Oral Administration Of Tracer Research Articles

Selenium Kinetics in Humans Change Following 2 Years of Supplementation With Selenomethionine.

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.

Substitution of Oral 18F-FDG for Intravenous 18F-FDG in PET Scanning

The aim of this study was to see whether oral administration of (18)F-FDG could be substituted-without significant loss of information-for intravenous injection of (18)F-FDG in patients with difficult intravenous access of any cause, such as that often seen in cancer patients after many cycles of chemotherapy. PET after both oral and intravenous administration of (18)F-FDG was performed on 2 healthy volunteers and 7 patients. An interval of 48 h was maintained between the oral administration and the intravenous administration. All scans were visually analyzed. Semiquantitative analysis of specific areas was done by calculating standardized uptake values (SUVs). Scanning was performed 60 min after intravenous tracer administration and 90 min after oral tracer administration. All lesions seen after intravenous administration were visualized on the oral study as well. SUVs were lower on the oral study than on the intravenous study. Oral (18)F-FDG can successfully be substituted for intravenous (18)F-FDG in patients with difficult intravenous access. However, because of the large amount of (18)F-FDG retained in the gut, careful interpretation will be required when disease of the gastrointestinal tract is being evaluated.

Quantification of incorporation of [15N]ammonia into plasma amino acids and urea.

The incorporation of 15N into individual plasma amino acids and urea was quantified in five human subjects who received 15NH4Cl either orally or intravenously for 6 h. After oral tracer administration, the highest enrichment was achieved by arginine, followed by urea and glutamine; distribution of 15N within glutamine was 55% amide and 45% amino N. Glutamine achieved the highest enrichment after the intravenous administration of tracer, with a distribution of 92% amide and 8% amino N. The relative distribution pattern of 15N incorporation was quantified from the rate at which 15N initially appeared in each plasma component. Amino acids (especially arginine, glutamine, and glutamate) accounted for greater than one-half (54%) of the orally administered tracer that was initially recovered in plasma components, compared with 46% initial appearance for urea; for the intravenous tracer, amino acids accounted for 78% of initial appearance of tracer compared with 22% for urea. Our results highlight the involvement of the splanchnic bed in the utilization of orally administered ammonia (preferential incorporation of oral tracer into arginine, urea, glutamate, and the amino N of glutamine) in contrast to the preferential incorporation of systemically administered ammonia into the amide N of glutamine and alanine.

Calculation of Zinc Absorption in Humans Using Tracers by Fecal Monitoring and a Compartmental Approach

Zinc absorption is often determined following oral administration of tracer by fecal monitoring as dose minus tracer excreted in feces. The value obtained for absorption with this method is influenced by excretion of absorbed tracer into feces during the fecal collection period and by any incomplete elimination of unabsorbed tracer. In the present study a published, physiologically based compartmental model of zinc metabolism has been used to calculate, after oral tracer administration, the fecal appearance of unabsorbed and absorbed tracer and the appearance when fractional rates of endogenous excretion and elimination were varied. Absorption was determined by fecal monitoring and compared to the value determined using parameter values of the compartmental model. The value calculated for absorption using fecal monitoring varied with length of fecal collection, rate of excretion of absorbed tracer (secretion) and rate of elimination of unabsorbed tracer. Absorption was determined correctly by fecal monitoring only when the amount of absorbed tracer excreted was equivalent to the amount of unabsorbed tracer remaining in the gut. Fecal monitoring determines a parameter representing the combined processes of absorption, endogenous excretion (or secretion) and fecal elimination.