Abstract
BackgroundSelenium (Se) is a nutritionally essential trace element and health may be improved by increased Se intake. Previous kinetic studies have shown differences in metabolism of organic vs. inorganic forms of Se [e.g., higher absorption of selenomethionine (SeMet) than selenite (Sel), and more recycling of Se from SeMet than Sel]. However, the effects on Se metabolism after prolonged Se supplementation are not known.ObjectiveTo determine how the metabolism and transport of Se changes in the whole-body in response to Se-supplementation by measuring Se kinetics before and after 2 years of Se supplementation with SeMet.MethodsWe compared Se kinetics in humans [n = 31, aged 40 ± 3 y (mean ± SEM)] studied twice after oral tracer administration; initially (PK1), then after supplementation for 2 y with 200 µg/d of Se as selenomethionine (SeMet) (PK2). On each occasion, we administered two stable isotope tracers of Se orally: SeMet, the predominant food form, and selenite (Na2 76SeO3, or Sel), an inorganic form. Plasma and RBC were sampled for 4 mo; urine and feces were collected for the initial 12 d of each period. Samples were analyzed for tracers and total Se by isotope dilution GC-MS. Data were analyzed using a compartmental model, we published previously, to estimate fractional transfer between pools and pool masses in PK2.ResultsWe report that fractional absorption of SeMet or Sel do not change with SeMet supplementation and the amount of Se absorbed increased. The amount of Se excreted in urine increases but does not account for all the Se absorbed. As a result, there is a net incorporation of SeMet into various body pools. Nine of the 11 plasma pools doubled in PK2; two did not change. Differences in metabolism were observed for SeMet and Sel; RBC uptake increased 247% for SeMet, urinary excretion increased from two plasma pools for Sel and from two different pools for SeMet, and recycling to liver/tissues increased from one plasma pool for Sel and from two others for SeMet. One plasma pool increased more in males than females in PK2.ConclusionsOf 11 Se pools identified kinetically in human plasma, two did not increase in size after SeMet supplementation. These pools may be regulated and important during low Se intake.
Highlights
Selenium is an essential nutrient for health, and there are indications that higher Se intakes may prevent certain diseases (1)
Selenium may have a role in some forms of cancer, cardiovascular disease, and cognitive decline (2) and relationships have been proposed between blood Se concentration and some health effects (1)
As well as by gender, we report on pharmacokinetic study 1 or baseline study (PK1) versus pharmacokinetic study 2 or supplemented study (PK2) for SeMet and Sel and for SeMet versus Sel within PK2
Summary
Selenium is an essential nutrient for health, and there are indications that higher Se intakes may prevent certain diseases (1). There are 25 proteins that contain Se, as the amino acid selenocysteine (Sec), distributed in tissues throughout the body (2). These selenoproteins function in many systems including the endocrine, nervous, and immune systems (2). An inorganic form of Se, selenite (Sel), often used as a supplement (3), has lower absorption than SeMet (4–6) and is only incorporated into functional selenoproteins (3). Previous kinetic studies have shown differences in metabolism of organic vs inorganic forms of Se [e.g., higher absorption of selenomethionine (SeMet) than selenite (Sel), and more recycling of Se from SeMet than Sel]. The effects on Se metabolism after prolonged Se supplementation are not known
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