Long-Term Excretion of Co57-Vitamin B12 and Turnover Within the Plasma

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Abstract
Eight control subjects were given an average of 1.0 µg. of Co57-vitamin B12 by different routes. The Co57 in plasma, urine and feces was measured for at least seventy days in all subjects and for more than 500 days in half of them with the following results: (1) The Co57-vitamin B12 disappeared rapidly from the plasma in a complex manner until about day 70. Subsequently, the loss could be described by a linear function with an average half-time of 690 days. (2) Urinary excretion was higher the first few days than subsequently regardless of route of intake. Fecal excretion did not become uniform until about one month after intravenous administration. (3) Urinary excretion during the period of uniform loss was 0.028 per cent of the retained dose per day. Simultaneous fecal excretion was 0.031 per cent per day. Comparison of urinary excretion of Co57 and total vitamin B12 as measured by microbiological assay suggested that only 10 per cent of the vitamin B12 excreted in the urine was in an active form. (4) The total daily excretion of vitamin B12 was calculated to be 0.66 to 2.1 µg., mean 1.3 µg.