Why Endurance Athletes Deplete Iron Faster
Endurance athletes have significantly higher iron requirements than sedentary individuals, and several mechanisms work simultaneously to deplete their stores:
- Foot-strike haemolysis: In runners, the repeated impact of footstrike on hard surfaces causes mechanical destruction of red blood cells in the capillaries of the foot — releasing iron that is then lost in the urine. This is well-documented and proportional to weekly mileage.
- Sweat losses: Iron is lost in sweat. High-volume training in warm conditions can produce meaningful iron loss over time.
- Gastrointestinal blood loss: Intense endurance exercise — particularly long runs — is associated with micro-haemorrhage in the gut, contributing to iron loss.
- Hepcidin elevation: After intense exercise, the liver releases hepcidin, a hormone that temporarily reduces gut iron absorption. Athletes who supplement with oral iron after training sessions may absorb less than they expect.
- High turnover of red blood cells: Training stimulates red blood cell production, increasing iron demand at the same time iron loss is elevated.
Who is at highest risk? Female athletes (due to menstrual iron loss on top of training losses), high-mileage runners, athletes in calorie restriction, vegetarians and vegans, and anyone who has recently increased training volume significantly.
How Iron Deficiency Affects Performance
Iron is central to oxygen transport. Haemoglobin — which requires iron — is the molecule that carries oxygen from the lungs to working muscles. Myoglobin, which stores oxygen within muscle cells, also requires iron. Even before anaemia develops, depleted iron stores impair:
Physiological impacts
- Reduced VO2 max
- Impaired mitochondrial function
- Reduced lactate threshold
- Slower muscle recovery
Subjective symptoms
- Unexplained fatigue during training
- Legs feeling heavy at same effort
- Breathlessness at familiar intensities
- Slower recovery between sessions
Research has shown that correcting iron deficiency — even without anaemia — improves VO2 max, endurance capacity, and perceived effort at the same workload. The gains from iron restoration in a genuinely deficient athlete can be substantial.
Why Oral Iron Often Fails in Athletes
Oral iron supplementation is the first-line treatment for iron deficiency — but for many endurance athletes, it falls short for several reasons:
- Hepcidin interference: Training raises hepcidin, which actively suppresses gut iron absorption. Taking tablets close to a training session means much of the dose may not be absorbed at all.
- GI sensitivity: Oral iron commonly causes nausea, constipation, and stomach cramps — which reduce compliance, particularly in athletes who are already sensitive to GI symptoms during running.
- Slow timeline: Oral iron takes 3–6 months to meaningfully restore ferritin. For an athlete preparing for a goal race in 6–8 weeks, this is too slow to help in the current training block.
- Ongoing losses outpace absorption: At high training volumes, iron losses may continue at a rate that oral supplementation cannot keep up with.
The Case for IV Iron in Athletes
An IV iron infusion delivers iron directly into the bloodstream, completely bypassing the gut. This means hepcidin, gut sensitivity, and absorption variability are irrelevant — the full dose is delivered and available for use immediately.
Ferritin typically rises substantially within four to six weeks of a single infusion. For an athlete preparing for a key event, this can make the difference between arriving at the start line with depleted stores and arriving with ferritin at an optimal level for performance.
IV iron is not a performance-enhancing drug when used to correct a confirmed deficiency. WADA does not prohibit therapeutic IV iron use, though the method of administration (IV) is listed — athletes competing at high levels should verify current rules with their sport's governing body.
What Blood Tests Do Athletes Need?
A comprehensive iron screen for an endurance athlete should include ferritin, haemoglobin, transferrin saturation (TSAT), serum iron, and full blood count. We also routinely add CRP (to account for inflammation elevating ferritin) and B12/folate, which affect red blood cell production independently of iron.
Use our Iron Deficit Estimator to calculate your iron requirement from your results, then book an assessment with our GP for a full clinical evaluation.