Rationale: Ectopic lipid deposition impairs muscle anabolic responseespecially during aging. We hypothesized that the anabolic efficiencyof dietary protein in skeletal muscle might be affected by high-fat diet.The objective of the study was to investigate muscle protein synthesisin response totwolevels of protein intake combined totwo levels of fatintake. Methods: Two groups of sixty adult and forty-nine old male Wistarrats were randomly divided into four groups: isocaloric standard diet (12% protein, 14% lipid, as STD12); isocaloric standard (high-protein)diet (25% protein, 14% lipid, STD25); hypercaloric high-fat (normal-protein) diet (12% protein, 45% lipid, HFD12); and hypercaloric high-fat (high-protein) diet (25% protein, 45% lipid, HFD25). The nutritionalintervention lasted 10 weeks. The fractional synthesis (FSR) andabsolute synthesis rates (ASR) of mixed muscle proteins werecalculated using isotopically labelled13C-Valine incorporation intibialis anterior (TA). Muscle lipid content was assessed usingchromatography method. Results: Rats in the high-fat diet groups self-limited their food intake,so that energy intake was not different among the groups. Regardlessof dietary intervention, TAweight was lower in old groups compared totheir adult counterparts (all p values Conclusions: Aging is characterized by reduced muscleweight despiteincreased FSR, suggesting specific alterations in the nutritionalregulation of muscle protein turnover. In isocaloric conditions, higherprotein intake modulates muscle lipid infiltration, without improvingage-related anabolic resistance in old rats fed a high-fat diet.