1. The long-term fate of an oral dose of [75Se]selenomethionine was studied in four women.
2. Urinary and faecal excretion, respiratory losses and whole-body retention of 75Se were measured, and also 75Se turnover in whole body, plasma and erythrocytes during a period of 33–44 weeks.
3. Intestinal absorption of [75Se]selenomethionine by the four subjects was 95.5–97.3% of the administered dose.
4. Urinary excretion accounted for 6–9% of absorbed 75Se in the first 2 weeks. No radioactivity was detected in expired air.
5. After the initial 8 weeks during which radioactivity decreased more rapidly, whole-body retention of 75Se decreased exponentially with a half-time of 207–290 d.
6. Plasma 75Se concentration reached a maximum level 3–4 h after the dose. Transient initial uptake of 75Se in erythrocytes during the first hour was followed by a gradual increase to a maximum concentration at 8–12 weeks.
7. These results are compared with the results of an earlier study of the metabolism of [75Se]selenite in two of the same women. 75Se from [75Se]selenomethionine was found to be more completely absorbed, had a greater retention and smaller endogenous urinary and faecal losses than 75Se from [75Se]selenite, and these differences persisted throughout the experimental period. These findings differed from those obtained in rats in which, after an initial period, 75Se from selenite was metabolized similarly to that from selenomethionine.
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