Digestibility and in vitro fecal fermentation of whole-wheat breads with different estimated glycemic indices.

How differences in whole wheat bread matrix properties and digestibility influence colonic fermentation remains unclear. The in vitro fecal fermentation of digested residues from four whole-wheat breads with different estimated GI (eGI) were compared. During fermentation, starch and protein contents decreased, gluten aggregates and monomeric proteins were hydrolyzed into peptides and amino acids, and starch chains containing 12-40 glucose units shifted toward shorter fractions. Broth pH was positively associated with eGI, whereas microbial richness and diversity were negatively associated with eGI. Total short-chain fatty acids reached 111.86 and 110.15 mmol/L in heat-moisture-treated and debranched-retrograded breads, respectively. Proteobacteria decreased, whereas Bacteroidetes increased, in relative abundance. Differences in bread matrix properties and digestibility may alter residual substrate availability and thereby contribute to distinct microbial and metabolic outcomes. Because the breads differed in multiple matrix attributes, these effects cannot be attributed to eGI alone; physiological relevance requires human validation.