KINETIC-MODEL OF FOLATE METABOLISM IN NONPREGNANT WOMEN CONSUMING [H-2(2)]FOLIC ACID - ISOTOPIC LABELING OF URINARY FOLATE AND THE CATABOLITE PARA-ACETAMIDOBENZOYLGLUTAMATE INDICATES SLOW, INTAKE-DEPENDENT, TURNOVER OF FOLATE POOLS

In a 10-wk study of folate metabolism in nonpregnant women (21-27 y, n =5-6 per group), subjects were fed a diet containing similar to 68 nm ol/d (30 mu g/d) folate from food. The remainder of the ingested folat e was provided as folic acid in apple juice (as nonlabeled during wk 1 -2, as [H-2(2)]folic acid during wk 3-10) to yield a constant intake o f 454, 680 or 907 nmol/d (200, 300 or 400 mu g/d). Isotopic enrichment of total urinary folate and the primary catabolite para-acetamidobenz oylglutamate (ApABG) was determined. Isotopic en rich ment of ApABG se rved as an indicator of labeling of tissue folates. A kinetic model co nsisting of fast- and slow-turnover nonsaturable pools and a saturable slow-turnover pool, with provisions for urinary and fecal excretion, catabolism and enterohepatic circulation, yielded a close fit to the d ata. Mean residence times for total body folate were 212, 169 and 124 d for folate intakes of 454, 680, and 907 nmol/d, respectively. The mo del predicted that variation in folate intake over this range had litt le effect on the mass of the large saturable folate pool; however, the fast-turnover nonsaturable pools increased in proportion to folate in take, whereas the slow nonsaturable pool also tended to increase. This model will aid in evaluation of folate turnover and in predicting kin etic consequences of physiologic conditions associated with altered fo late requirements.