Whole-body Retention, Tissue Distribution and Excretion of Selenium-75 after Oral and Intravenous Administration in Lambs Fed Varying Selenium Intakes

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Selenium metabolism was studied by measuring whole-body retention, urine and fecal losses and retention in selected tissues following administration of 75Se to lambs fed varying levels of dietary selenium. Whole-body loss of 75Se 48 to 336 hours after administration of the isotope could be described by a first-order rate constant which was inversely proportional to the dietary level of selenium. Intravenous injection of 75Se resulted in a higher retention than oral administration, especially on low dietary intakes of selenium. The concentration of 75Se in various tissues was inversely related to the dietary selenium level. Kidney and liver tissue were consistently higher in 75Se concentration than any other internal organ tissue. Of the total 75Se dose administered, the carcass retained the highest amount, being indirectly proportional to the dietary selenium level. The major pathways of 75Se excretion are the feces and urine. The kidney exhibited an unlimited ability to excrete 75Se. Since a significant amount of 75Se excretion was not accounted for as urine and fecal loss, respiratory loss of volatile selenium products formed in the rumen is offered as a possible explanation. Relative importance of these three excretory pathways depended upon dietary selenium intake level and route of 75Se administration.