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There has been a dramatic increase in obesity, which presents a risk of many chronic health problems. Recent studies have shown that obese individuals harbor an abnormal gut microbiota (dysbiosis), which has sparked interest in the gut microbiota as a target for weight management. Dietary fiber and whole grains that can be metabolized by gut microbiota have been shown to promote the growth of beneficial bacteria in the gut. Increased consumption of dietary fiber and whole grains may improve host / gut microbiota interactions in obesity and other metabolic diseases by normalizing gut dysbiosis. The present thesis describes two research projects to assess the impact of dietary fibers and whole grain on gut microbiota. In the first study, six dietary fibers [pectin, guar gum, inulin, arabinoxylan, β-glucan, and resistant starch type 2 (RS-2)] were subjected to in vitro digestion and fermentation using fecal samples from obese and normal weight individuals. Pyrosequencing was used to assess the impact of each dietary fiber on the gut microbiota community. Short/branched chain fatty acids (SCFA/BCFA) and carbohydrate utilization were correlated with proportions of bacterial taxa. The data showed that RS-2 caused the most dynamic change of the whole microbiota community and Bifidobacterium increased almost 10-fold on pectin substrate compared with the control. Certain taxa may be targeted to increase SCFA production or increase dietary fiber utilization. For instance, Ruminococcaceae and Faecalibacterium displayed positive correlations with butyrate production and while a strong positive relationship was shown between β-glucan utilization and Firmicutes. In the second study, since SCFA can influence hormones involved in energy absorption, utilization, and storage, the SCFA profile from five whole grains (wheat, rye, corn, rice, and oats) using an in vitro method was evaluated. There were large differences in fermentation profiles among individuals, even when supplied with the same dietary fiber. The obese type microbiota was less efficient at butyrate production and less metabolically active than the normal weight type microbiota initially, but given sufficient whole grain substrates the bacteria quickly became metabolically active.