Radioactive Vitamin B₁₂ Absorption Studies: Results of Direct Measurement of Radioactivity in the Blood

A new method for the determination of the absorption of vitamin B₁₂ has been described using measurement of radioactivity in the blood or plasma after the ingestion of physiologic test doses of Co⁶⁰ labeled vitamin B₁₂.

Although doses of 1.0 µc. (0.92 µg. vitamin B₁₂) gave higher counts, equally reliable results were obtained with 0.5 µc. (0.46 µg. vitamin B₁₂). The radioactivity was found in the plasma portion of the blood.

With this method it was possible to differentiate between all of nine patients with pernicious anemia and 36 control subjects.

In non-pernicious anemia subjects and in pernicious anemia patients given intrinsic factor, there was a relatively delayed rise in the blood or plasma radioactivity until a peak was reached in the 8 to 12 hour interval after the ingestion of the test dose. This absorption curve was quite different from the early rise observed by others after massive oral doses of vitamin B₁₂, indicating a different mode of absorption.

Following the peak blood concentration the radioactivity gradually declined and small amounts usually persisted for as long as one week, quite different from the rapid disappearance after parenteral administration previously reported.

This method appears valuable in the diagnosis of pernicious anemia and other vitamin B₁₂ malabsorptive states, in the evaluation of intrinsic factor activity, and in studies of various aspects of the metabolism of vitamin B₁₂.