Factors affecting odd- and branched-chain fatty acids in milk: a review

Odd- and branched-chain fatty acids (OBCFA) in milk fat are largely derived from bacteria leaving the rumen. The main OBCFA in milk of dairy cows are isomers of tetradecanoic acid (iso C14:0), pentadecanoic acid (C15:0, iso C15:0 and anteiso C15:0), hexadecanoic acid (iso C16:0) and heptadecanoic acid (C17:0, iso C17:0 and anteiso C17:0). There is an increasing interest in OBCFA as potential diagnostic tools of rumen function (e.g., rumen fermentation pattern, bacterial N). Other reasons for interest in OBCFA are their anticarcinogenic effects on cancer cells, their influence on milk fat melting point and their potential as indicators of dairy product intake by humans. In this paper, we review recent literature on the topic, particularly in relation to effects of dietary treatments on milk OBCFA. De novo synthesis of OBCFA in rumen bacteria and animal tissue is discussed briefly. Milk secretion of linear odd-chain fatty acids (C15:0, C17:0) was higher than their duodenal flow suggesting de novo synthesis from propionate in animal tissue, whereas regression analysis suggested cis-9 C17:1 to be a desaturation product of C17:0. Variation in milk OBCFA induced by dietary treatments is further evaluated and related to OBCFA composition of pure strains of rumen bacteria. An increase in the proportion of dietary forage generally increased milk OBCFA with the strongest effect on iso C14:0 and iso C15:0. In addition, forage source substantially affected milk OBCFA pattern with a decrease in iso C14:0 and iso C16:0 and increase in C17:0 and cis-9 C17:1 upon replacement of grass silage by maize silage. Finally, we relate the variation in milk OBCFA to dietary composition and rumen hydrogenation intermediates of dietary polyunsaturated fatty acids. Milk content of medium-chain fatty acids (C12:0, C14:0 and C16:0) was positively related with the linear odd-chain fatty acids and milk content of major hydrogenation intermediates (i.e., trans-11 C18:1; cis-9, trans-11 C18:2; trans-11, cis-15 C18:2) increased with increasing iso C17:0, whereas a negative relationship occurred with iso C14:0 and iso C16:0. This review illustrates the potential of OBCFA as a diagnostic tool for rumen function both in relation to nutrient supply and optimization of milk fatty acid composition