Nickel metabolism in humans investigated with an oral stable isotope

We report the results of the first complete study of nickel metabolism in human subjects using a stable nickel isotope (62Ni) as tracer. Four healthy adult subjects (two women and two men) fasted overnight before ingesting 10 microg 62Ni/kg body wt. Blood samples were drawn after fixed intervals of time and the total daily output of urine and feces was collected for the first 5 d after dose ingestion. 62Ni in plasma, urine, and feces was determined by isotope-dilution inductively coupled plasma-mass spectrometry with 61Ni. The direct measurement of the fecal excretion of the tracer allowed a reliable assessment of nickel absorption from the gastrointestinal tract and we found no evidence of the excretion of absorbed nickel via the gut. The percentage absorption calculated from the amount of 62Ni excreted in the feces ranged from 29% to 40%. Urinary excretion over 5 d ranged from 51% to 82% of the absorbed dose. Plasma 62Ni peaked between 1.5 and 2.5 h after ingestion and decreased by a factor of > 10 over the next few days. We observed low between-subject variability of nickel absorption and excretion. Confounding factors such as contamination and dietary intake of nickel, which hampered earlier measurements in subjects dosed with naturally abundant nickel, were eliminated by using the tracer isotope 62Ni.