Metals & the Microbiome
The gut is a metal economy — and we keep flooding the market.
Most of the iron in a supplement is never absorbed. It travels to the colon, where it becomes food for microbes — including the pathogens that iron feeds. This is where microbial metallomics stops being abstract and starts shaping human health.
Dietary iron and the infant gut
The clearest evidence comes from infant nutrition. In a randomized controlled trial of 115 Kenyan infants, iron-fortified porridge increased fecal enterobacteria — particularly pathogenic E. coli — raised the ratio of enterobacteria to protective bifidobacteria, and increased fecal calprotectin, a marker of gut inflammation. Infants on the higher-iron formula had substantially more diarrhea requiring treatment (27.3% vs 8.3%). An earlier trial in Ivorian children found the same pattern: iron shifted the community toward "a potentially more pathogenic gut microbiota profile."
The paradox
Iron is given to fight anemia — the world's most common nutritional disorder, affecting 40% of young children. Yet in the wrong setting, unabsorbed iron can feed the very pathogens that cause childhood diarrhea, a leading cause of under-five death. Getting metal supplementation right is a genuine metallomics problem.
Zinc shows a parallel effect from the other direction: in mice, excess dietary zinc reduced microbiome diversity and weakened colonization resistance to Clostridioides difficile, lowering the antibiotic threshold needed to trigger disease.
Toxic metals and dysbiosis
Metals that reach the gut without being absorbed reshape the community directly. Arsenic exposure perturbs the mouse microbiome and its metabolism — while, reciprocally, the microbiome protects the host from arsenic toxicity: germ-free animals accumulate more arsenic and suffer more. Cadmium and lead alter colonic microbiota composition even without changing overall richness. A 2025 systematic review of twelve human studies concluded that lead, arsenic and mercury exposure is associated with dysbiosis — expansion of pathobionts and depletion of beneficial Bifidobacterium.
Nutritional immunity, in the clinic
The same metal chemistry decides infections. The host practices nutritional immunity — a term coined by Eugene Weinberg in 1975 for the body's effort to withhold iron from invaders, and expanded by Hood, Skaar and colleagues into a full framework of transition-metal warfare at the host–pathogen interface. The host withholds iron, zinc and manganese, and in the macrophage phagosome floods trapped bacteria with toxic copper and zinc. Pathogens counter with metallophores and metal-detox systems (see The Science).
Calprotectin: both weapon and biomarker
One protein sits at the center of this story. Calprotectin (the S100A8/S100A9 heterodimer) is released by neutrophils and starves bacteria by chelating manganese and zinc; it accumulates to over a milligram per millilitre at infection sites. Corbin and colleagues showed in Science (2008) that mice lacking calprotectin form metal-rich abscesses that let Staphylococcus aureus flourish. The very same molecule, measured in stool as fecal calprotectin, is a standard clinical readout for intestinal inflammation — the identical marker used in those infant iron trials. Weapon and diagnostic in one.
Iron overload and infection risk
The dependence runs the other way too. In conditions of iron excess, "siderophilic" pathogens thrive: Vibrio vulnificus, harmless in normal serum, grows explosively in blood from patients with hereditary hemochromatosis, and mouse models show iron overload driving lethal responses to Yersinia. This is the mechanistic bridge from the supplementation concerns above to hard clinical outcomes — and a reminder that with metals, availability, not abundance, is what matters.
Sources
- Jaeggi et al. "Iron fortification adversely affects the gut microbiome … in Kenyan infants." Gut (2015). PubMed
- Zimmermann et al. Am. J. Clin. Nutr. (2010). PubMed
- Corbin et al. "Metal chelation and inhibition of bacterial growth in tissue abscesses." Science (2008). science.org
- Hood & Skaar. "Nutritional immunity: transition metals at the pathogen–host interface." Nat. Rev. Microbiol. (2012). nature.com
- Coryell et al. "The gut microbiome … protection against arsenic toxicity." Nat. Commun. (2018). PMC
Keep reading
Applications → siderophore antibiotics, gallium, bioremediation. · Research Library → reviews and case studies at this intersection.