Differentiation between Vitamin B₁₂—deficient and Folic Acid—deficient Megaloblastic Anemias with C¹⁴—Histidine

Intermediary metabolism of the monocarbon pool and histidine in normal subjects and patients with megaloblastic anemia was studied by continuous measurement of pulmonary excretion of C¹⁴O₂ and urinary excretion of C¹⁴ after injection of L-histidine-2(ring)-C¹⁴. Cumulative pulmonary and renal excretion of C¹⁴ for 1 month by two normal subjects approximates 45 per cent of the amount injected. Within 4 months after injection of the dose used in this study, the resultant average tissue radiation decreases below the average natural terrestrial and cosmic radiation level.

Simultaneous determination of two parameters, (1) cumulative 1-hour pulmonary C¹⁴ excretion and (2) the time of occurrence of maximum C¹⁴O₂specific activity (T_max), may permit rapid and unequivocal differentiation between folic acid deficiency and vitamin B₁₂ deficiency in the pathogenesis of megaloblastic anemia. Folio acid deficiency results in marked diminution of pulmonary C¹⁴ excretion (approximately 0.1 per cent of injection C¹⁴ in 1 hour) and marked prolongation of C¹⁴O₂-specific activity T_max (approximately 3 hours), while both parameters are normal (approximately 1 per cent and less than 1 hour, respectively) in patients with vitamin B₁₂ deficiency and megaloblastic anemia.

Measurement during periods of reticulocyte response to either folio acid or vitamin B₁₂ demonstrate normal C¹⁴O₂-specific activity T_max but decreased pulmonary C¹⁴ excretion. These observations suggest that prolongation of C¹⁴O₂-specific activity T_max is a sensitive index of folic acid deficiency or block and that if T_max is normal, pulmonary C¹⁴ excretion is a sensitive index of the relative partition of the active monocarbon pool between pathways for oxidation and pathways for nucleic acid synthesis.

This type of breath analysis seems to provide a quantitative dynamic representation of metabolic function which may be particularly useful in differentiating between the alterations of intermediary metabolism that occur in patients with folic acid-deficient megaloblastic anemia and in patients with vitamin B₁₂-deficient megaloblastic anemia.