INTRODUCTION: The yak (Bos grunniens) is uniquely adapted to the Qinghai-Tibet Plateau, yet nutritional guidelines for yaks remain limited and often follow cattle standards. This study aimed to clarify dietary energy requirements to improve yak feeding strategies. METHODS: Fattening yaks were assigned to three diets with different net energy for gain (NEg): low (LE, 4.06 MJ/kg), medium (ME, 4.46 MJ/kg), and high (HE, 4.87 MJ/kg), with 13% crude protein constant. Growth performance, apparent digestibility, serum biochemistry, hormones, and rumen fermentation were measured. Additional, 16S rDNA sequencing analyzed the rumen and fecal microbiota, and serum non-targeted metabolomics profiling was performed. RESULTS: Results showed that ME and HE diets significantly increased average daily gain and nutrient digestibility while reducing feed conversion ratios (P < 0.05). Serum glucose, triglycerides, and growth-related hormones were higher in ME and HE groups, while non-esterified fatty acids was significantly decreased (P < 0.05). Higher energy diets increased microbial protein and reduced ruminal ammonia nitrogen. The ME diet increased ruminal Proteobacteria, enhancing fermentation and soluble carbohydrate utilization, while promoting fiber-degrading UCG-005. High energy diets elevated fecal Spirochaetota and Treponema abundances. Metabolomics revealed differences mainly in lipidrelated metabolites correlated with specific microbial taxa. DISCUSSION: In conclusion, increasing dietary energy improves growth, nutrient utilization, and beneficial microbiota profiles in yaks. A medium-energy diet (NEg: 4.46 MJ/kg) optimizes fermentation and microbial balance, providing a scientific basis for precise nutritional strategies in yak production on the Qinghai-Tibet Plateau.