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Fecal donor diet influences <i>in vitro</i> fermentation properties. The purpose of this study was to determine how diet influences <i>in vitro</i> fermentation properties of whole grains and brans. Whole grain flour and bran from corn, oats, rye, and wheat were subjected to <i>in vitro</i> digestion followed by fecal fermentation. Fecal inocula were from subjects with recommended dietary fiber intakes (RecDF: 31±5 g/d) or normal dietary fiber intakes (NormDF: 11±3 g/d; n=4/group). Short/branched chain fatty acids (S/BCFA), ammonia, and total carbohydrate were analyzed during fermentation. Samples inoculated with the fecal microbiota from the RecDF group showed higher fermentability of carbohydrates and higher production of butyrate, accompanied by reduced acetate, propionate, and BCFA compared to those from the NormDF group (68% vs 62%; 3.47 mM vs 2.40 mM; 10.4 vs 25.4 mM; 2.31 mM vs 3.90 mM; 0.25 vs 0.47 mM, respectively). Fecal microbiota were better able to break down carbohydrates in the whole grain flours compared to brans (67% vs 62%) regardless of diet group. In the RecDF group, rye and wheat showed the highest carbohydrate fermentability (73% and 71% vs 64% and 65% for corn and oats); rye resulted in the highest butyrate production (5.53 mM compared with 2.19-3.12 mM for the other grains); and rye and oats resulted in the lowest ammonia production (11.7 mM and 12.6 mM compared with 19.4 mM and 19.9 mM for corn and wheat). In contrast, the microbiota from the NormDF group did not differ in their response to the grains. Thus, dietary fiber intake of the fecal donor has a dramatic influence on the ability of the gut microbiota to ferment the dietary fibers in grains, differentiate among grains, and produce metabolites that are beneficial to human health.