Feed costs are on the rise and are a major variable cost in swine production. As such, the overall objective of this proposal was to determine the extent to which energy sensing, insulin signaling and markers of muscle protein turnover contributes to feed efficiency (FE) in pigs divergently selected for residual feed intake (RFI). Residual feed intake is the residual portion of feed intake which indicates the extent to which animals deviate from their predicted or expected level of feed intake, with efficient animals having lower RFI index. Thus, RFI is a unique measure of FE because it represents true differences in the ability of animals to use feed energy for the metabolic processes of maintenance and growth. Using Iowa State University’s unique RFI pig lines, we hypothesized that pigs selected for low RFI are more efficient at nutrient utilization and lean tissue accretion because they have reduced protein degradation and turnover compared to higher RFI pigs. To test this hypothesis, we examined the extent to which liver and muscle AMP activated protein kinase (AMPK, energy sensor of tissues) contributes to enhanced metabolic efficiency in pigs selected for low RFI. Furthermore, we evaluated protein markers of muscle insulin signaling and protein turnover (degradation and synthesis) to understand the biological basis for the differences in RFI and FE in finisher pigs. Twelve high RFI and 12 low RFI finisher gilts were individually housed and used for this study. Muscle and liver samples were collected and AMPK, insulin signaling intermediates, protein degradation markers and markers involved in translation initiation of muscle protein synthesis were assessed by enzyme assays or western blots analysis of protein expression. The results of this project indicate that selection for improved FE and low RFI may select for reduced protein degradation and turnover. This is explained by the evidence that low RFI skeletal muscle had greater calpastatin activity, lower calpain and ubiquitin-proteasome activity compared to that of high RFI, less efficient pigs. No differences in insulin signaling intermediate markers and proteins involved in translation control of protein synthesis (mTOR pathway) were found in between our selection lines. Together, these data indicates that protein degradation and turnover may play a critical role in FE in swine. Nutritional and pharmacological strategies could be developed to attenuate protein turnover and degradation activity in finisher pigs to improve FE gains. Please contact Nicholas Gabler at Iowa State University for further detail (firstname.lastname@example.org).