Iron, Blood Health and Endurance Performance

Why fit endurance athletes can still feel fatigued

For endurance athletes, performance isn’t determined by training alone. Even when training volume is consistent, performance is multifactorial. One often-overlooked factor is blood health, more specifically iron status.

Blood health, the optimal amount of the key components of blood, governs how efficiently oxygen and nutrients are delivered to organs and tissues including working muscles. When this system is compromised, endurance performance can suffer regardless of how disciplined an athlete’s training programme may be.

Why blood health matters in endurance sport

Iron is central to endurance performance. It is required for the production of key heme proteins such as:

1. Haemoglobin

Which transports oxygen from the lungs to tissues

2. Myoglobin

The muscle store of oxygen: it allows the immediate release of oxygen to muscles when needed, sustains aerobic metabolism and thus delays lactate build up allowing more efficient performance overall.

3. Cytochromes

Mitochondrial enzymes involved in aerobic energy production.

Research led by sports scientists at the University of Zurich, Australian Institute of Sport, and reviewed in the British Journal of Sports Medicine and International Olympic Committee (IOC) consensus statements, suggests that 15–35% of endurance athletes have iron deficiency without anaemia. This level of deficiency is sufficient to blunt training adaptation and leave athletes feeling persistently fatigued despite “doing everything right”.

How iron metabolism is affected in endurance athletes

Endurance athletes are at risk of iron deficiency because of:

1. Increased iron loss

Iron is lost through sweat, gastrointestinal micro-bleeding and exercise-induced haemolysis, where repetitive mechanical stress damages red blood cells.

2. Increased iron demand

The upregulation of red blood cells and the need for muscle adaptation thus more myoglobin

3. Dietary absorption

Especially if vegetarian/vegan, absorption may be insufficient to meet demands

4. Females having menstrual bleeding

Will be at further increased risk due to additional blood loss

The role of hepcidin

Iron balance in the body is tightly regulated and influenced by both physiological demand and inflammation. Iron is absorbed in the small intestine, transported in the blood bound to transferrin, and stored primarily as ferritin in the liver, spleen and bone marrow. It is then used to synthesise heme proteins mentioned above.

Endurance exercise significantly increases iron demand through higher red blood cell turnover and increased mitochondrial activity. At the same time, repeated or intense training triggers a transient inflammatory response, increasing levels of hepcidin, a liver-derived hormone that plays a central role in iron regulation. Hepcidin reduces iron absorption from the gut and traps iron within storage sites, temporarily limiting its availability for red blood cell production.

The cumulative effect is a gradual depletion of iron stores that can impair oxygen delivery, energy metabolism and recovery long before anaemia develops.

Common blood imbalances that limit endurance performance

Iron depletion is the most common blood imbalance in endurance athletes, but it is not the only one. Research from Loughborough University and the University of Western Australia highlights that low haemoglobin, vitamin B12 or folate deficiency can impair red blood cell production and recovery. Electrolyte imbalance and low vitamin D are also associated with muscle fatigue, weakness and increased injury risk.

Athletes may notice declining power or pace, elevated heart rate at familiar intensities, slower recovery between sessions, or increased susceptibility to illness. These symptoms are frequently misattributed to overtraining rather than an underlying blood health issue.

How iron depletion affects endurance performance

When iron stores fall, oxygen delivery and utilisation are compromised. VO₂ max (the maximum rate at which the body can take in, transport and use oxygen expressed at ml O₂/kg body weight/min) can decline leading to an appreciable drop in performance and slower recovery. Athletes often report that familiar training intensities feel disproportionately hard, with higher perceived effort and rising heart rate.

Research published in Medicine & Science in Sports & Exercise, supported by laboratory studies from the University of Lausanne, demonstrates that iron deficiency without anaemia significantly impairs endurance performance and training efficiency.

Iron deficiency alone can cause early fatigue, exercise intolerance and shortness of breath on minimal exertion and you don’t need to be in a heavy phase of training to notice this.

Why female endurance athletes are at higher risk

Female athletes are particularly susceptible to iron depletion due to menstrual blood loss, higher iron demands from endurance training and, in some cases, lower dietary intake. Hormonal factors may also influence iron absorption and regulation.

Large cohort studies cited in the British Journal of Sports Medicine, including work from Stanford University School of Medicine, consistently show higher rates of iron deficiency in female endurance athletes. Iron stores can decline even when haemoglobin remains normal, leading to earlier fatigue, reduced endurance and impaired recovery.

When to test and what actually matters

Endurance athletes should consider blood testing if they experience unexplained fatigue, declining performance, poor recovery or rising perceived effort, particularly during heavy training blocks or ahead of key events.

Key markers to test

Beyond haemoglobin, ferritin is the most important marker, as it reflects iron stores in health and often falls first. IOC and BJSM guidance highlight that endurance athletes frequently perform best with ferritin levels above standard clinical cut-offs, even when haemoglobin remains within the normal range simply because one person can be symptomatic with a ferritin of 50 ng/ml while another may be ok until ferritin is 25 ng/ml.

Diet, supplements and absorption

Iron absorption is strongly influenced by diet and timing. We know that vitamin C significantly increases absorption of non-heme iron (plant based iron and supplements) while caffeine consumed close to meals significantly reduce absorption.

Vegetarian and vegan endurance athletes are at higher risk of inadequate iron intake and may require closer monitoring. Oral iron supplements can be effective but are often limited by gastrointestinal side effects and reduced absorption during periods of heavy training.

Optimise iron absorption

• Consume iron-rich foods with vitamin C sources
• Avoid tea, coffee and calcium around iron-rich meals
• Time supplementation away from hard training sessions (3-6 hour window post-exercise)

When iron infusions are considered

When symptomatic iron deficiency is confirmed and oral supplementation is ineffective, poorly tolerated or insufficient to meet physiological demands, iron infusions may be considered under medical supervision. Efforts should be made to rule out any other contributory causes of fatigue and with particular relevance to each individual case.

The International Olympic Committee and British Journal of Sports Medicine recognise intravenous iron as an appropriate treatment option in athletes with clinically low ferritin, particularly where more rapid restoration is required to support training consistency, recovery and performance. Clinical studies from University College London and the Australian Institute of Sport demonstrate that intravenous iron restores iron stores more reliably than oral supplementation, provided dosing is individualised and carefully monitored to minimise risk.

Persistent fatigue and underperformance are not always a training problem. Blood health, particularly iron status, plays a critical role in endurance capacity, recovery and consistency. Early recognition, appropriate testing and targeted treatment can unlock performance gains that training alone cannot deliver.