Submitted to: Journal of Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/15/2016
Publication Date: 10/19/2016
Citation: Kieffer, D.A., Piccolo, B.D., Marco, M.L., Kim, E., Goodson, M.L., Keenan, M.J., Dunn, T.N., Knudsen, K.E., Adams, S.H., Martin, R.J. 2016. Obese mice fed a diet supplemented with enzyme-treated wheat bran display marked shifts in the liver metabolome concurrent with altered gut bacteria. Journal of Nutrition. doi:10.3945/jn.116.238923.

Interpretive Summary: Dietary fiber has been shown to have a variety of positive effects on human health, but the mechanisms that underlie these outcomes remain largely unknown. Since fiber feeding alters the mix of naturally-occurring bacteria in the gut (the microbiome), factors produced by the microbiome are very likely involved and can make their way to the circulation. Once in the blood, one of the first organs coming in contact with microbiome-derived factors is the liver. The current studies sought to characterize how enzyme-treated wheat bran (ETWB, 20% by weight in the diet), a fermentable dietary fiber previously shown to decrease liver triglycerides and modify the gut microbiome in mice, influences liver biology and metabolism. We used a large-scale phenotyping approach to characterize ETWB-driven shifts in the microbiome, and identified for the first time candidate microbes and metabolites that may drive changes in liver metabolism and gene expression. Body weight and liver triglycerides were decreased by ETWB feeding (by 10% and 25%, respectively, p<0.001, relative to a control diet containing rapidly-digestible fiber), and an index of liver reactive oxygen species was increased (by 29%). The cecal microbiome had an increase in Bacteroidetes (by 42%) and decrease in Firmicutes (by 16%). Multivariate statistical analysis identified several metabolites as strong discriminators between the ETWB and control groups including: decreased liver antioxidants (glutathione and alpha-tocopherol), decreased liver carbohydrate metabolites including glucose, lower hepatic arachidonic acid, and increased liver and plasma beta-hydroxybutyrate (a ketone body). Liver gene expression profiling (transcriptomics) revealed key metabolic pathways impacted by ETWB, especially those related to lipid metabolism and some fed/fasting regulated genes. It is proposed that changes in specific bacteria elicit gut-derived signals that reach the liver via enterohepatic circulation, ultimately impacting host liver metabolism in a manner that mimics, in part, the fasted state and reduces liver fat. This study forms the foundation for testing which of the specific identified microbe-derived metabolites have activity on liver health.

Technical Abstract: Enzyme-treated wheat bran (ETWB) is a fermentable dietary fiber previously shown to decrease liver triglycerides and modify the gut microbiome in mice. It is not clear which mechanisms explain how ETWB feeding impacts hepatic metabolism, but factors (i.e., metabolites) associated with specific microbes may be involved. To address this question, we used a multi-omics approach to characterize ETWB-driven shifts in the cecal microbiome, and identified correlates between microbial changes and diet-related differences in liver metabolism in diet-induced obese mice that typically display steatosis. Male C57Bl/6 mice were fed a 45% fat diet supplemented with ETWB (20% w/w) for 10 wk and compared to control mice fed a rapidly digestible starch (n=15/grp). Body weight and liver triglycerides were decreased by ETWB feeding (by 10% and 25%, respectively, p<0.001), and an index of liver reactive oxygen species was increased (by 29%, p<0.01). The cecal microbiome had an increase in Bacteroidetes (by 42%, p<0.05) and decrease in Firmicutes (by 16%, p<0.05). Multivariate statistical analysis identified several metabolites as strong discriminators between the ETWB and control groups including: decreased liver antioxidants (glutathione and alpha-tocopherol), decreased liver carbohydrate metabolites including glucose, lower hepatic arachidonic acid, and increased liver and plasma beta-hydroxybutyrate. Liver transcriptomics revealed key metabolic pathways impacted by ETWB, especially those related to lipid metabolism and some fed/fasting regulated genes. Together, these changes indicate that dietary fibers such as ETWB regulate hepatic metabolism concurrent with specific gut bacteria community shifts. It is proposed that these changes elicit gut-derived signals that reach the liver via enterohepatic circulation, ultimately impacting host liver metabolism in a manner that mimics, in part, the fasted state.