Serial urine collections in a number of patients with pernicious anemia given 2 µg B12Co60 orally followed in two hours by 1000 µg nonradioactive vitamin B12 showed little urinary radioactivity at any time. When these tests were repeated together with a potent oral dose of intrinsic factor concentrate, there was little activity during the first four hours. Peak excretion rates occurred most commonly between 6 and 12 hours after ingestion of radioactive B12, sometimes even later. The time of peak excretion was fairly characteristic for the individual. Secondary peaks occasionally occurred, and only slight radioactivity usually remained after 24 hours. It is postulated that the delayed peak is related to the time it takes for B12 to be transported in the intestine to the point of absorption or to the duration of the intracellular metabolic processes of absorption. For most purposes the use of fractional urinary collections is not necessary.

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