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The Intestinal True Absorption of ⁶⁵Zn in Rats Is Adversely Affected by Diets Containing a Faba Bean (Vicia faba L.) Nonstarch Polysaccharide Fraction^1,2,3,

The Rowett Research Institute, Bucksburn, Aberdeen AB2 9SB, U.K.

The effects on the absorption of ⁶⁵Zn by two varieties of raw faba bean (Vicia faba L., minor) or seed components that may interfere with mineral metabolism in the gut, have been studied in growing rats. In bean diets all protein was supplied by the meals, and the fractions were tested by incorporating them in control diet at the same levels as they occur in the seeds. Absorption of ⁶⁵Zn was also measured in rats fed dephytinized bean meal produced by including phytase in the diet. Rats were pair-fed diets supplemented with amino acids and minerals to target requirements and containing 40 mg Zn/kg diet. True absorption of Zn was 50–70% lower in rats fed diets containing both cultívars of faba bean meals than in those fed the control diet. Although soluble nonstarch polysaccharides caused a significant reduction in the absorption of Zn, this effect disappeared after the removal of phytate by demineralization. In contrast, despite its negligible content of phytate, the insoluble nonstarch polysaccharides in the cell wall fraction of the cotyledon accounted for most of the reduction in Zn absorption in rats fed the faba bean diets. Addition of phytic acid to the control diet significantly reduced the absorption of ⁶⁵Zn but only from 44 to 36%. Moreover, the increase in the absorption of Zn was similarly small, from 21% to 29%, with the addition of phytase to the faba bean diet. These results suggest that, in addition to phytic acid, the insoluble nonstarch polysaccharides present in the cell walls of the cotyledon in Vicia faba seeds are mainly responsible for the lower Zn availability in rats fed faba bean diets.

KEY WORDS: • Vicia faba • phytate • rats • nonstarch polysaccharides • true absorption of ⁶⁵Zn

¹ Presented in part at Bioavailability '93, Ettlingen, Germany. [Rubio, L. A., Grant, G., Rahman, H. & Pusztai, A. (1993) Intestinal absorption of ⁶⁵Zn in growing rats fed diets containing faba bean or lupin seed meals as the only source of protein. In: Bioavailability '93, Nutritional, Chemical and Food Processing Implications of Nutrient Availability (Schlemmer, U., ed.), pp. 299–303. Bundesforschunganstolt für Ernahrung, Carlsruhe D-7500, Germany.

² The work has been supported by the Scottish Office Agriculture and Fisheries Department (SOAFD). A.P. is indebted to the Leverhulme Trust for an Emeritus Fellowship and to The Royal Society of Edinburgh for the Auber bequest Award. L.R. was awarded with a postdoctoral fellowship from the EEC. This collaborative work is a part of a European FLAIR Concerted Action Programme no. 9 coordinated by A.P. with financial support from the Commission of European Communities.

³ The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

⁴ To whom correspondence and reprint requests should be addressed.

Manuscript received 31 December 1993. Revision accepted 22 April 1994.