PubMed: https://pubmed.ncbi.nlm.nih.gov/25143342/
What the study examined
Jaeggi, Zimmermann and colleagues at ETH Zurich, working with partners in Kenya and the Netherlands, ran two linked double-blind randomized controlled trials in a rural Kenyan population where anemia and iron deficiency are common. A total of 115 infants, weaned at around six months of age, consumed a maize-based complementary porridge each day for four months.
The porridge was fortified in the home using a micronutrient powder (MNP). In the trials infants received either an iron-containing MNP or an otherwise identical MNP without iron. Two iron forms and doses were tested: 2.5 mg of iron as sodium iron EDTA (NaFeEDTA) and 12.5 mg of iron as ferrous fumarate. Randomization and blinding allowed the effect of iron itself to be isolated from the rest of the micronutrient mix.
The primary outcome was gut microbiome composition, characterized with both 16S rRNA pyrosequencing and targeted real-time quantitative PCR (qPCR) of key bacterial groups. Secondary outcomes included fecal calprotectin as a biomarker of intestinal inflammation and the incidence of diarrhea during the intervention.
The key findings
At baseline these infants had a bifidobacteria-rich microbiome typical of breastfed infants, with Bifidobacteriaceae making up roughly 63% of total 16S rRNA sequences. This composition is generally considered protective against enteric pathogens.
Iron fortification shifted that balance. Iron-containing MNPs significantly increased the abundance of enterobacteria, particularly Escherichia/Shigella, including pathogenic E. coli strains (p=0.029), and raised the enterobacteria-to-bifidobacteria ratio (p=0.020). Clostridium was also increased, while the relative dominance of protective bifidobacteria declined.
Crucially, the microbial shift was accompanied by host inflammation. Fecal calprotectin, a neutrophil-derived marker of intestinal inflammation, rose significantly in the iron groups (p=0.002). Diarrhea incidence was numerically higher with the 12.5 mg ferrous fumarate dose (27.3% vs 8.3% in controls), a difference that did not reach statistical significance in this sample (p=0.092) but pointed in the same direction as the microbiome and inflammation data.
The mechanism: unabsorbed iron in the colon
Iron absorption in the upper small intestine is incomplete, and in iron-replete or inflamed states it is downregulated further, so a large fraction of fortificant iron passes into the colon. There it becomes a shared resource that the resident microbiota compete for. This is a problem of nutritional immunity in reverse: the infant gut normally withholds free iron to restrain pathogens, and flooding the lumen with bioavailable iron relaxes that restraint.
Many enteric pathogens, including pathogenic E. coli, Salmonella and Shigella, are avid iron scavengers equipped with high-affinity siderophore systems and other metallophore machinery that let them capture luminal iron efficiently. Beneficial bifidobacteria and lactobacilli have comparatively low iron requirements and lack these systems, so added iron tends to favor the potential pathogens over the commensals.
The resulting bloom of enterobacteria, together with the loss of bifidobacterial dominance, is thought to compromise colonization resistance and irritate the gut epithelium, which is consistent with the observed rise in fecal calprotectin. In short, a dietary metal acted as a selective growth factor that reorganized the community and triggered a measurable inflammatory response in the host.
How it fits the metal-microbiome-disease axis
This trial is one of the clearest human demonstrations of the first two links in the metal-microbiome-disease axis operating together within a single study: a change in metal exposure (iron intake) drives a change in the microbiome (enterobacterial bloom, bifidobacterial loss), and that dysbiosis is coupled to a disease-relevant readout (intestinal inflammation measured by calprotectin, with a signal toward diarrhea).
The example is instructive precisely because iron is an essential nutrient rather than a toxic heavy metal. It shows that the axis is about metal availability and microbial competition for it, not simply about toxicity. The same siderophore-mediated competition that a fortificant iron dose exploits is the mechanism pathogens use to overcome host metal withholding during infection.
The findings do not argue against treating iron deficiency, which carries serious risks of its own. Rather, they underscore why dose, chemical form and delivery matter, and they have motivated work on lower doses, alternative iron compounds such as NaFeEDTA, and co-administration of prebiotics to blunt the microbiome cost. For the broader thesis of this site, the study is a well-controlled proof of concept that what reaches the colon in metal form can reshape microbial ecology and inflame the gut.
Key findings
- In 115 Kenyan infants, iron-fortified maize porridge increased pathogenic Escherichia/Shigella, including E. coli strains (p=0.029), versus non-iron controls.
- The enterobacteria-to-bifidobacteria ratio rose significantly with iron (p=0.020), eroding the protective bifidobacteria-dominated baseline (~63% of 16S sequences).
- Fecal calprotectin, a marker of intestinal inflammation, increased significantly in the iron groups (p=0.002).
- Diarrhea was numerically more common with 12.5 mg ferrous fumarate (27.3% vs 8.3% control), though not statistically significant (p=0.092).
- Both 2.5 mg NaFeEDTA and 12.5 mg ferrous fumarate produced adverse microbiome shifts, isolated from the rest of the micronutrient mix by the trial design.
- Mechanistically, unabsorbed luminal iron is preferentially captured by siderophore-equipped pathogens, favoring them over low-iron-requirement commensals.
Frequently asked questions
Does iron fortification really change an infant's gut bacteria?
Yes. In this double-blind randomized controlled trial in Kenyan infants, four months of iron-fortified porridge significantly increased potentially pathogenic enterobacteria such as Escherichia/Shigella and reduced the relative dominance of protective bifidobacteria, compared with an identical porridge without added iron.
Why does dietary iron favor pathogens like E. coli?
Much fortificant iron is not absorbed and passes into the colon. Pathogens such as E. coli, Salmonella and Shigella have high-affinity siderophore systems to scavenge that iron, while beneficial bifidobacteria have low iron needs, so added luminal iron gives the potential pathogens a competitive growth advantage.
Did the iron cause gut inflammation?
Iron-containing micronutrient powders significantly raised fecal calprotectin (p=0.002), a validated marker of intestinal inflammation, and were associated with a trend toward more diarrhea at the higher 12.5 mg ferrous fumarate dose (27.3% vs 8.3% in controls).
Does this mean iron-deficient infants should not receive iron?
No. Untreated iron deficiency carries serious developmental risks. The study argues for careful attention to iron dose, chemical form and delivery, and it has spurred research into lower doses, compounds like NaFeEDTA, and pairing iron with prebiotics to reduce the microbiome and inflammation cost, rather than abandoning iron interventions.