Energy and the blood

Red cells divide fast and need methylation to copy DNA cleanly, so a shortage makes them come out large, pale and too few. Less haemoglobin means less oxygen to every muscle, and that is the breathlessness on stairs and the heaviness in the legs. Refilling B12 lets the marrow build normal cells again, and the blood count climbs over weeks as the old cells are replaced. The mitochondrial fuel door opening again adds to it from the other side.