A 3-month randomized controlled trial in 23 female soccer players (median age 21) found that 27 mg/day elemental iron supplementation attenuated seasonal declines in iron stores, raised serum iron (p=0.05), lowered systemic IL-6 inflammation (p=0.04), and — notably — increased gut microbial α-diversity (p=0.01) compared to unsupplemented controls. Baseline dietary iron averaged just 11.2 mg/day, and 43% of players entered the study with ferritin below 35 µg/L, indicating iron depletion. No gastrointestinal complaints or liver-damage biomarkers differed between groups.
This finding challenges the conventional clinical reflex to prescribe high-dose iron (typically 100–200 mg/day) for iron-depleted athletes. High-dose protocols are well-documented to increase luminal iron availability for pathogenic bacteria, reducing microbial diversity — the opposite of what appeared here at the lower dose. The IL-6 reduction is intriguing, potentially linking improved iron metabolism to dampened training-induced inflammation, though causality cannot be established from this design.
Critical caveats: the per-protocol sample shrinks to just 18 participants, severely limiting statistical power and generalizability. No significant between-group difference was achieved for the primary marker, serum ferritin. The study is also a preprint posted on medRxiv and has not yet undergone peer review — findings should be interpreted cautiously until independently validated. Still, the gut-diversity signal at a nutritionally plausible dose makes this an incremental but genuinely interesting contribution to sports-nutrition science, warranting larger confirmatory trials.
