Iron Fortification, Infant Microbiome and Diarrhea Risk

Clinical Overview

This narrative review synthesizes clinical, mechanistic, and epidemiologic data on how oral iron fortification and supplementation (typically 10–12.5 mg Fe/day as ferrous fumarate, electrolytic iron, or NaFeEDTA) affect the gut microbiome and diarrhea in infants and children, mainly in Sub-Saharan Africa. Iron-containing micronutrient powders (MNPs) and supplements effectively reduce iron-deficiency anemia but increase colonic iron because fractional absorption is usually <20%. Across multiple trials, high-dose iron interventions increased fecal enterobacteria, pathogenic Escherichia coli (e.g., 6.0 vs 4.5 log gene copies/g feces, P = 0.029), gut inflammation (fecal calprotectin), and diarrhea or diarrhea-related hospitalizations (risk ratios ~1.1–1.6), especially in low-hygiene settings. review article

What was reviewed and who was studied

The authors reviewed animal models, in vitro colonic fermentation systems, and randomized trials of iron fortification and supplementation in infants and children, primarily in African and other low-resource settings, with limited data from high-income countries. The focus is on iron speciation (ferrous fumarate, NaFeEDTA, electrolytic iron) delivered via MNPs, fortified foods, or supplements, and consequent changes in fecal microbiota composition, pathogenic enterobacteria, short-chain fatty acids, gut inflammation (calprotectin), and clinical diarrhea.

Major findings

This review integrates diverse evidence showing dose- and context-dependent effects of oral iron on the pediatric gut microbiome and diarrheal morbidity.

Finding	Detail
High-dose iron (≈12.5 mg Fe/day) reduces anemia but increases colonic iron	Sprinkles and similar MNPs (12.5 mg Fe as ferrous fumarate) and supplements (10–12.5 mg Fe/day) improve iron status but leave >10 mg Fe/day unabsorbed to the colon when added to low-bioavailability complementary foods.
Iron drives microbiome shifts favoring enterobacteria over bifidobacteria/lactobacilli	In Ivorian children, wheat flour fortification (20 mg Fe/day, 4 days/week) increased fecal enterobacteria, decreased lactobacilli, and raised fecal calprotectin, indicating iron-associated dysbiosis and gut inflammation.
Infant MNPs increase pathogenic Escherichia/Shigella and diarrhea in high-pathogen settings	In Kenyan 6-month-old infants receiving maize porridge with 12.5 mg Fe/day or 2.5 mg Fe/day, iron-containing MNPs increased enterobacteria, Escherichia/Shigella, pathogenic E. coli (6.0 vs 4.5 log gene copies/g, P = 0.029), fecal calprotectin, and diarrhea requiring treatment (27.3% vs 8.3%).
Low-dose NaFeEDTA (2.5 mg Fe/day) is microbiologically safer but less efficacious	MixMe MNP (2.5 mg Fe as NaFeEDTA plus ascorbic acid and phytase) showed poor anemia efficacy in Kenyan infants and did not significantly alter iron biomarkers, suggesting suboptimal absorbed iron dose despite potentially lower colonic iron burden.
Population context modifies microbiome and inflammation responses	In South African schoolchildren with better hygiene, 50 mg Fe supplements (4 days/week) did not significantly alter fecal calprotectin or qPCR-defined microbiota, contrasting with Ivorian data where baseline enterobacteria predominance predicted adverse iron effects.
MNPs modestly but consistently increase diarrhea risk at population level	Large trials in Ghana and Pakistan (12.5 mg Fe/day with or without Zn) and meta-analyses show higher hospitalizations (RR 1.23; 95% CI 1.02–1.49), bloody diarrhea (IRR 1.63; 95% CI 1.12–2.39), severe diarrhea, and small but significant pooled diarrhea risk increases (RR ~1.04–1.15).

Implications for Microbial Metallomics

The review links luminal iron flux to shifts in infant and child gut microbial ecology, pathogenic colonization, and inflammatory tone in iron-deficient and iron-replete hosts.

