Iron and Endurance Performance: Deficiency, Signs and Sources

Iron is central to endurance performance because it carries oxygen in the blood and helps muscles use that oxygen to produce energy. Iron deficiency is one of the most common nutritional issues in athletes, particularly endurance athletes and women. Yet iron is also a nutrient where more is not better and supplementing without testing can be harmful. This article explains the balance.

Why iron matters for endurance

Iron is a key component of haemoglobin, the protein in red blood cells that transports oxygen from the lungs to the tissues. It is also part of myoglobin in muscle and of enzymes involved in energy production. When iron stores fall, the body's ability to deliver and use oxygen declines, which directly undermines endurance capacity, and can cause fatigue, breathlessness and reduced training tolerance.

Stages of iron deficiency

Iron deficiency develops in stages, and performance can suffer before full-blown anaemia appears:

• Depleted iron stores, shown by low ferritin, before haemoglobin falls.
• Iron-deficient without anaemia, where stores are low and function begins to be affected.
• Iron-deficiency anaemia, where haemoglobin drops and oxygen delivery is clearly impaired.

Because early deficiency can affect wellbeing and training before anaemia, endurance athletes with symptoms benefit from checking iron status rather than waiting.

Who is at risk?

Several factors make athletes more prone to low iron:

• Female athletes, due to menstrual blood losses.
• Endurance athletes, through mechanisms such as foot-strike red blood cell breakdown, sweat losses and increased iron regulation during heavy training.
• Vegetarians and vegans, because plant iron is less well absorbed than iron from meat.
• Athletes in growth phases or those on low-energy diets.

Common signs of low iron

Warning signs can include unusual tiredness, declining performance despite consistent training, breathlessness during efforts that used to feel easy, pale skin, frequent minor illness and poor recovery. These symptoms are non-specific and can have many causes, from inadequate sleep and overtraining to other nutritional gaps, which is exactly why testing is essential before assuming iron is to blame.

Why you should test before supplementing

Iron is a case where self-prescribing is unwise. The body has no efficient way to excrete excess iron, so taking supplements when you are not deficient can lead to a build-up that may harm the liver and other organs, and some people carry genetic conditions that cause iron overload. High-dose iron can also cause constipation, nausea and stomach upset. For these reasons, the sensible approach is to confirm deficiency with a blood test, typically including ferritin and haemoglobin, and to supplement only under medical guidance at an appropriate dose. Retesting helps confirm that stores are recovering.

Food sources of iron

There are two dietary forms of iron. Haem iron, from animal foods, is absorbed more efficiently, while non-haem iron, from plants, is absorbed less well but can be boosted by pairing with vitamin C.

• Red meat, poultry and fish, the best-absorbed haem iron sources.
• Legumes such as lentils, beans and chickpeas.
• Tofu and other soy foods.
• Dark leafy greens such as spinach.
• Fortified cereals and wholegrains, and nuts and seeds.

Absorption of non-haem iron improves when eaten with vitamin C-rich foods such as citrus, peppers or tomatoes, while tea, coffee and calcium-rich foods eaten at the same time can reduce absorption. Timing these drinks away from iron-rich meals is a simple way for at-risk athletes to get more from the iron they eat.

Practical guidance

Endurance and female athletes should prioritise iron-rich foods and pair plant sources with vitamin C. If you notice persistent fatigue or declining performance, ask a doctor or sports physician to check your iron status rather than starting supplements on your own. When deficiency is confirmed, targeted supplementation under supervision can restore performance, but for athletes who are not deficient, iron supplements offer no benefit and carry real risks.

Key takeaways

• Iron enables oxygen transport and energy production, making it vital for endurance.
• Deficiency progresses in stages and can hurt performance before anaemia appears.
• Female, endurance and plant-based athletes are at greatest risk.
• Signs like fatigue and declining performance are non-specific, so testing is essential.
• Only supplement iron when deficiency is confirmed and under medical guidance, since excess iron is harmful.
• Choose iron-rich foods and pair plant sources with vitamin C to aid absorption.

References

1. Iron — Health Professional Fact Sheet — NIH Office of Dietary Supplements
2. Sim M et al., Iron considerations for the athlete: a narrative review, European Journal of Applied Physiology, 2019 — European Journal of Applied Physiology
3. WHO Guideline: Daily iron supplementation in adult women and adolescent girls (2016) — World Health Organization
4. Nutrition and Athletic Performance — Joint Position Statement (2016) — Academy of Nutrition and Dietetics, Dietitians of Canada, ACSM