Iron and the Triathlete: Ferritin, Fatigue and When to Get Tested

Ferritin has become something of a buzzword among endurance athletes. Podcasts, social media and training forums are full of athletes comparing numbers, and many triathletes take iron supplements "just in case". At the same time, real iron deficiency is common, often missed, and can make training feel much harder than it should.

This article explains what iron does, how common deficiency is in triathletes, what ferritin can and can't tell you, why exercise affects iron absorption, and what the evidence says about testing and treatment. It isn't a guide to diagnosing or treating yourself. Iron decisions belong with a clinician, after a blood test.

What iron does

Iron is essential for carrying oxygen. It's a core part of haemoglobin, which carries oxygen in red blood cells, and of myoglobin, which holds oxygen in muscle. It's also needed by enzymes involved in producing energy in the mitochondria.

For an endurance athlete, that makes iron central to performance. If you don't have enough, your ability to deliver and use oxygen can fall, and training at the same pace or power can feel harder.

The stages of iron deficiency

Iron deficiency isn't all-or-nothing. A 2019 narrative review by Sim and colleagues in the European Journal of Applied Physiology described a widely used three-stage system for athletes, originally set out by Peeling and colleagues in 2007:

• Stage 1, iron deficiency: ferritin below 35 µg/L, with haemoglobin above 115 g/L and transferrin saturation above 16%. Iron stores are low, but other markers are normal.
• Stage 2, iron deficiency without anaemia: ferritin below 20 µg/L, haemoglobin still above 115 g/L, but transferrin saturation below 16%. The supply of iron to make red blood cells is now limited.
• Stage 3, iron-deficiency anaemia: ferritin below 12 µg/L, haemoglobin below 115 g/L and transferrin saturation below 16%.

Ferritin reflects the body's iron stores. Transferrin saturation shows how much iron is available in the blood. Haemoglobin tells you whether red blood cell production has been affected. A full picture needs all three, and ideally a marker of inflammation.

How common is it?

Sim and colleagues summarised the research as showing that iron deficiency affects roughly 15–35% of female athlete cohorts and 5–11% of male athlete cohorts. Women are at higher risk, and endurance athletes are particularly affected.

Triathlon-specific data are scarce. One study, by Coates, Mountjoy and Burr, published in the Clinical Journal of Sport Medicine in 2017, looked back at routine blood tests taken between 2009 and 2015 in 38 elite runners and triathletes aged 21 to 36. Over that period:

• 60% of female triathletes and 37.5% of male triathletes had at least one episode of iron deficiency.
• 55.6% of female runners and 31.3% of male runners had at least one episode.
• Iron-deficiency anaemia occurred in 25% of male triathletes and 20% of female triathletes, compared with 6.3% of male runners and none of the female runners.

The authors noted that the higher rate of anaemia in men than in their female teammates hadn't been reported before. They also found that the athletes were taking oral iron, averaging 94 mg a day, but there was no correlation between iron intake and ferritin or haemoglobin.

These findings need careful reading. The sample was small, with only 38 athletes split across four sex and sport groups, so each percentage represents a handful of people. The abstract doesn't state the ferritin threshold used, and these were elite athletes, not age-groupers. But the study makes two useful points: iron deficiency is common even in well-monitored athletes, and men aren't immune.

Why endurance athletes are at risk

Several factors combine to put endurance athletes at higher risk:

• Higher needs. Sim and colleagues noted that exercise may increase iron losses by around 1–2 mg a day, on top of standard recommended intakes of 8 mg a day for men and 18 mg a day for women.
• Menstruation. Monthly blood loss is a major reason women are at higher risk.
• Diet. Iron from plant foods is absorbed less well than iron from meat and fish. Compounds such as phytates in whole grains and legumes, polyphenols in tea and coffee, and calcium reduce absorption, while vitamin C improves it.
• Low energy and carbohydrate availability. The review notes research on how diet, including manipulation of carbohydrate and overall energy, affects iron regulation. TriForward's "Under-Fuelled, Not Undertrained" covers low energy availability in detail.
• Altitude. Training at altitude increases demand for iron to build red blood cells.
• Exercise itself, through the hormone hepcidin.

Hepcidin: why exercise affects absorption

Hepcidin is the main hormone that controls iron absorption. When hepcidin is high, less iron is absorbed from the gut and more is held in storage.

