L-Theanine ameliorates high-protein diet-associated hepatic glucose and lipid metabolic alterations by promoting fatty acid β-oxidation and suppressing lipid synthesis and gluconeogenesis in rats.

High-protein diets (HPDs) have the potential to reduce body weight, yet excessive protein intake can induce hepatic metabolic dysregulation and insulin resistance. L-theanine (LTA), a unique amino acid abundantly present in tea leaves, regulates protein metabolism under HPD conditions. However, its influence on glucose and lipid metabolism remains unclear. In this study, Sprague-Dawley rats were fed either a standard maintenance diet (20% of energy from protein) or HPDs containing 30%, 40%, or 50% of energy from protein, and were administered LTA at different doses (0, 100, 200, or 400 mg kg-1·body weight-1) for 40 days. A combination of physiological and biochemical assessments, metabolomics, proteomics, and Western blot techniques was used to investigate the regulatory effects and mechanisms of LTA on glucose and lipid metabolism under HPD conditions. The 30% HPD did not cause notable metabolic changes, whereas the 40% and 50% HPDs led to hyperinsulinemia and hepatic lipid accumulation by enhancing gluconeogenesis and fatty acid synthesis while inhibiting fatty acid β-oxidation. LTA alleviated insulin elevation, hepatic steatosis, and lipid droplet accumulation caused by the 40% and 50% HPDs. Mechanistically, LTA enhanced peroxisome proliferator-activated receptor alpha (PPARα) expression and increased acyl-CoA dehydrogenase medium chain (ACADM) expression to promote mitochondrial fatty acid β-oxidation, while decreasing acetyl-CoA carboxylase alpha (ACACA) and phosphoenolpyruvate carboxykinase 1 (PCK1) expression, thereby suppressing lipid synthesis and gluconeogenesis. These findings highlight the potential of LTA to mitigate HPD-associated metabolic alterations, supporting its application as a functional food ingredient for metabolic health management.