The urinary excretion of orally administered tritium-labeled folic acid as a test of folic acid absorption.

By FREDERICK A. Kuis nn I HE ABSORPTION of folic acid occurs primarily in the proximal small intestine and is thought to be mediated, in part at least, by an active transport 2 l)iminished absorption of folic acid commonly occurs in disease characterized by diffuse atrophy of the jejunal mucosal villi, such as idiopathic steatorrhea ( non-tropical sprue, gluten enteropathy ) and tropical sprue and may occur occasionally in diseases involving the jejunal submucosa, such as lymphoma.4 Previous studies of tolic acid absorption have utilized microbiological technics to assay folic acid concentrations either in the serum or urine of subjects following an oral test dose of the vitamin. Chanarin and co-workers3’5 have favored the assay of serial changes in hourly serum folic acid concentrations, whereas Girdwood and co-workers#{176} have advocated assay of the urinary excretion of folic acid following the oral test dose. Although both of these technics have been shown to be reliable tests of folic acid absorption, they have the disadvantage of requiring elaborate microbiological technics for assay and, in addition, provide little quantification of the degree of malabsorption. Recently tritium-labeled folic acid ( H3FA ) has become available,7 and balance studies using this isotope have been reported in detail by Anderson, Belcher, Chanarin, and Mollin.8 When an oral test dose of H3FA of 40 j.tg. per Kg. body weight was employed, control subjects were found to excrete, on the average, three times as much radioactivity in the urine as patients with idiopathic steatorrhea. The urinary excretion of radioactivity was enhanced in both control subjects and, to a lesser degree, in patients with malabsorption when an injection of non-radioactive folic acid was given following the oral tracer dose. Under these circumstances, control subjects excreted more than 28 per cent of the oral dose in the urine, whereas the majority of patients with idiopathic steatorrhea excreted less than 25 per cent. The purpose of the present study is to further evaluate the H3FA urinary