Milk Odd- and Branched-Chain Fatty Acids as Biomarkers of Rumen Fermentation

In recent years, research has focused on determining the contents of odd- and branched-chain fatty acids (OBCFAs) in milk from various ruminant species, examining the relationship between rumen fermentation processes and changes in feed composition. The content of OBCFAs in cow's milk fat depends on the composition of the rumen microbiota, influenced by factors such as the feeding system, feed composition, proportion of roughage to concentrate, and rumen content pH. Therefore, the profiling of FAs in milk is of the utmost importance, as they can be considered as a noninvasive biomarker for predicting the proportions of volatile fatty acids (VFAs) in the rumen, microbial protein synthesis, or susceptibility to ruminal acidosis in dairy cows. High-fiber diets generally have a lower milk OBCFA content than lower-fiber diets. Some OBCFAs found in milk fat can be markers of ruminal acidosis or a negative energy balance. In this review, we present an up-to-date summary of research on the role and significance of OBCFAs in rumen fermentation processes and the link between these relationships and the potential for diagnosing metabolic disorders in dairy cows. Cow's milk and dairy products are the primary sources of OBCFAs, which have beneficial health properties. The goal of this study was to identify the factors that influence the content of OBCFAs in cow's milk and to indicate which OBCFAs can serve as biomarkers for fermentation processes. The content of OBCFAs in milk depends on the species of ruminants, with studies showing that this varies between 3.33% (in goat's milk) and 5.02% (in buffalo's milk). These differences also stem from the animals' energy balance, lactation phases, forage-to-concentrate ratio, and the presence of bioactive compounds in feeds, as well as management practices and environmental conditions. The OBCFAs in milk fat mainly come from rumen bacteria, but can also be synthesized de novo in the mammary gland, making them potentially useful noninvasive indicators of rumen fermentation. The concentration of BCFA is lower in colostrum and transitional milk than in full lactation milk. The proportions of total OBCFAs are higher in first- and second-parity cows. The most effective predictors of the biohydrogenation of fatty acids in the rumen are likely C18:2 cis-9, trans-11, iso-C16:0, and iso-C13:0. OBCFAs have been identified as potential biomarkers for rumen function, because their synthesis depends on specific bacteria. Strong predictors of subclinical ruminal acidosis include iso-C14:0, iso-C13:0, and C15:0. The concentration of & sum; OBCFA >C16 in milk is associated with fat mobilization and serves as a significant marker of the energy balance in cows.