Iron deficiency is common in triathletes — more so in women — follows a defined staging system, and alternate-day or single-morning-dose supplementation absorbs measurably better than split or consecutive-day dosing
Interpretation
Sim et al.'s narrative review estimates iron deficiency affects 15-35% of female and 5-11% of male athletes, with staging based on ferritin, haemoglobin and transferrin saturation, and notes exercise-induced hepcidin blocks iron absorption for several hours post-exercise — timing supplementation away from training and toward morning is the practical implication. Coates et al.'s elite-triathlete-specific data shows the real scale: 60% of female and 37.5% of male elite triathletes in their cohort had at least one iron-deficiency episode, with oral iron dosing in that group uncorrelated with ferritin or haemoglobin outcomes — supplementing without a defined protocol doesn't reliably work. Stoffel et al.'s RCT provides the protocol fix: alternate-day dosing absorbed significantly better than consecutive-day dosing (21.8% vs 16.3%, p=0.0013) at the same total dose.
Practical implication
Recommend iron screening be routine (not just symptom-triggered) for triathletes, especially women, and that supplementation — when indicated — follow an alternate-day or single-morning-dose protocol timed away from training sessions, rather than the daily split-dosing pattern many athletes default to.
Sources
3 sources, independently verified.
Estimates iron deficiency affects 15-35% of female and 5-11% of male athlete cohorts, and lays out a three-stage severity system (by ferritin, haemoglobin and transferrin saturation) along with practical dosing guidance: roughly 100mg/day for 8-12 weeks can raise ferritin 40-80%, timed away from exercise (hepcidin, which blocks iron absorption, rises for several hours post-exercise) and ideally in the morning. The reference framework the rest of this article's recommendations sit inside.
At least one episode of iron deficiency occurred in 60% of female triathletes and 37.5% of male triathletes in this elite cohort (vs 55.6% and 31.3% for runners); iron-deficiency anaemia specifically hit 25% of male triathletes and 20% of female triathletes. Oral iron dosing in this group averaged 94±115mg/day with no correlation to ferritin or haemoglobin outcomes — supplementing without monitoring doesn't reliably work.
Alternate-day dosing (60mg every other day) was significantly better absorbed than consecutive-day dosing of the same amount (fractional absorption 21.8% vs 16.3%, p=0.0013), and a single morning dose absorbed similarly to a split twice-daily dose while provoking a smaller hepcidin response. The direct evidence behind recommending alternate-day, single-dose iron supplementation over daily split dosing.