Dose-Dependent Alterations of the Human Gut Microbiome During Oral Iron Supplementation: A Randomized Study in Iron-Deficient Non-Anaemic Women

Background/Objectives: Oral iron supplementation is widely used to treat iron deficiency but frequently causes gastro-intestinal side effects that limit treatment adherence. Unabsorbed luminal iron has been proposed to influence intestinal microbial communities, yet the effects of different oral iron doses on the human gut microbiome remain insufficiently characterized. Methods: In this randomized open-label study, 30 healthy premenopausal women with iron deficiency without anaemia received either low-dose oral iron supplementation (6 mg twice daily) administered under fasting conditions or standard-dose iron supplementation (100 mg once daily) taken with a meal for four weeks. Stool samples were collected before and after treatment and analyzed using 16S rRNA sequencing to evaluate microbiome composition. Results: Baseline characteristics, including age, body mass index, hemoglobin concentration and serum ferritin, were comparable between groups. After four weeks of treatment, distinct alterations in gut microbiome composition were observed between the low-dose and standard-dose groups. The genera Colidextribacter and GCA-900066575 decreased in the low-dose group but increased in the standard-dose group, whereas Oscillospira showed the opposite pattern. Gastrointestinal adverse events were reported by 87% of participants receiving standard-dose iron supplementation compared with 7% receiving low-dose iron supplementation (p < 0.0001). Conclusions: Oral iron supplementation induces dose-dependent changes in the intestinal microbiome and higher doses are associated with substantially increased gastrointestinal intolerance. These findings suggest that lower iron doses may reduce microbiome disruption and improve treatment tolerability.