Dietary composition can impact colonization and composition of gut microbiota. The first aim of this dissertation was to evaluate the effect of dietary Se deprivation, sex and, aging on taxonomic composition of gut microbiota in mice. We have previously shown that long-term dietary Se deprivation promotes health span deteriorate including type-2 diabetes like symptoms in later state Terc-/- mice carrying humanized telomeres. In the present study, the data suggested that the dietary Se deprivation and aging significantly and comparably altered the gut microflora composition. The ratio of Firmicutes/Bacteroidetes was decreased by dietary Se deprivation or aging in both sexes. Abundance of selective gut bacteria genus was associated with dietary Se status and aging as evidenced by heat map and principal coordinate analyses. Moreover, the abundance of Akkermensia muciniphila, a bacterium in association with obesity, differed by Se-deprivation, aging and sex. Taken together, changes in gut microbiota composition by dietary Se deprivation may accelerate mouse aging in a sex-specific manner. The second aim of this dissertation was to investigate the effects and underlying mechanism of fecal fermentation of non-digestible fibers isolated from common beans on adipocyte differentiation and fat accumulation to elucidate the cellular mechanisms. Treatment of 3T3-L1 murine preadipocytes with fermented product resulted in a reduction of triglyceride accumulation in a dose- and time-dependent manner. The fermentation product exhibited antiipogenic effects through suppression of the adipogenesis-associated key transcription factors or activators peroxisome proliferator-activated receptor gamma (Ppar gamma), CCAAT enhancer-bind protein alpha (C/ebp alpha) and fatty acid binding protein 4 (Fabp4). Moreover, the mRNA expression lipolytic genes including peroxisome proliferator-activated receptor delta (Ppar delta) and mitochondrial uncoupling protein 2 (Ucp2) were induced by the fermentation products of non-digestible fiber isolated from common beans counteract adipogenesis through regulation of key targets.Altogether, optimizing gut microbiota by nutritionally adequate level of dietary Se and non-digestible fiber appears to represent a promising strategy to control or prevent early onset of aging and age-related metabolic diseases.