Production, Transport, and Metabolism of Volatile Fatty Acids in the Yak Rumen: Unraveling the Unique Mechanisms Underpinning High-Altitude Adaptation.

Volatile fatty acids (VFAs), the primary end-products of microbial fermentation in the ruminant forestomach, supply approximately 70% of the host's energy requirements and play a pivotal role in maintaining energy homeostasis. While the mechanisms governing ruminal VFA production, absorption, and metabolism are well-characterized in common ruminants like dairy and beef cattle, a systematic integration of these processes in yaks, an iconic species long-adapted to the extreme Qinghai-Tibet Plateau, remains incomplete. This review synthesizes current knowledge on the entire VFA pathway in the yak rumen, from production to tissue metabolism. We detail the critical roles of functional microbes, including fibrolytic bacteria and Prevotella, in VFA synthesis and how their activity is dynamically regulated by dietary composition and seasonal shifts. Building on the unique structural features of the yak rumen epithelium, the review analyzes VFA absorption mechanisms involving both passive diffusion and carrier-mediated transport. Furthermore, we systematically outline the metabolic fates and energy partitioning strategies of VFAs across the rumen epithelium, liver, and peripheral tissues. This synthesis aims to elucidate the highly efficient and adaptive physiological basis of VFA metabolism that underpins the yak's exceptional ability to utilize energy under the low-energy conditions of the high-altitude environment. Ultimately, this work seeks to provide a theoretical foundation for understanding plateau-adapted energy efficiency and to inform precision nutritional strategies for ruminants in alpine regions.