HaemoglobinHemoglobin · Hb

Haemoglobin is the iron containing protein inside red blood cells that binds oxygen in the lungs and releases it in the tissues. It is what makes blood red and what makes oxygen transport possible at the volumes the body requires.

Its concentration is the central number in diagnosing anaemia, and anaemia matters for performance long before it matters clinically: reduced oxygen carrying capacity shows up as breathlessness, fatigue and a distinct inability to hold paces that used to be comfortable.

It is read alongside the red cell indices. Mean corpuscular volume indicates whether the cells are small, normal or large, which narrows the cause, and red cell distribution width shows how variable they are, which often changes earlier than the average does.

Endurance athletes present a specific complication called dilutional pseudoanaemia. Training expands plasma volume more than red cell mass, so haemoglobin concentration falls while total oxygen carrying capacity is unchanged or improved. That is an adaptation rather than a deficiency, and treating it with iron is a mistake.

Genuine iron deficiency is common in menstruating women, endurance athletes and vegetarians, and ferritin catches it earlier than haemoglobin does, because stores are drained before the cells are affected.

Altitude raises it over weeks by stimulating red cell production, which is the legitimate basis of altitude training and the biology that blood doping abuses.

In practice, low haemoglobin always needs a cause found rather than only correcting, and iron supplements are a medical decision rather than a purchase.