Abstract: The present invention relates to characterizing changes in mammalian intestinal microbiota associated with associated with high-fat and low-fat diets and with diets containing hydroxypropylmethylcellulose (HPMC) and related methods for diagnosing, preventing and treating obesity and related conditions such as metabolic syndrome and diabetes mellitus. Therapeutic methods of the invention involve the use of probiotics, and/or prebiotics, and/or narrow spectrum antibiotics/anti-bacterial agents that are capable of restoring healthy mammalian bacterial intestinal microbiota.