Exercise, particularly hard or long sessions, causes inflammation that raises hepcidin. Sim and colleagues described hepcidin as rising for roughly 3–6 hours after exercise. Hepcidin is also naturally lowest in the morning.

That has practical implications for anyone who has been prescribed iron. The review suggests taking iron as far away from hard exercise as possible, with a preference for the morning. It also notes early research suggesting that iron taken within 60 minutes of finishing a morning session may still be absorbed well, before hepcidin has risen, but says this needs confirming.

Inference for triathletes: if you train hard in the evening, a morning dose is naturally well away from the session. If you train hard in the morning, taking iron soon afterwards or the next morning may both be reasonable options to discuss with your clinician.

Alternate-day dosing

Hepcidin also responds to iron itself. A dose of iron raises hepcidin for some time afterwards, which may reduce absorption from the next dose.

Stoffel and colleagues tested this in two randomised trials published in The Lancet Haematology in 2017. They studied iron-depleted women aged 18 to 40 with ferritin of 25 µg/L or less, using labelled iron to measure absorption precisely.

In the first trial, 40 women took 60 mg of iron either every day for 14 days or every other day for 28 days, so both groups took the same number of doses:

• Fractional absorption was higher on alternate days: 21.8% versus 16.3%.
• Total iron absorbed was higher on alternate days: about 175 mg versus 131 mg.
• Hepcidin was higher with daily dosing during the first 14 days.

In the second trial, 20 women compared a single 120 mg morning dose with two 60 mg doses (morning and afternoon). Splitting the dose raised hepcidin but didn't increase absorption.

The authors concluded that single doses on alternate days appeared to make the most of absorption, but that the results needed confirming in people with iron-deficiency anaemia. The participants weren't athletes, and the study measured absorption, not changes in ferritin or performance over months. Sim and colleagues also mention alternate-day dosing as an option that may improve absorption.

Does treating it help performance?

For athletes with clear iron-deficiency anaemia, treatment is a medical necessity. The more debated question is whether treating iron deficiency without anaemia helps.

Burden and colleagues addressed this in a 2015 systematic review and meta-analysis in the British Journal of Sports Medicine. They pooled 17 studies of iron-deficient but non-anaemic endurance athletes. Iron treatment:

• Raised ferritin with a large effect (Hedges' g 1.09).
• Raised haemoglobin with a moderate effect (g 0.70).
• Improved VO2max with a moderate effect (g 0.61).

Treatments lasting longer than about 80 days appeared to have the smallest effect on ferritin. The abstract doesn't report the total number of participants or a sex breakdown, and VO2max isn't the same as race performance. But it supports treating confirmed deficiency, even before anaemia develops.

Why the ferritin thresholds are debated

Ferritin is the most commonly used marker, but it has limitations that matter for athletes.

• Different cut-offs are used. Sim and colleagues note values of 35, 30, 20 and 12 µg/L used in different contexts. Laboratory reference ranges for the general population are often lower than the levels sports clinicians aim for.
• Ferritin rises with inflammation. It's an acute-phase protein, so it can go up after hard exercise, illness or injury, making iron stores look healthier than they are.
• Sex-specific cut-offs have been criticised for setting the lower limit for women too low.
• Ferritin alone doesn't tell the full story. Haemoglobin, transferrin saturation and a marker of inflammation help interpret it.

Inference: a ferritin result is most useful when it's taken in consistent conditions, ideally rested, not straight after a race or hard block, and when it's compared with your own previous results. One number on its own can mislead in either direction.

When to get tested

Sim and colleagues suggested routine screening once a year for athletes with no symptoms and no history of iron deficiency, and more often, every three to six months, for those at higher risk. Inference: based on the risk factors above, that probably includes female athletes, anyone with previous deficiency, vegetarians and vegans, athletes in heavy training, and those planning altitude training.

Reasons to ask for a test sooner include:

• Persistent fatigue that doesn't improve with rest.
• Unexplained drops in performance, especially in heart rate response or ability to sustain efforts.
• Breathlessness, dizziness or pale skin.
• Heavy periods.
• A restricted diet or a period of under-eating.

These symptoms aren't specific to iron. They can also point to under-fuelling, overreaching, illness, thyroid problems and other conditions, which is exactly why testing is better than guessing. TriForward's "Reading the Trend, Not the Number" covers how HRV and other monitoring can help track fatigue.