Concept	Implication
Colonic iron as a growth-limiting micronutrient	Increasing unabsorbed Fe in the colon selectively enriches enterobacteria (including Salmonella, Shigella, pathogenic E. coli) while relatively disadvantaging low-iron–requiring bifidobacteria and lactobacilli, defining an iron-sensitive microbial niche structure in early life.
Iron formulation and speciation (ferrous fumarate vs NaFeEDTA)	Highly bioavailable but lower-dose NaFeEDTA reduces colonic Fe exposure compared with equimolar ferrous salts, suggesting that metallomic speciation and chelation can decouple anemia treatment from pathogenic overgrowth.
Microbiome–inflammation coupling via iron	Iron-driven increases in enterobacteria and Escherichia/Shigella correlate with fecal calprotectin, supporting an iron-dependent axis from microbial community composition to mucosal inflammation and potentially to diarrheal pathogenesis.
SCFA production under iron deprivation	Animal and in vitro models show that iron depletion reduces butyrate and propionate and alters Bacteroides and Roseburia/Eubacterium rectale, highlighting how iron availability shapes microbial metabolic outputs relevant to barrier integrity.
Prebiotic galacto-oligosaccharides as metallomic moderators	Adding galacto-oligosaccharides to a 5 mg Fe MNP mitigated iron-related microbiome disturbances while maintaining anemia improvement, indicating that manipulating carbon substrates can buffer iron’s ecological perturbations.
Hygiene and baseline microbiome as metallomic context	Divergent responses between high-pathogen, low-hygiene African settings and better-resourced environments show that background microbiome composition and exposure burden condition iron–microbe–host interactions, which must be considered when designing iron delivery strategies.

Limitations

The review depends mainly on a limited number of randomized trials in specific African and Asian settings and several meta-analyses with heterogeneous designs, iron doses, and formulations. Microbiome assessments vary (qPCR vs 16S sequencing), and mechanistic studies rely on rodent and in vitro models that omit host factors such as mucosal immunity. Data on iron-replete infants in high-resource settings are sparse, and redox speciation beyond compound names is not systematically characterized.

Future perspectives

Next steps include dose–response trials of lower iron loads using highly bioavailable formulations (e.g., NaFeEDTA combinations) to define the minimal effective absorbed iron dose that prevents iron-deficiency anemia while minimizing colonic Fe. Studies should integrate high-resolution microbiome profiling, SCFA quantification, calprotectin, and clinical diarrhea endpoints in both low- and high-income settings. Factorial designs adding prebiotics, probiotics, or lactoferrin could test strategies that stabilize bifidobacteria and lactobacilli under increased iron supply. Longer-term follow-up is needed to assess growth, neurodevelopment, and potential persistence of iron-induced microbiome alterations beyond infancy.

Key takeaways for Researchers and Clinicians

This review centers on infants and young children, primarily in Sub-Saharan Africa, receiving oral iron via MNPs, fortified foods, or supplements. Iron (as ferrous fumarate, electrolytic iron, or NaFeEDTA) emerges as the key metal shaping the early-life colonic ecosystem, with higher doses consistently associated with increased enterobacteria, pathogenic Escherichia/Shigella, elevated fecal calprotectin, and modest but measurable elevations in diarrhea and diarrhea-related hospitalizations.

Methodologically, integrating fecal microbiome profiling (qPCR, 16S sequencing), calprotectin, and diarrheal outcomes offers a practical template for metallomic safety evaluation of iron interventions. Clinically, the take-home is that iron remains essential for preventing iron-deficiency anemia but should be delivered in formulations and doses that limit colonic Fe exposure and, where possible, be paired with prebiotics to maintain a protective bifidobacterial–lactobacillary barrier. For microbial metallomics, the translational hook is clear: the infant gut acts as an iron-tunable ecosystem, where iron speciation and dose directly modulate pathogen–commensal balance, inflammation, and potentially survival.

Citation

Paganini D, Zimmermann MB. The effects of iron fortification and supplementation on the gut microbiome and diarrhea in infants and children: a review. Am J Clin Nutr. 2017;106(Suppl):1688S–1693S. doi:10.3945/ajcn.117.156067