Vitamin C,
does it really help with Isotope-measured fractional iron absorption in nonanemic women with low iron stores?

Useful facts when choosing a product

• The tested vitamin C was single-ingredient food-grade L-ascorbic acid (Sigma Aldrich); manufacturing origin, lot and exact purity were not reported. [S01: Methods 2.3]
• The iron capsule was Ferrum Hausmann 100 mg Kaps (Vifor). Naming a brand does not establish product donation or manufacturer funding; product provision was not reported. [S01: Methods 2.3, Acknowledgments]
• The 100 mg refers to elemental iron in the capsule. Total elemental iron including the reported 3 mg label was not separately tabulated, so absolute absorbed iron in mg was not invented. [S01: Methods 2.3; S17: registry-data mirror]
• Baseline vitamin C status, habitual diet and total vitamin C intake were not reported. This is not classified as a vitamin C deficiency-correction trial. [S01: Methods 2.1–2.5]

What the research actually shows

The difference between the condition medians is 6.1 percentage points, and their ratio is 1.296, approximately 29.6% higher. These are descriptive comparisons of marginal medians, not the mean or median individual change or an adjusted crossover treatment estimate. The IQRs are 19.1–32.6% around the 26.7% median and 12.3–25.5% around the 20.6% median; they are not confidence intervals. A paired-effect CI, SD and SE were not verified. [S01: Results 3; CALC01] The question is locked to adding only ascorbic acid to otherwise identical non-heme iron exposure in the same participants. Erythrocyte incorporation of stable 54Fe, 57Fe and 58Fe was measured, then converted to absorption using isotope dilution and an assumed 80% incorporation fraction. Serum iron or hemoglobin changes were not substituted for fractional absorption. [S01: Methods 2.3; S16: section 7.4, pp.37–38] Eligibility was restricted to women aged 18–45 years with ferritin ≤30 μg/L, Hb >12 g/dL and CRP <5 mg/L. Of 35 enrolled, 34 completed all 6 conditions. At 07:00–09:00 after an overnight fast, participants received the same ferrous fumarate capsule (100 mg elemental iron), reported 3 mg isotopic label and 200 mL water; the test condition added 80 mg food-grade L-ascorbic acid. The author-reported molar ratio was 0.24:1. Methods specify blood sampling on days 22/43, with isotope-specific incorporation integrated 17–21 days after dosing. [S01: Methods 2.1–2.3, Results 3]

Why this is classified as C (50)

C / 50. The seven axes are B·S·R1·I2·EX·CX·B2 (claim, endpoint, replication, independence, effect, precision, bias). Isotope-derived absorption is a surrogate. Academic funding is documented, but only 1 fully verified directly comparable trial family is decisive. Medians/IQR and limited P reporting cannot yield a valid standardized effect or paired CI, supporting EX(a)/CX. The supplied rubric counts n<200 and completer per-protocol analysis as B2. I2 is the sole strength; the fixed C/1 anchor gives 50. The score is neither a treatment-success probability nor an official GRADE score. [S01; supplied rubric/calculator]

Counterpoint. Absence of registration is not asserted. The printed NCT04074707 link appears to concern pregnancy, but a registry-data mirror identified NCT05414474 with matching 6 conditions, investigators and enrollment. Official history was inaccessible, and discrepancies in recruitment year, blood sampling day and registry versus report eligibility remain unresolved. Condition-specific adverse-event counts were not reported. [S01: Methods 2.1–2.5, Figure 1, Results 3; S04; S17]

Rejudgment record. Supplied calculator C agrees with assigned C; fixed score anchor agrees. — Supplied rubric and grade calculator used unmodified.

Review performed and remaining limitations

Absence of registration is not asserted. The printed NCT04074707 link appears to concern pregnancy, but a registry-data mirror identified NCT05414474 with matching 6 conditions, investigators and enrollment. Official history was inaccessible, and discrepancies in recruitment year, blood sampling day and registry versus report eligibility remain unresolved. Condition-specific adverse-event counts were not reported. [S01: Methods 2.1–2.5, Figure 1, Results 3; S04; S17]

Search scope and limitations. 2026-09-16; 59 general web queries and one failed native PubMed query. Direct openings and failures logged separately. No exhaustive-search assurance.

Evidence Table

Receipt — 22 References

Evidence access cutoff: 2026-09-16. Each citation states its actual access level (full-text transcription, abstract, index, or other) and remaining limitations. Bibliographic checking does not certify the study results or treatment efficacy.

Correction log — 6

Corrections applied to this verdict, in chronological order. Changes are logged, not erased.

• 2026-09-16 · Boundary correction — Did not label 907 a duplicate merely because vitamin C and oral route match.
• 2026-09-16 · Estimand guard — Kept 6.1 percentage points descriptive; did not invent paired CI from IQR.
• 2026-09-16 · Registration linkage correction — Separated the pregnancy link from probable matching NCT05414474 mirror; no assertion of absent registration.
• 2026-09-16 · Conflicting source values retained — Retained recruitment-year, blood-day, eligibility, coffee-P and reference-median discrepancies.
• 2026-09-16 · Unmeasured extrapolation removed — Did not create orange-juice-alone, absolute absorbed-mg, optimal-dose or clinical-health results.
• 2026-09-16 · Supporting-file URL corrected — Used the actual publisher download link verified by final click; retained the 403 failure. No numeric or grading change. (grade C unchanged)

What this document does and does not do

Chamgap is an information source. It reports what research has and has not confirmed; it does not tell readers what to take or buy. That decision belongs to readers and, when needed, medical or legal professionals. This verdict reflects literature available up to the search date and may change as new research appears. Nothing here is medical advice.