Invited review: A quantitative, whole-animal integration of metabolism of palmitic, stearic, and oleic acids in dairy...

Milk fatty acid (FA) composition reflects the integrated metabolism of dietary FA, ruminal biohydrogenation, tissue fluxes, and mammary synthesis. Despite extensive knowledge of individual processes, a quantitative, whole-animal framework linking FA supply to milk output remains limited. This review introduces a mass-balance approach based on fatty acid balance (FAB), defined as metabolizable total C16 and total C18 FA intake minus milk total C16 and C18 FA secretion, to evaluate constraints on milk fat synthesis in dairy cows. Using this framework, FA metabolism is partitioned by chain length, recognizing that C18 FA originate almost exclusively from dietary and adipose sources, whereas C16 FA derive additionally from de novo synthesis in the mammary gland. This distinction imposes fundamental constraints: deficits in C18 FA must be met by adipose tissue mobilization, while remaining C16 FA requirements after adipose mobilization are fulfilled through de novo synthesis. Application of the framework to published data demonstrates that metabolizable FA supply is often insufficient to meet milk FA output, resulting in negative FAB and necessitating sustained endogenous FA contributions beyond early lactation. Quantitative evaluation of cis-9-C18:1 biohydrogenation further reveals that limited ruminal escape (18.7% of intake) restricts dietary supply to a minor fraction of milk cis-9-C18:1, with the majority derived from adipose tissue and mammary desaturation of stearic acid. Sensitivity analysis indicates that these conclusions are robust to variation in key assumptions, including ruminal biohydrogenation, intestinal digestibility, and efficiency of use. Overall, this approach establishes FA mass balance as a central constraint on milk fat synthesis and highlights the need to incorporate dynamic, whole-animal FA partitioning into nutritional models. Recognition of persistent endogenous FA contributions challenges static representations of milk fat synthesis, which should be taken into account in models predicting milk fat composition.