Why supplementing without testing is risky

Taking iron "just in case" has real downsides:

• Iron overload. The body has no easy way to get rid of excess iron. Haemochromatosis, an inherited condition in which iron slowly builds up over many years, is particularly common in people of northern European background, especially in Ireland, Scotland and Wales, according to the NHS. Its symptoms include tiredness and joint pain, which can easily be mistaken for training fatigue or low iron. People with haemochromatosis are usually advised to avoid iron supplements.
• Missing the cause. Iron deficiency in an adult always has a reason. That may be diet, heavy periods or training, but it can also be blood loss from the gut or a condition that affects absorption. A supplement can correct the numbers while the cause goes undiagnosed.
• Side effects. Sim and colleagues note that oral iron commonly causes gut disturbance. For an athlete trying to train gut tolerance for race fuel, that matters.
• It may not work anyway. In the Coates study, athletes were taking oral iron, but intake didn't correlate with their ferritin or haemoglobin.

What treatment usually looks like

Treatment should be guided by a clinician. Sim and colleagues described typical oral regimens of around 100 mg of elemental iron a day for 8–12 weeks, producing ferritin increases of roughly 40–80%. Intravenous or intramuscular iron raised ferritin much more quickly in the studies they cited, without gut upset, but it needs medical supervision and isn't the first option for most athletes. Reactions to injected iron range from mild rash to, rarely, anaphylaxis.

Diet is still important. Including well-absorbed iron sources, pairing plant iron with vitamin C, and keeping tea and coffee away from iron-rich meals can all help, although diet alone may be too slow to correct an established deficiency.

A typical triathlon scenario

The following is illustrative, not drawn from a case study. A female age-group triathlete in a heavy Ironman build notices that her easy runs feel hard and her heart rate is higher than usual. She's been eating less meat, has heavy periods, and is training twice a day. A friend suggests iron tablets.

Instead, she books a blood test through her GP, taken on a rest day. Her ferritin is low and her transferrin saturation is borderline, but haemoglobin is normal. Her doctor investigates her periods, reviews her diet with her, and prescribes iron with advice on timing. She also reviews her fuelling with a sports dietitian. After a few months, a repeat test shows improvement.

A male training partner with similar fatigue tries the same tablets without a test. His symptoms don't improve. When he's finally tested, his iron is high. He has haemochromatosis.

Common mistakes

• Taking iron without a test.
• Testing straight after a race or a hard block, when inflammation can push ferritin up.
• Assuming men don't need to think about iron.
• Comparing your ferritin with a friend's rather than with your own previous results.
• Taking iron with tea, coffee or calcium-rich foods.
• Treating the number but not the cause.

What we don't know

• The right ferritin threshold for athletes. Cut-offs vary and there's no single agreed value.
• How common iron deficiency is in age-group triathletes. Most data come from elite or small cohorts.
• Whether alternate-day dosing improves performance outcomes in athletes, rather than just absorption in non-athletes.
• How best to time iron around twice-daily training.

References

• Sim M, Garvican-Lewis LA, Cox GR, Govus A, McKay AKA, Stellingwerff T, Peeling P. Iron considerations for the athlete: a narrative review. European Journal of Applied Physiology, 2019;119(7):1463–1478. doi:10.1007/s00421-019-04157-y
• Coates A, Mountjoy M, Burr J. Incidence of iron deficiency and iron deficient anemia in elite runners and triathletes. Clinical Journal of Sport Medicine, 2017;27(5):493–498. doi:10.1097/JSM.0000000000000390
• Burden RJ, Morton K, Richards T, Whyte GP, Pedlar CR. Is iron treatment beneficial in, iron-deficient but non-anaemic (IDNA) endurance athletes? A systematic review and meta-analysis. British Journal of Sports Medicine, 2015;49(21):1389–1397. doi:10.1136/bjsports-2014-093624
• Stoffel NU, Cercamondi CI, Brittenham G, Zeder C, Geurts-Moespot AJ, Swinkels DW, Moretti D, Zimmermann MB. Iron absorption from oral iron supplements given on consecutive versus alternate days and as single morning doses versus twice-daily split dosing in iron-depleted women: two open-label, randomised controlled trials. The Lancet Haematology, 2017;4(11):e524–e533. doi:10.1016/S2352-3026(17)30182-5
• NHS. Haemochromatosis. NHS website, accessed 11 October 2026. nhs.uk