In Hemopyrrollactamuria (HPU), the body excretes a substance called HPL-complex into the urine. The excretion of this substance indicates an inborn error of haem synthesis, in which excess Copro I is spontaneously synthesized. This compound is broken down and excreted, causing yet more Copro I to be excreted. The excretion of both HPL-complex and Copro I increases in a carbohydrate-free diet. The use of pyridoxal-5-phosphate, zinc and manganese supplementation for a period of 3-4 months allows a gradual decrease in the excretion of Copro I.