The Urinary Excretion of Orally Administered Tritium-labeled Folic Acid as a Test of Folic Acid Absorption

The urinary excretion of radioactivity following an oral test dose of tritium-labeled folio acid was determined in a group of 40 subjects which included controls and patients with a variety of malabsorptive disorders. The results were compared with the occurrence of folic acid deficiency, as detected by subnormal serum L. casei folate levels, and with a variety of other tests available for the evaluation of intestinal absorption, including the determination of folio acid absorption based on the microbiological assay of peak serum Str. faecalis folate concentrations following the oral test dose.

The urinary excretion of H³FA in 15 subjects with normal intestinal absorption ranged from 26.0 to 57.8 per cent of the oral test dose, with a mean of 40.7 per cent. The H³FA excretion in 25 patients with intestinal malabsorption ranged from 1.9 to 38.7 per cent, with a mean of 15.3 per cent. Twenty patients in this group excreted less than 26 per cent. Those five patients who excreted greater than 26 per cent were considered to have normal absorption of folic acid since absorption studies of peak serum Str. faecalis folate levels were within the normal range.

The results of the H³FA urinary excretion test correlated well with other parameters of intestinal absorption, particularly in the instances of severe malabsorption, although three cases with malabsorption were observed in which H³FA excretion was subnormal and peak serum Str. faecalis folate concentrations were normal. Folic acid deficiency was observed in 12 patients and its incidence appeared to be related to the severity of impairment of folic acid absorption in the majority of cases.

It is concluded that the H³FA urinary excretion test is a simple, rapid and reliable index of folic acid absorption.