Metabolic origin and bioactive properties of odd and branched-chain fatty acids in ruminants' milk

Milk odd and branched-chain fatty acids (OBCFA) are a group of lipids that represents less than 5 % of the total fatty acids (FA) and that includes a group of molecules, among which the most abundant are the isomers of the pentadecanoic (15:0, iso-15:0 and anteiso15:0), hexadecanoic (iso-16:0), and heptadecanoic (17:0, iso-17 :0 and anteiso-17:0) FA. OBCFA are synthesized by rumen microorganisms from the molecules produced during feed fermentation processes. Recent research indicates the possibility of endogenous synthesis of some odd (15:0 and 17:0) and branched-chain (iso-1:0 and anteiso-17 : 0) FA. The presence of these FA in milk is influenced by dietary factors, mainly the starch vs fiber proportion, forage to concentrate ratio, and the supplementation with fat sources that change the lipid metabolism, which modifies the OBCFA profile of milk. Milk and dairy products are the main and almost only source of OBCFA in the human diet. Despite their low concentration, OBCFA possess bioactive properties that have been shown in different investigations. This article reviews the metabolic origin, bioactive properties, and most recent nutritional strategies directed to manipulate the contents and profiles of OBCFA in milk fat.