Regulation of metabolism The gut microbiota obtains its nutrients from various sources, such as consumed food ingredients and host-derived requirements such as epithelial cells and mucus. Microorganisms use these substrates to produce energy to modulate cellular processes, metabolism and growth. The gut includes pathways for carbohydrate, lipid, and amino acid metabolism (6). Interestingly, the metabolic action of the gut flora is both adaptive and renewable. Through the synthesis of short-chain fatty acids (SCFAs), B vitamins, and vitamin K, commensal bacteria interact mutualistically with intestinal epithelial cells to promote differentiation, proliferation, and absorption from the gut. Together, this metabolic mechanism preserves valuable energy and digestible substrates for the host, all while providing energy and nutrients for growth (9). SCFAs (mainly acetate, butyrate and propionate) are the end products of carbohydrate fermentation of dietary fiber and show many positive influences in counteracting metabolic and inflammatory disorders, for example obesity, diabetes and inflammatory bowel diseases. This occurs through the activation of G protein-coupled receptors and alteration of transcription factors (17, 18). The microbial ensemble of SCFAs, TMAs, acetaldehyde, and inflammatory regulators controls host metabolic fitness through pathways that influence intestinal motility, immune function, and satiety ( 19 ). An example of lipid metabolic activity that relates the intestinal flora to dyslipidemia (excessive quantity of lipids) is the hydrolysis of bile salts, which are steroid acids produced in the liver from cholesterol and secreted into the bile to accelerate the absorption of fat-soluble vitamins, together with the metabol...... middle of paper ......ience, 336, 1262-1267.21. Ramakrishna, B. S. (2013). Role of the intestinal microbiota in human nutrition and metabolism. Journal of Gastroenterology and Hepatology, 28(S4), 9-17.22. Cani, P. D., Everard, A., & Duparc, T. (2013). Intestinal microbiota, enteroendocrine functions and metabolism. Current Opinion in Pharmacology, 13(6), 935-940.23. Xie, G., Zhang, S., Zheng, X., & Jia, W. (2013). Metabolomics approaches for characterizing metabolic interactions between the host and its commensal microbes. Electrophoresis, 34(19), 2787-2798.24. Chassard, C., & Lacroix, C. (2013). Carbohydrates and human intestinal microbiota. Current opinion in clinical nutrition and metabolic care, 16(4), 453-460.25. Aziz, Q.Q., Doré, J.J., Emmanuel, A.A., et al. (2013). Gut microbiota and gastrointestinal health: current concepts and future directions. Neurogastroenterology and motility, 25(1), 4-15.