A runner reduces their training load, sleeps more, and still can't hit splits they were hitting six months ago.
The most common diagnosis: overtraining. The actual cause, in a significant proportion of cases: iron deficiency.
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The Iron-Performance Link Most Coaches Miss
Iron is the central component of haemoglobin — the protein that carries oxygen in red blood cells — and myoglobin, which stores oxygen in muscle tissue. But its performance relevance extends beyond oxygen transport.
Iron is also essential for:
- Mitochondrial respiratory chain function (cytochromes b and c)
- Oxidative phosphorylation efficiency
- Thyroid hormone synthesis (which regulates metabolic rate)
- Immune function and recovery
The problem is that standard full blood counts (FBC) measure haemoglobin, not iron stores. You can have normal haemoglobin and severely depleted ferritin — the storage form of iron — for months before anaemia develops.
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Iron Deficiency Without Anaemia: The Missed Diagnosis
Clinical iron deficiency anaemia is defined by haemoglobin below 120 g/L (women) or 130 g/L (men). Most GPs only flag a problem when this threshold is crossed.
But iron deficiency without anaemia — also called pre-latent or latent iron deficiency — begins affecting performance at ferritin levels below 30–35 ng/mL, well before haemoglobin falls.
At ferritin below 20 ng/mL, endurance athletes typically show:
- Reduced VO2max relative to training load
- Impaired lactate clearance
- Elevated heart rate at submaximal intensities
- Disproportionate perceived effort
- Disrupted sleep quality
The 2012 Burden et al. study found that iron supplementation in female athletes with ferritin below 35 ng/mL (but normal haemoglobin) improved time to exhaustion and reduced heart rate at equivalent workloads — without any change in haemoglobin. The performance benefit came from restoring iron's non-haematological functions.
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Why Runners Are at High Risk
Runners have a unique iron loss pathway that most other athletes don't: foot-strike haemolysis.
Repetitive impact — particularly on hard surfaces — mechanically ruptures red blood cells in the capillaries of the foot, releasing haemoglobin into the urine (haematuria) and accelerating iron turnover. This effect is significant in high-mileage runners and explains why distance runners have substantially higher iron requirements than sedentary populations.
Additional runner-specific risk factors:
- Gut iron absorption is suppressed for up to 6 hours post-exercise (hepcidin response)
- Inflammation from heavy training suppresses iron bioavailability
- Female runners with menstruation: iron losses can reach 1–2 mg/day during menstruation, against dietary absorption of only 1–1.5 mg/day under normal conditions
- Plant-based and low-red-meat diets: non-haem iron from plant sources has 2–15% bioavailability vs. 15–35% for haem iron from meat
What to Measure — and What the Numbers Mean
A standard FBC is insufficient. The panel that actually captures iron status:
| Marker | Optimal for Athletes | Flag Below |
|---|---|---|
| Ferritin | > 50 ng/mL | < 30 ng/mL |
| Serum iron | 60–170 μg/dL | — |
| Transferrin saturation | 20–50% | < 15% |
| Haemoglobin | ♀ >130 g/L, ♂ >140 g/L | ♀ <120, ♂ <130 |
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Correcting Iron Deficiency Without Overloading
Iron supplementation in athletes requires care: excess iron generates oxidative stress and impairs training adaptation.
For confirmed deficiency (ferritin < 30 ng/mL):
- Dietary first: Red meat 2–3x/week, liver occasionally, legumes with vitamin C to enhance non-haem absorption
- Supplementation if dietary intervention is insufficient: Ferrous sulfate 100–200mg every other day is now preferred over daily dosing — the hepcidin response means daily dosing reduces net absorption compared to alternate-day protocols (Moretti et al., 2015)
- Retest ferritin at 8–12 weeks: Haemoglobin response is faster (4–6 weeks) but ferritin stores take longer to replenish
- Avoid iron supplements within 6 hours post-exercise: Hepcidin peak suppresses absorption
When did you last check your ferritin specifically — not just a standard blood count?