Erythropoiesis is the formation of red blood cells in bone marrow — the continuous manufacture of the body’s oxygen carriers. Stem cells in the marrow divide and differentiate through a precisely staged lineage, expelling their nuclei and loading up with haemoglobin, until some two million new red cells enter the blood every second. The hormone erythropoietin, released by the kidneys when oxygen runs low, throttles production up or down. Nutrition is woven through the process: iron builds the haem, vitamin B12 and folate drive the DNA synthesis of the dividing precursors, copper and vitamin B6 play supporting roles — and deficiency at any point produces anaemia, the clinical face of failed erythropoiesis. Athletes’ illicit EPO use hijacks the same hormone. It is manufacturing at biological scale, and diet supplies the raw materials.