1. A 45 d metabolic study was carried out in four young adult male North American residents consuming a controlled diet based on an amino acid mixture. During the initial 10 d, total daily selenium intake was adjusted to 107.7 (SE 0.1) μg/d, which was reduced to 11.4 (SE 0.1) μg/d for the remaining 35 d.
2. Two doses of a stable isotope (74SeO32-) were administered orally in the post-absorptive state on days 4 and 39 of the study.
3. Se balance (faecal + urinary excretion) as well as stable isotope excretion studies were carried out for the entire 45 d period; blood plasma and erythrocyte Se concentrations were also monitored.
4. Plasma Se concentrations (μg/ml) fell progressively from the initial value of 0.132 (SE 0.007) to 0.083 (SE 0.008) at the end of the study. The erythrocyte concentrations of Se did not vary in a consistent manner (average value for the entire study 0.147 (SE 0.002) μg/ml).
5. Faecal excretion of unenriched Se decreased from 66 (SE 6) μg/d for days 1–10 to 10.2 (SE 0.8) μg/d for days 14–40. Mean urinary excretions of the unenriched Se were 43.9 (SE 2.8) μg/d (days 1–10) and 26.9 (SE 4.6) μg/d (days 14–40). Total balance (intake - faecal excretion - urinary excretion) for unenriched Se was μg/d):-18 (SE 7) days 10–19, -17 (SE 2) days 19–39, -5 (SE 1) days 38–45.
6. Fractional absorption of the ingested label was 0.529 (SE 0.032) and 0.542 (SE 0.038) for the Se-adequate and Se-restricted phases of the study. However, urinary excretion of the absorbed label was reduced from 6.57 (SE 0.73)% for day 1 of the Se-adequate phase to only 3.32 (SE 0.26)% for day 1 of the Se-restricted phase. Similar observations were also made for day 7 of each phase.
7. These findings indicate that immediate contribution of ingested Se to the urinary Se pool is small.