Effects of low-dose iron supplementation on iron status, safety outcomes and gut microbiota in female soccer players: a randomized controlled study

Purpose: Iron deficiency impairs sports performance, and female athletes are particularly vulnerable. High-dose iron supplements, commonly used to prevent iron depletion and performance impairments, may cause gastrointestinal side effects and disrupt the gut microbiota. Whether lower doses can improve iron status without adverse effects remains unclear. The aim of this study was to characterize iron intake and iron status in female soccer players during the competitive season and investigate effects of low-dose iron supplementation on iron status, safety-related outcomes and gut microbiota. Methods: In a two-arm parallel randomized controlled trial, female soccer players (median age 21) were randomized to an intervention group (n=12) receiving 3-month low-dose iron supplementation (27 mg elemental iron/day) or a control group (n=11) without supplementation. Blood/fecal samples were collected at baseline and 3-month follow-up. Dietary intake was estimated using 7-day food diaries. Between-group differences were analyzed per protocol (n=18) using ANCOVA with baseline adjustment. Results: The players had inadequate baseline iron intake (median 11.2 mg/day) and 43% had serum ferritin indicating iron depletion (<35 g/L). At follow-up, no significant between-group difference was found for serum ferritin, but fewer athletes in the intervention group experienced decreases from baseline to follow-up (P<0.05). Moreover, serum iron was higher in the intervention group (Pgroup=0.05). No between-group differences were observed for gastrointestinal symptoms or liver damage biomarkers. On the contrary, the intervention led to lower IL-6 (Pgroup=0.04) and higher gut microbial -diversity (Pgroup=0.01) compared to controls. Conclusions: The low-dose iron supplementation was well tolerated, attenuated decreases in iron stores, increased gut microbial diversity and attenuated systemic inflammation in female soccer players with suboptimal dietary iron intake. However, potential adverse effects of long-term exposure cannot be excluded.