REF-1421 · Barney et al., 2022 · The Journal of Nutrition · Paper · PubMed

Does a long run block iron from my next meal?

In 28 collegiate runners, a meal 2 h after a ~99-min run gave 36% less iron absorption than at rest, with hepcidin up 51%. One meal, fixed order, abstract only.

Main caveat: Abstract only; the publisher's free page returned a bot check and was not tried further.
limited evidenceMeasured on highly trained athletes

Barney and colleagues (Florida State University) asked whether a prolonged run raises hepcidin and lowers dietary iron absorption in trained runners.

Twenty-eight collegiate cross-country runners (age 19.7 ± 1.2 years; VO2max 66.1 ± 6.1 mL/kg/min; ferritin 21.9 ± 13.3 ng/mL) ran 98.8 ± 14.7 min (21.2 ± 3.8 km at 4.7 min/km) during a team practice, ate a standardised meal containing a stable iron isotope 2 h later, and gave blood 1 h after the meal; two weeks later they repeated the protocol without exercise.

Red cells collected 15 days after each condition gave isotope incorporation. After the run, plasma hepcidin was 51% higher (45.8 ± 34.4 vs 30.3 ± 27.2 ng/mL, P = 0.0010), IL-6 higher (0.660 vs 0.457 pg/mL, P < 0.0001) and fractional iron absorption 36% lower (11.8 ± 14.6% vs 18.5 ± 14.4%, P = 0.025).

Exploratory analyses suggested the hepcidin rise was driven by the male runners. The authors conclude that a prolonged run raises hepcidin and lowers dietary iron absorption in trained runners with low stores, supporting a role for IL-6, and call for work on sex differences.

Our reading: the other half of the timing question — with REF-1418 (iron 30 min after a moderate morning run absorbed best) it suggests iron soon after, or well away from, a long session; neither study tested a window directly.

Protocol

Participants: n = 28 trained female and male collegiate cross-country runners; age 19.7 ± 1.2 y; VO2max 66.1 ± 6.1; serum ferritin 21.9 ± 13.3 ng/mL.
Exercise condition: a prolonged run during team practice, 98.8 ± 14.7 min, 21.2 ± 3.8 km, 4.7 ± 0.3 min/km.
Meal: a standardised meal with a stable iron isotope 2 h after the run (or at the same point on the rest day); blood 1 h after the meal.
Rest condition: the same protocol 2 weeks later without exercise (fixed order).
Absorption: isotope enrichment of red cells collected 15 days after each condition.
Funding and interests: Crossref lists Florida State University as funder; PubMed lists "Research Support, Non-U.S. Gov't"; no conflict-of-interest statement in the abstract record.

Key Results

MeasureAfter the runAt restChangeP
Plasma hepcidin (ng/mL)45.8 ± 34.430.3 ± 27.2+51%0.0010
Fractional iron absorption11.8 ± 14.6%18.5 ± 14.4%−36%0.025
Plasma IL-6 (pg/mL)0.660 ± 0.3540.457 ± 0.212—< 0.0001
Sex (exploratory): the hepcidin increase with exercise "may be driven by a response in males but not females".
Spread: absorption SDs (±14.6 and ±14.4 percentage points) show wide differences between runners in each condition; the abstract gives no SD of the paired change, so how much individual responses differed cannot be read from it (our reading).

Mechanisms

Measured: after a run of about 100 min, IL-6 and hepcidin were higher and absorption from a meal 2 h later was lower than on a rest day.
Authors' interpretation: IL-6 contributes to the exercise-induced hepcidin rise, and hepcidin lowers dietary iron absorption; sex differences in the hepcidin response need study.
Inference: REF-1418 fed 30 min after a 90-min run at moderate intensity and found absorption higher than at rest, before the 3-h hepcidin rise; this study fed at 2 h after a longer run. The two fit a short early window and a later dip, but they differ in feeding time, run, iron form (fortified drink vs meal iron) and design, so the base cannot say where the turning point lies.

Practical Application

Does a long run block iron from my next meal? In 28 collegiate runners, a meal 2 h after a ~99-min run gave 36% less iron absorption than at rest, with hepcidin up 51%. One meal, fixed order, abstract only.
The timing row: keeping iron out of the hours after a long session now has a direct measure for meal iron 2 h after a long run (B); the base's SYNTH-06 line ("not in the hours after a hard session") was an inference from the hepcidin mechanism. Taking iron soon after a morning session was better in REF-1418 (B+). The schedule is the prescriber's.
Women: the hepcidin rise looked smaller in the women (exploratory); whether absorption differs by sex is not in the abstract.
What it does not show: effects on ferritin over weeks, on supplements taken fasting, or after shorter or easier runs.

Limitations

Abstract only; the publisher's free page returned a bot check and was not tried further.
Fixed order: rest always followed the run by 2 weeks; no randomisation or counterbalancing.
Field run: a team practice, so intensity and conditions are not standardised.
One meal per condition; meal iron, not a supplement; no ferritin outcome.
Sex split not given in the abstract; the sex analysis is exploratory.
Funding and interests: Florida State University per Crossref; no conflict statement in the record.
How this entry was checked and changed
Why this level

(from the abstract) — a paired comparison of 28 runners (a controlled crossover of at least 15 would be B+) held at B because the conditions were not randomised or counterbalanced (rest always came 2 weeks after the run), the run was a team practice rather than a standardised bout, and the outcome is absorption from one meal