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The effect of wheat bran or California small white beans in the diet on absorption of monoglutamyl (PteGlu) and heptaglutamyl folic acid (PteGlu7) was studied in six men confined to a metabolic unit. Relative folate absorption was determined by measuring 24-h urinary folate excretion and serum folate levels at 0, 1, and 2 h after ingestion of a formula meal containing 1.13 mumol PteGlu or PteGlu7 (500 micrograms PteGlu equivalent). Serum data showed PteGlu absorption was more rapid than PteGlu7 absorption. Urinary excretion of PteGlu7 was 63% (50 less than or equal to mean less than or equal to 76%) of PteGlu excretion. Addition of 30 g wheat bran to the formula meal accelerated PteGlu absorption whereas PteGlu7 absorption was not significantly affected by either food. Effects of the two foods were qualitatively different. Wheat bran increased the absorption of PteGlu relative to PteGlu7 whereas beans minimized the difference between PteGlu and PteGlu7 serum areas.

As little as 5% of pectin added to a fiber-free diet elevates urinary methylmalonic acid (MMA) severalfold in vitamin B-12--deprived rats. The present study examines whether increased urinary MMA reflects lower vitamin B-12 status or occurs only because of fermentation of pectin by intestinal bacteria and increased production of propionate, a precursor of MMA. By monitoring urinary and fecal excretion of 57Co after a tracer dose of [57Co]vitamin B-12, we found the biologic half-life of vitamin B-12 to be 59 d for rats fed a fiber-free diet and only 19 d for rats fed a 5% pectin diet. Also, pectin-fed rats oxidized only 12% of a 1-mmol dose of [14C]propionate to 14CO2 in 2 h, whereas rats fed the fiber-free diet expired 33% of the dose. Finally, high urinary MMA persisted even after the removal of pectin from the diet. We conclude that dietary pectin accelerates vitamin B-12 depletion in rats, possibly by interfering with enterohepatic recycling of vitamin B-12. By stimulating microbial propionate production, pectin and other fermentable fibers may also contribute to increased urinary MMA in vitamin B-12 deficiency, but a larger propionate pool does not account for the other effects of pectin on vitamin B-12 status.

This study was designed to examine the effect of several sources of dietary fiber on the utilization of folic acid added to the diets of rats. Weanling rats were given a low folacin basal diet for 21 days, divided into various groups and then fed folic acid (pteroylglutamic acid) and/or fiber-supplemented diets for 7 or 8 days. The slope of the liver folacin response was measured as an indicator for utilization of added folic acid. Fecal folacin excretion was measured as an indicator of unabsorbed dietary folacin plus folacin synthesis by intestinal bacteria. There was no detectable effect of cellulose, xylan, pectin or wheat bran on the utilization of added folic acid. Liver and fecal folacin content indicated that some fiber sources contributed additional available folacin to the animals from intestinal synthesis or naturally occurring folacin. Cellulose acted as a simple dietary dilutant and had no significant effect on the utilization of added folic acid or total fecal folacin excretion. Xylan stimulated intestinal folacin synthesis, and this was reflected in higher fecal and liver folacin content. Wheat bran and beans (two varieties of each) contained measurable folacin or stimulated synthesis of bacterial folacin, which appeared to be available to the rat.

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