Role of biohydrogenation intermediates in milk fat depression

Fat is the most variable constituent in milk and is reduced on low-fiber/high-concentrate diets or on rations containing lipid supplements rich in polyunsaturated fatty acids. The biohydrogenation theory attributes the causal mechanism underlying diet-induced milk fat depression (MFD) to changes in ruminal lipid metabolism leading to increased formation of specific biohydrogenation intermediates that exert anti-lipogenic effects. Trans-10, cis-12 conjugated linoleic acid (CLA) is the only intermediate shown unequivocally to inhibit milk fat synthesis. However, increases in ruminal trans-10, cis-12 CLA formation do not provide a universal explanation for the reductions in milk fat during diet-induced MFD, suggesting that other biohydrogenation intermediates may also be involved. Post-ruminal infusion experiments have provided tentative evidence that cis-10, trans-12 CLA and trans-9, cis-11 CLA also exert anti-lipogenic effects. Diet-induced MFD is consistently associated with an increase in milk fat trans-10 18:1 concentrations, but recent direct evidence offers little support for a role of this intermediate in the regulation of milk fat synthesis. Further research is required to characterize the structure and function of other biohydrogenation intermediates, as well as considering the contribution of more global changes in ruminal lipid metabolism to provide a more universal explanation of diet-induced MFD.