Iron and endurance sport: deficiency signs, testing and safe supplementation

For an endurance athlete, fatigue, breathlessness or a performance dip can have many causes. Iron deficiency may occur with or without anaemia, but symptoms alone cannot establish it. Testing and supplementation therefore need clinical guidance. [1][2][6]

Iron supports haemoglobin, which carries oxygen, and myoglobin in muscle. These roles matter during endurance exercise, but fatigue after a hard block may also reflect sleep, low energy availability, infection, excessive training or another condition. [3][6]

In some athletes, sweat losses, impact-related haemolysis, small gastrointestinal losses, insufficient intake, growth, pregnancy or heavy menstrual bleeding may contribute to risk. Heavy periods deserve attention; they should not be reduced to a generic “low iron” explanation. [2][6]

Heme iron occurs only in animal foods and is, on average, absorbed better than nonheme iron. Plant foods provide nonheme iron, whose absorption is more affected by iron stores and meal composition. [2][3]

Deficiency can begin with depleted stores while haemoglobin remains in the reference range. When haemoglobin production becomes insufficient, iron-deficiency anaemia can develop. These are not interchangeable labels: deficiency without anaemia exists, and fatigue can have other causes. [1][6]

Ferritin gives information about iron stores. Haemoglobin helps identify anaemia. Depending on the context, a clinician may also discuss transferrin saturation, a blood count or inflammation markers. [1][6]

Sports thresholds are not perfectly uniform. A value advertised online as “optimal for every runner” is an oversimplification. Timing of the sample, recent infection, training load and medical context matter.

Animal sources include meat, poultry, fish, seafood and liver. Plant sources include legumes, nuts, seeds, grains and leafy green vegetables. The aim is varied, sufficiently energetic food that fits the person’s choices. [2][3]

Vitamin C can support nonheme iron absorption. Pairing legumes or grains with citrus, kiwi, peppers, tomatoes or cabbage is a simple food strategy. [2]

Tea and coffee taken with a meal, and some phytates, can reduce nonheme absorption. That is not a reason to remove legumes, grains, tea or coffee; meals and timing can be varied according to tolerance and context. [2][3]

Trials in athletes mainly show higher ferritin when initial stores are low; they do not justify taking iron to improve performance without documented deficiency. Studied protocols differ in form, dose, duration and population. [5][6]

Intravenous iron is a targeted medical decision; it is not a performance shortcut and should not be used without documented deficiency and indication. The IOC notes that supplements do not replace a sound diet or appropriate follow-up. [4]

Iron can cause nausea, constipation or diarrhoea, reduce absorption of some medicines and become toxic in excess; dose and form therefore need supervision. Chronic excess can damage organs including the liver, and iron-overload disorders change the risk. [1][4][7]

Extra care is warranted with pregnancy, children, kidney or digestive disease, bleeding, inflammatory disease, regular medication or a highly restrictive diet. Seek advice promptly with pregnancy, a child concerned, very heavy periods, digestive disease, bleeding, persistent fatigue or suspected deficiency. [1][2]

1. 01Describe the symptom, its duration and its course instead of attributing it immediately to iron.
2. 02Review risks: heavy periods, low-energy or poorly planned plant-based eating, losses, growth, pregnancy, digestive disease and training load.
3. 03Ask whether testing is appropriate; do not choose markers or interpret one result alone.
4. 04If deficiency is confirmed, investigate the cause and follow the prescribed plan.
5. 05Check elemental iron, interactions and duration with the clinician.
6. 06Review tolerance and progress rather than increasing the dose yourself.

Iron is essential for oxygen transport, but an endurance athlete’s fatigue is not automatically iron deficiency. Ferritin and haemoglobin answer different questions, and inflammation and context can complicate interpretation. Food can support intake; supplementation should follow documented deficiency and medical guidance.