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The primary structures of ispaghula husk and wheat bran were investigated in order to determine how and why these fibres are among the most beneficial dietary fibres. To this end, the polysaccharide preparations have been subjected to enzymic hydrolysis and methylation analysis.The results have shown ispaghula husk and wheat bran to be very-highly-branched arabinoxylans consisting of linear f-D-(1-4)-linked xylopyranose (Xylp) backbones to which a-L-arabinofuranose (AraJ3 units are attached as side residues via a-(l13) and a-(1-02) linkages.Other substituents identified as present in wheat bran include P-D-glucuronic acid attached via the C(O)-2 position, and arabinose oligomers, consisting of two or more arabinofuranosyl residues linked via 1-2, 1-3, and 1-4 linkages. Ispaghula-husk arabinoxylan is more complex having additional side residues which include a-D-glucuronopyranose (GalAp)-(1-42)-linked-a-L-rhamnopyranose-(1-04)-0-D-Xylp, a-D-GalAp-(l-o3)-linked-a-L-Araf-(l-4)-[3-D-Xylp, and a-L-Araf-(l-43)-linked-P-D-Xylp-(1l -4)--D-Xylp. The beneficial effects of increased faecal bulk and water-holding capacity are undoubtedly related to the structures of the arabinoxylans, with differences in their efficacy to treat various functional bowel disorders due to their specific structural features. [ABSTRACT FROM AUTHOR]