Human urinary excretion and metabolism of 82Se-enriched selenite and selenate determined by LC-ICP-MS

Urinary excretion of selenium after ingestion of isotope labeled selenite and selenate was studied in seven healthy volunteers, 4 men and 3 women (age 28-50 years). An aqueous solution containing 330 mu L Se-82-selenate (corresponding to 74.3 mu g Se-82) was given orally and urine samples were subsequently collected during the following 24 hours. The scheme was repeated four weeks later with a 280 mu L Se-82-selenite solution (corresponding to 74.4 mu g Se-82). The amount of total Se in the urine samples was determined by inductively coupled mass spectrometry. The mean total urinary excretion of Se-82 following Se-82-selenate administration was 33.7% (range 15.6-42.5%) while the mean total excretion of Se-82 after Se-82-selenite administration was 3.2% (range 2.8-3.9%) of the ingested amount. LC-ICPMS analysis of the urine samples showed that the majority of the selenium excreted after selenate ingestion was unchanged selenate for 6 of the individuals while one individual had metabolized a fraction (approx. 20%) of the selenate to selenosugar. Ingestion of 10 times larger doses of selenite in two individuals resulted in 13-23% excretion primarily excreted as selenosugar. These results show that the human metabolic pathways of selenite and selenate are different and indicate that not all selenate, although well absorbed, may be available for the beneficial health effects.