Validity of the Assumption of Tracer Equilibrium with respect to the Excretion of Vitamin B₁₂

Evidence apparently refuting the assumption of tracer equilibrium shortly after administration of vitamin B₁₂ has been critically examined. It is fundamentally impossible to demonstrate differences between the excretion rate of labelled B₁₂ and unlabelled B₁₂ in the studies described. The apparent changes in the excretion rate of B₁₂ are shown to be due to the pooling of data from several patients. The excretion data from whole-body monitoring of individual patients can be adequately described by a single exponential term, if the data from the first few days after administration are not considered. For a single pool system, the assumption of tracer equilibrium is justified. It is shown that excretion rates, calculated on the assumption of tracer equilibrium, agree with those estimated from independent clinical findings based on the time required to develop symptoms of hypovitaminosis following total gastrectomy. The data which dispute the assumption lead to excretion rates which are almost certainly underestimates.