BackgroundGut microbiome composition profoundly impacts host physiology and is modulated by several environmental factors, most prominently diet. The composition of gut microbiota changes over the lifespan, particularly during the earliest and latest stages. However, we know less about diet-aging interactions on the gut microbiome. We previously showed that diets with different glycemic indices, based on the ratio of rapidly-digested amylopectin to slowly-digested amylose, led to altered composition of gut microbiota in male C57BL/6J mice. ObjectiveHere, we examined the role of aging in influencing dietary effects on gut microbiota composition and to identify gut bacterial taxa that respond to diet and aging. MethodsWe studied three age groups of male C57BL/6J wild-type mice: young (4 months), middle-aged (13.5 months), and old (22 months), all fed either high glycemic (HG) or low glycemic (LG) diets matched for caloric content and macronutrient composition. Fecal microbiome composition was determined by 16S rDNA metagenomic sequencing and was evaluated for changes in alpha and beta diversity and bacterial taxa that change by age, diet, or both. ResultsYoung mice displayed lower alpha diversity scores than middle-aged counterparts but exhibited more pronounced differences in beta diversity between diets. In contrast, old mice had slightly lower alpha diversity scores than middle-aged mice, with significantly higher beta diversity distances. Within-group variance was lowest in young, LG-fed mice and highest in old, HG-fed mice. Differential abundance analysis revealed taxa associated with both aging and diet. Most differential taxa demonstrated significant interactions between diet and aging. Notably, several members of the Lachnospiraceae family increased with aging and HG diet, while taxa from the Bacteroides_H genus increased with the LG diet. Akkermansia muciniphila decreased with aging. ConclusionsThese findings illustrate the complex interplay between diet and aging in shaping the gut microbiota, potentially contributing to age-related disease.