Oral Antibiotics Reverse Maladaptive Metabolic Responses to a Low Protein Diet

Background: The colonic microbiome is remarkable in that it is amongst the most densely populated microbial habitats on Earth. The study of gut microbial ecology and its interplay with the host metabolome has emerged as a critical frontier in contemporary nutritional and metabolic research. Classic studies in physiology have shown that the gut microbiome plays an important role in nitrogen balance of the host. Urea, produced by the host, is hydrolyzed in the colon by urease-producing gut bacteria and the resultant ammonia is absorbed by the host and utilized for protein metabolism. In this study, we determined the effect of the gut microbiome on the metabolic alterations induced by a low protein diet (LPD). Methods and Results: Female C57B/6J mice were fed either a normal chow (20% kcal protein) or low protein diet (3% kcal protein) for 1 month. Metabolic phenotyping revealed that mice fed a low protein diet failed to gain weight despite a 40% increase in food intake. Compared to mice fed a normal chow diet, mice fed a LPD also showed significant alterations in body composition, as assessed by MRI, with a decrease in lean mass and a proportional increase in fat mass as well as a significant increase in energy expenditure, as determined by indirect calorimetry. In addition, LPD fed mice suffered a profound defect of water conservation manifested as severe polydypsia and polyuria. Remarkably, except for growth retardation, the administration of oral antibiotics to mice fed a LPD dramatically prevented all of these adverse metabolic sequelae. GC-MS of urinary samples revealed that oral antibiotics did not lead to profound alterations in urinary amino acid excretion suggesting that gut bacteria were not competing with the host for dietary nitrogen sources in a significant way. This notion is supported by the failure of LPD mice to increase serum IGF-1 levels and normalize body weight when placed on oral antibiotics. Preliminary determination of gut microbiome composition, by 16S rDNA sequencing using the 454/Roche method and taxonomic group assignment at the genus level using RDPclassifier, revealed modest alterations induced by a LPD. Conclusions: These results suggest that the gut microbiome evokes deleterious metabolic consequences in the host in the setting of a low protein diet that are independent of competition for dietary nitrogen. The gut microbiome may, therefore, play an important role in the regulation of host metabolic physiology in settings where the consumption of dietary protein is severely limited.