REF-1420 · Pengelly et al., 2025 · Journal of Sport and Health Science · Paper · PubMed · Free full text

Does low iron slow female athletes?

In 23 small studies of trained women with ferritin under 40, deficient athletes were 3-4% slower in rowing time trials; iron improved some tests within groups but rarely beat placebo. Weak evidence.

Main caveat: No pooling; within-group percentages carry most of the conclusions.
moderate evidenceMeasured on trained to world-class athletes

Pengelly and colleagues (University of Canberra) reviewed what iron deficiency and iron supplementation do to sports performance in high-level female athletes with ferritin below 40 µg/L.

They found 23 studies (669 athletes, 16 sports): 7 compared iron-deficient with iron-sufficient athletes (267 athletes, 95% in aerobic sports) and 16 tested supplementation (402 athletes; daily doses from 4 to 100 mg of elemental iron, totals 336 to 8400 mg). Heterogeneity prevented a meta-analysis.

In the comparisons, iron-sufficient athletes completed time trials 1-19% faster, with the two rowing studies showing 4% and 3% slower 2-km and 4-km times in deficient rowers; effects on VO2max were inconsistent (−6% to +4%).

With supplementation, VO2max changed by −3% to +15% "with no significant differences in performance between athletes receiving FeSup or placebo"; time to exhaustion improved about 20% within groups in two trials giving 100 mg of iron, one orally for 56 days, one as five intramuscular injections over 8-10 days; energy efficiency, lactate and work-rate measures improved by 2-30% within groups at 16-60 mg a day for 42-168 days; and the VO2max gains of 6-15% were in studies with ferritin criteria below 16 µg/L.

Quality was moderate (77%). The authors conclude that iron deficiency impairs endurance performance in high-level female athletes and that about 100 mg of elemental iron a day, or bi-daily, can improve it.

Our reading: the women-specific answer is weaker than the abstract reads — the cost of deficiency is cross-sectional, most improvements are within-group changes in tiny samples with training going on, and the "parenteral" protocol was intramuscular, not intravenous; the pattern that benefit concentrates in the most depleted matches REF-1409.

Protocol

Search and eligibility: five databases, July 2023, plus reference lists; female athletes at Tier 2 or above (VO2max >45 mL/kg/min, or >5 h a week / >20 h a month); performance assessed; iron deficiency by ferritin <40 µg/L; English, peer-reviewed.
Selection: 1221 records, 282 full texts, 23 studies; screening by two reviewers (Covidence), a third for disagreements.
Participants: n = 669 (experimental 341, control 328); age 22 ± 6 years (range 13-47); VO2max 49.4 ± 6.1 mL/kg/min; training 6.4 ± 2.8 h a week.
Supplementation studies: 16 studies, 402 athletes; oral iron in most (4 to 100 mg a day), intramuscular injections in two (100 mg every second day, 500 mg in total), one dietary (low-phytate pea protein); most used ferritin <20 µg/L as the criterion; diet, menstruation and training reported in 48%, 35% and 78% of all studies.
Quality: modified Downs and Black (21 items), 77% ± 13% (range 57% to 100%); 10 studies high, 13 moderate.
Synthesis: narrative, with percentage changes; no meta-analysis (heterogeneity).
Funding and interests: no funding statement in the paper; "The authors declare that they have no competing interests."

Key Results

QuestionWhat the studies showedStudies behind it
Does deficiency slow endurance?Time trials 1-19% slower in deficient athletes; collegiate rowers 4% slower over 2 km (p = 0.004) and 3% over 4 km (p = 0.03)4 comparisons (2 of them cross-sectional rowing studies with 149 and 48 rowers)
Does deficiency lower VO2max?Inconsistent, −6% to +4%3 small comparisons
Strength and powerIsokinetic strength up to −23%, peak and mean power 6-10% lower; not significant2 studies (deficient n = 8 and 5)
Supplementation and VO2max−3% to +15%, no significant difference from placebo; 6-15% gains within groups in studies with ferritin <16 µg/L criteria16 studies; gains from 3 (oral 16-100 mg a day, 36-126 days)
Supplementation and endurance testsTime to exhaustion about +20% within groups with 100 mg (oral, 56 days; or intramuscular, 8-10 days), both with no between-group difference; time trials slower or unchanged with <100 mg a day2 trials (n = 18 and 16); 3 trials
Supplementation and lactate, efficiency, work rateCompared with baseline: lactate 7-30% better (16-100 mg a day, 42-56 days); energy efficiency, maximal work rate and velocities 2-8% better (47-60 mg a day, 56-168 days); velocity at lactate threshold or OBLA 4-10% better (16-60 mg a day, 42-168 days)2-3 studies each
Power after supplementation−5% to +9%, no difference from placebo3 studies
Side effectsReported in 4 studies; one athlete stopped for gastric discomfort; about 40% had symptoms on the dietary intervention4 studies
The 100 mg protocol, study by study (Table 2): Klingshirn 1992, oral 100 mg a day for 56 days, 18 runners: time to exhaustion +20% with no difference between groups; Peeling 2007, intramuscular 100 mg "bi-daily" over 8-10 days (500 mg total), 16 endurance athletes: time to exhaustion +18% (p ≤ 0.05), no difference between groups. The abstract calls the second "parenteral".
Low-dose studies: three studies gave 4-6 mg a day (one of them a dietary pea-protein intervention), and one of them was a probiotic trial in which both groups received the same iron.

