the carbonate among citrate, citrate-malate, phosphate, hydroxyapatite and dietary sources, found no significant hemoglobin reduction (0.44 g/L, 95% CI -0.90 to 1.78) and concluded effects on longer-term iron status markers were unlikely to be biologically significant [34]; it is not stratified to the carbonate, so it is Tier C. These are null findings and do not support a yellow grade. Tooling-limited for case-report searches (web search budget exhausted before a methemoglobinemia or hemolysis query could run).

Limits on what this shows

• Why no grade: Dual condition met: no Tier B or C evidence and no circulating "low energy" claim for calcium carbonate found. Not a finding of safety. Kept independent of Mitochondrial burden and Oxygen delivery/utilization (Hard Rule 7).

What was searched

Same web-search phrasing and PubMed "mitochondrial OR ATP OR respiration" listing as Mitochondrial burden: none concerned oral calcium carbonate antacid use. One listing title, "Calcium-regulation of mitochondrial respiration maintains ATP homeostasis" (J Neurosci 2013), concerns physiological intracellular calcium and was not linked to antacid intake; it was not opened.

Channel outcomes

Same as Mitochondrial burden, including the 2026-10-04 repair pass (no relevant Europe PMC or web search results; later queries rate limited). Tooling-limited for database searches; clean zero where reached.