Radioactive Vitamin B₁₂ Absorption Studies: Comparison of the Whole-Body Retention, Urinary Excretion, and Eight-hour Plasma Levels of Radioactive Vitamin B₁₂

This paper reports the results of investigations designed to compare the whole-body retention, urinary excretion, and 8-hr plasma level of ⁵⁸Co vitamin B₁₂ in some 80 absorption tests on 50 subjects. Residual levels of radioactivity in the body were measured using a whole-body counter. A proportional relationship is demonstrated between the 9-day whole-body retention, the 8-hr plasma radioactivity and the 48-hr urinary excretion of vitamin B₁₂. The constants of proportionality are determined, enabling the results of absorption tests carried out by different methods to be compared. The mean values and the range of the results obtained are tabulated for subjects in each of four clinical categories. It is shown that approximately one third of the activity absorbed from a 1-µg dose is excreted in the urine following a single intramuscular injection of 1 mg nonradioactive vitamin B₁₂ at 3 hr. The administration of this flushing dose does not significantly alter the activity absorbed as represented by that retained together with that excreted in the urine. The plasma activity at 8 hr is increased by a factor of about 2 over that obtained without the flushing dose. Factors affecting the accuracy of the individual techniques have been studied and the poor reproducibility of successive tests is discussed.