A human model of selenium that integrates metabolism from selenite and selenomethionine.

Selenium (Se) metabolism is affected by its chemical form in foods and by its incorporation (specific vs. nonspecific) into multiple proteins. Modeling Se kinetics may clarify the impact of form on metabolism. Although the kinetics of Se forms have been compared in different participants, or the same participants at different times, direct comparisons of their respective metabolism in the same participants have not been made. The aim of this study was to simultaneously compare kinetics of absorbed Se from inorganic selenite (Sel) and organic selenomethionine (SeMet) in healthy participants (n = 31). After oral administration of stable isotopic tracers of each form, urine and feces were collected for 12 d and blood was sampled over 4 mo. Tracer enrichment was determined by isotope-dilution-GC-MS. Using WinSAAM, a compartmental model was fitted to the data. Within 30 min of ingestion, Se from both forms entered a common pool, and metabolism was similar for several days before diverging. Slowly turning-over pools were required in tissues and plasma for Se derived from SeMet to account for its 3-times-higher incorporation into RBC compared with Se from Sel; these presumably represent nonspecific incorporation of SeMet into proteins. Pool sizes and transport rates were determined and compared by form and gender. The final model consisted of 11 plasma pools, 2 pools and a delay in RBC, and extravascular pools for recycling of Se back into plasma. This model will be used to evaluate changes in Se metabolism following long-term (2 y) Se supplementation.

Effects of selenomethionine supplementation on selenium status and thyroid hormone concentrations in healthy adults.

BACKGROUND: Selenium, a potential cancer prevention agent currently being tested against prostate cancer in the Selenium and Vitamin E Cancer Prevention Trial (SELECT), plays an integral role in thyroid metabolism. The effects of long-term selenium supplementation on thyroid hormone concentrations are unknown. OBJECTIVE: The objective was to investigate the effects of long-term selenium supplementation on thyroid hormone concentrations. DESIGN: Twenty-eight healthy adults took 200 microg selenomethionine/d for 28 mo. The thyroid hormones triiodothyronine (T3), thyroxine (T4), and thyrotropin (TSH) were measured in plasma for 4 mo before supplementation and quarterly during supplementation. The assay methods were changed midstudy; the results of the 2 methods were not comparable. Therefore, one analysis was conducted based on the results of the first method, and a second analysis was based on all of the data, adjusted for the change. Serial data collection permitted a test for trends rather than simply a difference between initial and final values. RESULTS: By 9 mo, mean (+/-SEM) plasma selenium concentrations had increased from 1.78 +/- 0.07 micromol/L at baseline to 2.85 +/- 0.11 micromol/L for men and from 1.64 +/- 0.04 to 3.32 +/- 0.1.2 micromol/L for women. T3 concentrations in men increased 5% per year (P = 0.01). T4 and TSH concentrations were unchanged. CONCLUSIONS: Selenium supplementation produced no clinically significant changes in thyroid hormone concentrations. A small but statistically significant increase in T3 concentrations was noted in men, with no corresponding decreases in TSH. A subset of SELECT subjects might be monitored periodically for changes during long-term selenium supplementation.