Haemoglobin (HGB)
Haemoglobin is the protein inside red blood cells that carries oxygen. Higher haemoglobin means more oxygen delivery per unit of blood — which translates directly to aerobic capacity and endurance. The standard male range is 130–175 g/L. For athletes optimising aerobic performance, the upper portion of this range — 155–175 g/L — is generally associated with better outcomes. Below 130 g/L, even if technically 'normal', is often associated with noticeable fatigue and reduced VO2 max.
Haematocrit (HCT)
Haematocrit is the percentage of your blood volume made up of red blood cells. It moves in parallel with haemoglobin. The standard male range is 38–50%. Altitude training, adequate iron stores, and good hydration all support haematocrit. Dehydration can falsely elevate haematocrit — which is why a high result should always be retested with proper hydration before drawing conclusions. Above 52–54% warrants investigation, as this level increases blood viscosity and cardiovascular risk.
Red Blood Cell Count (RBC)
The actual count of red blood cells per litre of blood. Standard male range is 4.5–5.9 ×10¹²/L. RBC count works alongside haemoglobin and haematocrit to give a complete picture of oxygen-carrying capacity. Low RBC with normal haemoglobin may indicate macrocytosis — larger than normal red blood cells, often caused by B12 or folate deficiency.
What drives these markers down?
Iron deficiency is the most common culprit — particularly in female athletes and those with high training volumes. Ferritin below 30 µg/L is strongly associated with reduced haemoglobin. B12 and folate deficiency impair red cell production. Chronic inflammation suppresses erythropoiesis — the production of new red blood cells. Overtraining syndrome can also reduce RBC production via neuroendocrine disruption.
What you can do
If haemoglobin and haematocrit are low without an obvious cause, always check ferritin first — it's the most commonly missed issue. Check B12 and folate. Ensure adequate sleep and recovery. Altitude training or altitude simulation can drive meaningful improvements in red cell mass over 3–4 weeks. Test these markers twice a year at minimum, or after any period of unexplained fatigue or performance decline.