Effect of fish oil on in vitro rumen lipolysis, apparent biohydrogenation of linoleic and linolenic acid and accumulation of biohydrogenation intermediates

The effect of fish oil (FO) on lipolysis and apparent biohydrogenation of linoleic (C18:2n-6, LA), linolenic (C18:3n-3, LNA), eicosapentaenoic (C20:5n-3, EPA) and docosahexaenoic (C22:6n-3, DHA) acid was studied in 6 and 24 h in vitro incubations with diluted sheep rumen contents (50 ml) as inoculum and six substrates (0.50 g) containing compound feeds based on barley, sugar beet pulp and soybean meal. Feeds containing sunflower oil (SO, 80 g/kg DM) or linseed oil (LO, 80 g/kg DM), as C18:2n-6 and C18:3n-3 source, respectively, were further enriched with three levels of fish oil (0, 20 and 40 g/kg DM), whereas addition of lard (40, 20 and 0 g/kg DM) ensured equal amounts of added fat (120 g/kg DM) in all feeds. Lipolysis was determined by the difference in triacylglycerol before and after incubation, whereas apparent biohydrogenation was calculated by the decrease of free polyunsaturated fatty acids (PUFA). no influence on the degree of lipolysis or apparent biohydrogenation of C18:2n-6 and C18:3n-3, or on lipolysis of EPA and DHA in the different incubations. Biohydrogenation of EPA and DHA, however, was reduced with increasing FO dose. Compared to C18:2n-6 and C18:3n-3, lipolysis as well as biohydrogenation of EPA and DHA were significantly lower. Although C18:2n-6 and C18:3n-3 disappeared to the same extent, the amount of stearic acid (C18:0), the end-product of their biohydrogenation, was significantly lower in substrates with FO supplementation. Higher amounts of t11C18:1 and t11c15C18:2 in incubations with these substrates suggest that FO mainly inhibits the final step of biohydrogenation of C18:2n-6 and C18:3n-3. on of FO to diets containing C18:2n-6 or C18:3n-3 could increase ruminal accumulation and, hence, duodenal flow of t11C18:1. This could result in a higher uptake by the tissues and more substrate for the endogenous synthesis of c9t11CLA.