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Methylmalonic Acid and Vitamin B₁₂ Excretion of Rats Consuming Diets Varying in Cellulose and Pectin¹

Department of Nutritional Sciences, University of California, Berkeley, California 94720

The purpose of this study was to compare the effects of various levels of two components of dietary fiber, cellulose and pectin, on the utilization of vitamin B₁₂ by the rat. Compared with the fiber-free group, the rats fed purified diets of 40 or 50% of cellulose excreted about twice as much methylmalonic acid (MMA) after 10 weeks of B₁₂ depletion. In contrast, rats consuming purified diets containing 5% to 15% of pectin experienced a 9- to 21-fold elevation of group average MMA excretion, respectively. Compared with the fiber-free group, rats offered cellulose- or pectin-containing diets exhibited significantly greater fecal excretion of the radioactive B₁₂ dose that was injected after several weeks of B₁₂ depletion. However, despite the large difference in MMA excretion between the cellulose and pectin groups, the two fibers supported surprisingly similar rates of fecal excretion of ⁵⁷Co. Urinary MMA excretion decreased significantly in all groups following the injection of 0.5 µg of vitamin B₁₂, thus indicating that the observed defect in MMA metabolism was B₁₂-responsive. Mechanisms consistent with these results are discussed. These data indicate that two dietary fibers, cellulose and pectin, have a deleterious effect on vitamin B₁₂ status which is due in part to increased fecal excretion of the vitamin. The dramatically greater influence of pectin than of cellulose on MMA excretion emphasizes the importance of investigating separately the metabolic effects of the various components of dietary fiber.

KEY WORDS: • vitamin B₁₂ deficiency • methylmalonic acid • dietary fiber • cellulose • pectin • intestinal flora • fiber-B₁₂ interaction

¹ Portions of these data were presented at the annual meeting of the Federation of American Societies for Experimental Biology, April 1977, Chicago, Illinois. Cullen, R. W. & Oace, S. M. (1977) Cellulose and pectin enhance vitamin B₁₂ depletion in rats. Federation Proc. 36, 1118 (abstract). The work was supported in part by NIH Grant AM 19970.

² To whom reprint requests should be sent.

Manuscript received 25 August 1977.