In HPU the body excretes a substance, called HPL-complex, in the urine. This excretion indicates an inborn error in the haem synthesis in which more Copro I is spontaneously synthesized. This compound is broken down and excreted, as likewise more Copro I is excreted. The excretion of the HPL-complex as well the Copro I, increases in a carbohydrate-free diet. The use of pyridoxal-5-phosphate, zinc and manganese let the excretion diminish in three to four months.