Mechanisms

Measured: deficient rowers were slower and used more energy (2% less efficient) in a 4-km time trial; supplemented athletes showed lower lactate and better efficiency in some endurance tests.
Authors' interpretation: non-anaemic deficiency impairs muscle oxidative capacity (iron-sulphur enzymes, cytochromes), which shows in sustained endurance efforts more than in a graded VO2max test; VO2max falls mainly when haemoglobin drops (≤119 g/L) or ferritin is very low; strength and power may suffer only in anaemia.
Inference: most improvements are pre-post changes in both groups during training, so training and iron effects are mixed; where a placebo group existed, differences were mostly absent.

Practical Application

Does low iron slow female athletes? In 23 small studies of trained women with ferritin under 40, deficient athletes were 3-4% slower in rowing time trials; iron improved some tests within groups but rarely beat placebo. Weak evidence.
What to expect, honestly (B+): a performance cost of deficiency is plausible for sustained endurance; the review links its VO2max gains to very low ferritin criteria, but they are within-group changes, significant in one of three studies; REF-1409 likewise saw the clearest ferritin rise from the lowest starts; most numbers here (2-20%) are within-group changes, not effects against placebo.
Correction for the iron guide: the review's "parenteral" 100-mg protocol was intramuscular injections, not IV iron; IV iron in active women is REF-1422 (no VO2peak change).
Doses quoted (about 100 mg a day orally, or 16-60 mg a day for efficiency) are the review's synthesis of small studies; the dose is the prescriber's, and the authors themselves send iron-deficient athletes to a sports physician or dietitian and remind them of anti-doping rules.
Women's context: heavy menstrual bleeding is common in female runners (REF-573); a heavy-bleeding question belongs before any iron discussion (SYNTH-37). Not from this paper.

Limitations

No pooling; within-group percentages carry most of the conclusions.
Small studies: 18 of 23 with groups of 20 or fewer; the authors note low power.
Ferritin-only definition (<40 µg/L at inclusion, mostly <20 in the studies); haemoglobin not always measured; inflammation not addressed.
Mixed designs, routes and sports; training rarely controlled; diet and menstruation reported in a minority.
Reporting: "parenteral" in the abstract for intramuscular injections; "bi-daily" for every second day; no protocol registration reported.
Funding and interests: no funding statement; no competing interests declared.
How this entry was checked and changed
Why this level

a systematic review held below A- because nothing was pooled, most reported changes are within-group, the studies are small and of mixed designs (18 of 23 with groups of 20 or fewer), and between-group differences were mostly absent; read from the full text (Europe PMC, CC BY-NC-ND 4.0); the supplementary result tables were not read

Full text (checked 2026-10-28)
A total of 23 studies comprising 669 athletes (age range: 13–47 years) across 16 sports were included; iron deficiency negatively affects endurance performance by 3%–4% (full text)
Most studies (n = 18) contained group sizes ≤ 20 athletes; the quality of studies was moderate (77%), ranging from low (57%) to high (100%) (full text)
The effect of FeSup on maximal aerobic capacity varied from –3% to +15%, with no significant differences in performance between athletes receiving FeSup or placebo (full text)
Time-to-exhaustion improved by ∼20% when FeSup protocols involved 100 mg of elemental iron either per day, following oral supplementation across 56 days, or bi-daily via parenteral administration over 8–10 days (full text)
Peeling et al. (2007): n = 16, sFer <35 µg/L, Intramuscular, 100 mg bi-daily (8–10, 500 mg); time to exhaustion +18% (p ≤ 0.05), no difference between groups (full text)
Maximal aerobic capacity improved by 6%–15% following 16–100 mg/day for 36–126 days in studies utilizing a sFer criterion of <16 µg/L (full text)
Endurance performance was compromised by 3%–19% among ID athletes compared to their IS counterparts (full text)
DellaValle et al. (2011): 2 km TT –4% (p = 0.004); Dellavalle et al. (2012): 4 km TT –3% (p = 0.03) (full text)
The authors declare that they have no competing interests (full text)