Reference ranges and female athletes
A laboratory reference range is a statistical description of a sampled population, and trained female athletes were not the population it was built on.
An athlete's iron demand differs in ways that compound: higher red cell turnover, losses through sweat and gut, foot-strike haemolysis in running sports, and for many, menstrual blood loss on top. The same ferritin value means something different in a sedentary population and in someone training fifteen hours a week.
The useful question is what threshold the research on athletes uses when it decides who counts as deficient.
Ferritin thresholds in athlete research
The most relevant work is a 2024 systematic review in the Journal of Sport and Health Science, covering 23 studies and 669 female athletes aged 13 to 47 across 16 sports.
It defined its population as high-level female athletes with serum ferritin below 40 micrograms per litre, where high-level meant a maximal oxygen uptake above 45 mL/kg/min or more than five hours of training a week.
The review opens by describing iron deficiency as common in female athletes.
The mismatch with your lab slip
A 2024 systematic review of ferritin reference intervals found that the laboratory lower limit for healthy adults consistently sits below 30 micrograms per litre. A threshold of 40 sits above them.
An athlete gets a result of 25, is told it is within range, and is left with no explanation for why her sessions feel harder than they should.
Iron deficiency and performance
The same review put numbers on it.
| Measure | What the review found |
|---|---|
| Endurance performance, when deficient | reduced by 3 to 4 per cent |
| Endurance performance, after treatment | improved by 2 to 20 per cent |
| Maximal aerobic capacity, after treatment | improved by 6 to 15 per cent |
| Isokinetic strength and anaerobic power | may be impeded, but the range spans -23 to +4 per cent |
In a three-hour event, 3 to 4 per cent is five to seven minutes.
The treatment figures came from about 100 mg of elemental iron per day for up to 56 days in the oral studies, and aerobic capacity gains came from 16 to 100 mg a day across 36 to 126 days.
Strength and anaerobic power
The review found effects on isokinetic strength and anaerobic power ranging from a 23 per cent decrement to a 4 per cent improvement, and the effect of supplementation on anaerobic power varied from -5 to +9 per cent.
A range that crosses zero in both directions means the effect is still unknown. Anyone quoting you a confident strength benefit from iron is reading a range as a result.
My meta-analysis and its limits
I led a systematic review and meta-analysis on intravenous iron in adults with iron deficiency without anaemia, published in the Journal of Cachexia, Sarcopenia and Muscle. It pooled 21 randomised controlled trials and 3,514 participants.
Intravenous iron significantly improved physical function, measured as peak oxygen consumption, by a mean difference of 1.77 mL/kg/min, with a 95 per cent confidence interval of 0.57 to 2.97. Fatigue improved. Quality of life did not shift overall.
We rated the quality of the evidence low or very low for every outcome except fatigue, mostly because most included studies carried a high risk of bias. That should temper how hard anyone leans on the number.
A second meta-analysis, in women of reproductive age specifically, found daily oral iron improved relative maximal oxygen uptake by 2.35 mL/kg/min, 95 per cent confidence interval 0.82 to 3.88, across 18 studies. It also found submaximal benefits, a lower heart rate and a lower percentage of maximum required to hold a given workload. Only three of its studies were at overall low risk of bias.
The direction of effect is consistent across three independent bodies of work, with wide margins of uncertainty.
The ceiling
Everything above is about correcting a deficiency. Raising iron status in someone who is not deficient has not been shown to consistently improve performance.
Iron also carries costs. The body has no mechanism for actively excreting it, oral supplements carry a real side-effect burden, and iron overload is a clinical problem.
The benefit comes from avoiding deficiency.
Tests to ask for
- Ask for ferritin with transferrin saturation, which shows iron in circulation, and C-reactive protein (CRP), which shows whether inflammation is inflating the ferritin reading.
- Ferritin rises with inflammation, and hard training is inflammatory, so a sample taken the morning after a brutal session can read higher than your real status.
- If you are training seriously and your result is in the twenties or thirties, ask how your number sits against the thresholds used in athlete research.
- Allow months: the trials that showed benefit ran 36 to 126 days.
- If you are losing iron faster than you can absorb it, find the cause, because supplementation only offsets the loss.
If you are training hard, feeling flat and have been told your bloods are fine, look at the number again.
Related articles
- Iron depletion over time
- Iron tablets and a stalled ferritin
- A Practical Guide to Iron Deficiency and Anaemia
Sources
The study figures on this page come from these four papers.
- Pengelly M, Pumpa K, Pyne DB, Etxebarria N. Iron deficiency, supplementation, and sports performance in female athletes: a systematic review. Journal of Sport and Health Science 2025;14:101009. doi:10.1016/j.jshs.2024.101009
- Dugan C, Cabolis K, Miles LF, Richards T. Systematic review and meta-analysis of intravenous iron therapy for adults with non-anaemic iron deficiency: an abridged Cochrane review. Journal of Cachexia, Sarcopenia and Muscle 2022;13(6):2637-2649. doi:10.1002/jcsm.13114
- Pasricha SR, Low M, Thompson J, Farrell A, De-Regil LM. Iron supplementation benefits physical performance in women of reproductive age: a systematic review and meta-analysis. The Journal of Nutrition 2014;144(6):906-14. doi:10.3945/jn.113.189589
- Truong J, Naveed K, Beriault D, Lightfoot D, Fralick M, Sholzberg M. The origin of ferritin reference intervals: a systematic review. The Lancet Haematology 2024;11(7):e530-e539. doi:10.1016/S2352-3026(24)00103-0
This article is educational and not medical advice. Never begin iron supplementation without testing, and always discuss changes to supplementation or testing with a qualified clinician who knows your full history.