REF-1417 · McCormick et al., 2020 · International Journal of Sport Nutrition and Exercise Metabolism · Paper · PubMed

Is every-other-day iron enough for a runner?

In 31 runners with ferritin under 50, ferritin rose about 20 µg/L in 8 weeks on either schedule, with fewer severe gut complaints on alternate days (1 vs 6). Small; abstract only.

Main caveat: Abstract only; the full text was not open.
limited, mixed evidenceMeasured on trained to highly trained athletes

McCormick and colleagues (University of Western Australia, Western Australian Institute of Sport) compared daily with alternate-day oral iron in 31 endurance-trained runners (nine men, 22 women) whose serum ferritin was below 50 µg/L. Runners supplemented for 8 weeks; ferritin was measured at baseline and every two weeks, and haemoglobin mass before and after in 10 of them.

Ferritin rose by 19.7 µg/L over the 8 weeks (p < .001) with no difference between schedules (treatment p = .928, interaction p = .877), and was 21.2 µg/L higher in men than in women. Haemoglobin mass showed no treatment or interaction effect; it was 140.85 g higher in men and was affected by training load and dietary iron intake.

There were six complaints of severe gastrointestinal side effects on daily iron and one on alternate days. The authors conclude that both schedules raised ferritin and that alternate days came with less gastrointestinal upset.

Our reading: it narrowed the SYNTH-06 gap "alternate-day iron in athletes" (that line was corrected on 2026-10-29) with a small, abstract-only trial; it matches the larger outcome trial in non-athlete women (REF-1416: same ferritin at the same total dose, fewer GI effects), and it cannot by itself show that the schedules are equivalent.

Protocol

Participants: n = 31 endurance-trained runners (9 men, 22 women), serum ferritin <50 µg/L.
Groups: daily (DAY) or alternate-day (ALT) oral iron for 8 weeks; group sizes, allocation and dose not given in the abstract.
Measurements: ferritin at baseline and fortnightly; haemoglobin mass pre and post in a subset (n = 10); gastrointestinal side effects (method not described).
Analysis: linear mixed-effects models for ferritin and haemoglobin mass.
Funding and interests: not in the PubMed record; affiliations: The University of Western Australia, the Western Australian Institute of Sport, Catalyst Dietitian, Edith Cowan University.

Key Results

Ferritin: no treatment (p = .928) or interaction (p = .877) effect; increased 19.7 µg/L (p < .001) over 8 weeks in both groups.
Sex: ferritin 21.2 µg/L higher in men than in women (p < .001); haemoglobin mass 140.85 g higher in men (p = .004).
Haemoglobin mass (n = 10): no treatment (p = .146) or interaction (p = .249) effect; training load (p = .001) and dietary iron intake (p = .015) affected it.
Gastrointestinal: six complaints of severe gastrointestinal side effects with daily iron, one with alternate days (complaints, not necessarily people; no statistics in the abstract).

Mechanisms

Measured: equal ferritin gains on both schedules; fewer severe GI complaints on alternate days.
Authors' interpretation: both strategies increase ferritin in athletes with suboptimal iron; the alternate-day approach goes with lower GI upset.
Inference: if the alternate-day group received half the iron (as REF-1416 describes this trial), an equal ferritin rise fits the better absorption of a dose taken 48 h after the previous one (REF-1415; REF-970); this trial did not measure absorption or hepcidin.
Inference: REF-1416 found ferritin higher with daily dosing at 6 weeks and 3 months; with 31 runners split two ways, this trial may have been too small to see such a difference.

Practical Application

Is every-other-day iron enough for a runner? In 31 runners with ferritin under 50, ferritin rose about 20 µg/L in 8 weeks on either schedule, with fewer severe gut complaints on alternate days (1 vs 6). Small; abstract only.
What it adds: the first athlete data in the base on the schedule question (SYNTH-06 lists it as a gap); with REF-1416 it supports alternate days as an option for gut-sensitive runners, labelled B- here, A in non-athlete women. The schedule is the prescriber's.
Men vs women: ferritin was 21.2 µg/L higher in men than in women (a difference in level; the abstract does not say whether the rise differed by sex), so a woman's lower value is not by itself a failure to respond (inference).
What it does not show: performance or haemoglobin-mass benefit (no effect, n = 10), the dose, or what happens beyond 8 weeks.

Limitations

Abstract only; the full text was not open.
Allocation, dose, iron form, GI method and group sizes are not in the abstract; randomisation is not stated.
Small: 31 runners; haemoglobin mass in 10.
Short: 8 weeks; no performance outcome.
Complaints, not people: six vs one severe complaints, without a test.
Funding and interests: not in the record.
How this entry was checked and changed
Why this level

(from the abstract) — a small two-arm trial (B design) with key methods missing from the abstract: how runners were allocated, the dose and form of iron, how gastrointestinal complaints were recorded and graded, and baseline ferritin by group; haemoglobin mass in only 10