Polyphenols are often ingested alongside dietary fibres. They are both catabolised by, and may influence, the intestinal microbiota; yet, interactions between them and the impact on their resultant microbial products are poorly understood. Dietary fibres (inulin, pectin, psyllium, pyrodextrin, wheat bran, cellulose—three doses) were fermented in vitro with human faeces (n = 10) with and without rutin (20 µg/mL), a common dietary flavonol glycoside. Twenty-eight phenolic metabolites and short chain fatty acids (SCFA) were measured over 24 h. Several phenolic metabolites were produced during fibre fermentation, without rutin. With rutin, 3,4-dihydroxyphenylacetic acid (3,4diOHPAA), 3-hydroxyphenylacetic acid (3OHPAA), 3-(3 hydroxyphenyl)propionic acid (3OHPPA) and 3-(3,4-dihydroxyphenyl)propionic acid (3,4diOHPPA; DOPAC) were produced, with 3,4diOHPAA the most abundant, confirmed by fermentation of 13C labelled quercetin. The addition of inulin, wheat bran or pyrodextrin increased 3,4diOHPAA 2 2.5-fold over 24 h (p < 0.05). Rutin affected SCFA production, but this depended on fibre, fibre concentration and timepoint. With inulin, rutin increased pH at 6 h from 4.9 to 5.6 (p = 0.01) but increased propionic, butyric and isovaleric acid (1.9, 1.6 and 5-fold, p < 0.05 at 24 h). Interactions between fibre and phenolics modify production of phenolic acids and SCFA and may be key in enhancing health benefits.
Dietary polyphenolics have been linked to a range of systemic effects, which may, in part, be attributable to their colonic bacterial metabolites. Both the gut bacteria and food matrix (e.g. dietary fibre alongside polyphenolics) may be important factors in the variability in human responses to polyphenolic-rich foods (1) . Quercetin is a commonly consumed dietary flavonol found in onions, with limited bioavailability. Specific glycosides of quercetin, e.g. quercetin-3-O-rutinoside (rutin), escape intestinal absorption and are catabolised by the gut microbiota to a range of phenolic acids (2) . Little is known about the interactions between polyphenolics / quercetin and dietary fibres commonly occuring in foods -or the impact of such interaction on the colonic production of phenolic acids with demonstrated bioactivity (2).Dietary fibres (inulin, pectin, psyllium, pyrodextrin, wheat bran and cellulose, 0.8, 1.7 and 3.3 g/50mL fermentation fluid) were fermented in vitro with human faeces (n = 10) in the presence or absence of rutin (1 mg/50mL). Twenty eight potential phenolic metabolites were analysed by GC-MS and short chain fatty acids by GC-FID. Results were confirmed by fermentation of 13C labelled quercetin.3,4-Dihydroxyphenylacetic acid, 3-hydroxyphenylacetic acid, 3-(3 hydroxyphenyl) propionic acid and 3-(3,4-dihydroxyphenyl)propionic acid were identified as metabolites, with 3,4-dihydroxyphenylacetic the most characteristic. Addition of inulin, wheat bran or pyrodextrin changed the amount and rate of breakdown of 3,4-dihydroxyphenylacetic acid over 24 h (P < 0.05). Several phenolic metabolites were not specific to rutin, but resulted from fibre fermentation. Rutin had a minimal impact on pH and short chain fatty acid production.Faecal bacterial production of 3,4DHPAA with rutin (R+, 20 µg/mL) and without rutin (R-) in presence of fibre at four concentrations (0, 0.8,1.7 and 3.3 g/50mL) over 24 hours (areas under the curve); INU, inulin, PEC, pectin, ISP, psyllium, RM, pyrodextrin, WB, wheat bran, CEL, cellulose; means ± S.E.M.Fermentable fibres impacted on the bacterial catabolism of rutin and also significantly contributed to the colonic pool of phenolic acids. Inter-individual variations between individual donor's capacity to ferment rutin and fibers warrant further investigation.
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