Low red blood cell count: real anaemia or sports anaemia?

A low red blood cell count sounds like anaemia, and in most people it is. In a trained endurance athlete the most common cause lies elsewhere: your blood is diluted. Your body has made extra plasma, the number of red cells has stayed the same, and the count per litre therefore comes out lower.

That difference is not academic. It is the difference between taking iron you do not need and missing a real deficiency.

I see this misunderstanding in runners and cyclists more than in any other group. Their value drops at precisely the moment their fitness climbs.

What does a low red blood cell count mean?

It means fewer erythrocytes are counted per litre of blood than expected, often alongside a lower haemoglobin and haematocrit. Those three values almost always move together, because they measure the same cells from a different angle. Anaemia is the clinical term once your haemoglobin drops below the lower limit.

There are broadly three ways that happens. You make too few red cells, you lose them, or you dilute them.

That last category is the forgotten one. And in athletes it is the largest.

What is sports anaemia?

Sports anaemia, or pseudo-anaemia, is a dilution effect and not a deficiency. Endurance training makes your body hold on to more blood plasma, sometimes ten to twenty percent more. The total number of red cells in your body stays the same or even rises, but because they swim in more fluid the lab counts fewer per litre (PMID 10688281).

That extra plasma is not a defect. It is an adaptation. More plasma means a larger stroke volume and better heat dissipation, which is exactly what you want when you run a marathon.

The confusion arises because the test shows the same number as in real anaemia. In medicine, telling true anaemia apart from dilutional anaemia is a well known and clinically important distinction (PMID 38863867).

Imagine two runners, both with a haemoglobin of 8.2 mmol/l, just under the lower limit for men. The first trains ten hours a week, feels strong and has seen his times improve. The second has been tired for two months, is short of breath on the stairs, and his ferritin is 8 µg/l. The same haemoglobin, opposite situations.

How do you tell sports anaemia from real anaemia?

By looking at your ferritin, your symptoms and your performance, not at your haemoglobin alone. Dilution does not touch your performance; a real deficiency does. That is the most usable distinction you can draw without a doctor.

The row most athletes skip is the ferritin row. Without that value this distinction is largely guesswork.

Can an athlete have diluted blood and an iron deficiency at once?

Yes, and that is the combination where things genuinely go wrong. Dilution lowers your haemoglobin, and an iron deficiency lowers it too. If you assume your low value is purely dilution, you miss the deficiency sitting underneath it.

Endurance athletes carry exactly that risk. They lose iron through sweat, through small bleeds in the gut and, in runners, through damage to red cells in the soles of the feet. In female endurance athletes the prevalence of iron deficiency is high (PMID 36554486), and among university athletes iron deficiency without anaemia proved anything but rare (PMID 38018828).

So I keep it simple: if your haemoglobin is low, you want your ferritin next to it. Full stop.

When should you discuss a low value with your GP?

When you have symptoms, when your ferritin is low, or when your value keeps sliding while your training stayed the same. Fatigue that rest does not lift, breathlessness during efforts you used to handle easily, pallor or palpitations belong in a conversation. The NHG guideline on anaemia describes anaemia not as one disease but as a finding with many possible causes.

Above all, do not start taking iron on your own based on a single low haemoglobin. Without a measured deficiency that is a gamble, and excess iron accumulates.

My experience: the athletes who gained most from a conversation were not the ones with the lowest number, but the ones whose number did not match how they felt.

Where do you start?

Measure your red series alongside your ferritin, and measure again after a calm period. A single low value is a question, not an answer. Our complete blood count test gives you erythrocytes, haemoglobin, haematocrit and the indices, with an assessment by a BIG-registered doctor.

How the values connect is in complete blood count explained. What your MCH says about your iron is in the MCH value. Why endurance athletes in particular lose iron is in iron deficiency in athletes, and the mirror image (a value that is too high) is in high haematocrit. The size of your red cells itself, and when it runs too high, is covered in high MCV explained.

References

1. Clinical importance of distinguishing true anemia from dilutional pseudo-anemia. Clinical Case Reports. 2024. PMID 38863867.
2. Fallon KE, Fallon SK, Boston T. Sports haematology. Sports Medicine. 2000. PMID 10688281.
3. High prevalence of iron deficiency in competitive female endurance athletes. International Journal of Environmental Research and Public Health. 2022. PMID 36554486.
4. Prevalence of iron-deficient but non-anemic university athletes. Journal of the International Society of Sports Nutrition. 2023. PMID 38018828.
5. NHG. NHG guideline on anaemia. Accessed 2026.
6. RIVM. Blood testing and reference values. Accessed 2026.

Disclaimer

Every blood test result includes a professional assessment by a BIG-registered doctor. This article gives general information and is not a substitute for medical advice. For treatment decisions, discuss your results with your GP.