Effects of diets containing sun fl ower oil and fi sh oil on lipid metabolism and fatty acid fl ow to the duodenum of beef steers

Duodenal fi stulated steers were offered grass silage at 14 g/kg liveweight and one of three concentrates at a ratio of 60:40 (forage:concentrate on a dry matter basis): FISH0, FISH1 or FISH2 designed to be iso-lipid and to provide the same amount of sunfl ower oil but increasing amounts of fi sh oil: 0, 1 and 4%, respectively. Fatty acid intakes and duodenal fl ows were examined to determine the effects of fi sh oil on polyunsaturated fatty acid (PUFA) metabolism. Fish oil signifi cantly increased the fl ow of long chain PUFA, conjugated linoleic acid and trans vaccenic acid to the duodenum and decreased the fl ow of stearic acid. Biohydrogenation of linoleic and linolenic acids was not affected by fi sh oil inclusion.


INTRODUCTION
Clinical research has shown that the intake of polyunsaturated fatty acids (PUFA) and in particular long chain PUFA such as C20:5n-3 and C22:6n-3 found in fi sh oil are benefi cial to human health (Tapiero et al., 2002).Previous studies have shown that fi sh oil inclusion in the diet of ruminants has increased the concentration of long chain PUFA in milk (Shingfi eld et al., 2003) and muscle (Scollan et al., 2001a).Fish oil also signifi cantly increased the post-ruminal fl ow of trans vaccenic acid (TVA), an intermediate in the biohydrogenation of linoleic and linolenic acid (Scollan et al., 2001b;Shingfi eld et al., 2003).This may have been responsible for the observed increase in the concentration of conjugated linoleic acid (CLA) in milk through the bioconversion of TVA to cis 9 trans 11 CLA in the mammary gland (Shingfi eld et al., 2003).This study was designed to report the effect of graded levels of fi sh oil on the fl ow of long chain PUFA and biohydrogenation intermediates such as TVA and CLA, when steers were offered a fl at rate of linoleic acid, supplied from sunfl ower oil.It was hoped this would create a greater understanding of the effect of fi sh oil in the biohydrogenation of C18 PUFA.

Animals and experimental design
Six Hereford x Friesian steers (about 410 kg), prepared with rumen and duodenal cannulae were offered a fi rst cut perennial ryegrass silage plus one of three concentrates: FISH0, FISH1 or FISH2 (Table 1).The total daily feed allowance was 14 g DM/kg liveweight (about 90% ad libitum) with a forage : concentrate ratio of 60:40 (DM basis).The experiment design was a Latin square consisting of 3-periods with two animals per treatment.Each 21d period consisted of 14 d adaptation to the diet and 7 d for digesta collection.Animals received their daily forage allocation at 09.00 and their daily concentrate allocation in 2 equal meals at 09.00 and 15.00.Digesta fl ow at the duodenum was estimated using a dual-phase marker system with ytterbium acetate and chromium EDTA as the particulate and liquid phase markers, respectively (Faichney, 1975).

Chemical and statistical analysis
Chemical and fatty acid compositions of the silages and digesta were determined as described by Lee et al. (2003).Digesta fl ows were calculated after mathematical reconstitution of true digesta as described by Faichney (1975).Biohydrogenation of C18 PUFA was assessed as the difference between daily intake and duodenal fl ow (g/day).Data were subjected to ANOVA (Genstat 7 ©, 2004) with diet as the treatment effect and blocking according to period + animal.

RESULTS
Dry matter and major fatty acid intake and duodenal fl ow are given in Table 2.There were no signifi cant differences in nutrient and total fatty acid intake and duodenal fl ow.Increasing the concentration of fi sh oil in the diet signifi cantly increased the intake and duodenal fl ow of long chain PUFA.It also increased the fl ow of TVA and CLA, but not the isomer cis 9 trans 11, and decreased the fl ow of stearic acid.Biohydrogenation of linoleic and linolenic acid were not signifi cantly different across diets, averaging 90.8 and 91.8%, respectively.

DISCUSSION
All three diets in the present study resulted in net synthesis of fatty acids across the rumen as previously reported by Scollan et al. (2001b) when feeding a fi sh oil LEE M.R.F.ET AL. supplement, and this maybe due to endogenous lipid or microbial synthesis.Fish oil had no effect on the extent of biohydrogenation of either linoleic or linolenic acid, but signifi cantly increased the fl ow of the intermediate products TVA and total CLA and signifi cantly reduced the fl ow of the end product stearic acid.However, there was no signifi cant difference in the fl ow of cis 9 trans 11 CLA, the product of the initial isomerisation of linoleic acid in biohydrogenation, in these diets and so the effect of fi sh oil appeared to be an inhibition to the fi nal reduction of TVA into stearic acid.Wallace et al. (2004) have identifi ed two species of ruminal bacteria responsible for the biohydrogenation of both linoleic and linolenic acid due to their extreme sensitivity to PUFA, namely Butyrivibrio fi brisolvens and Fusocillus spp.These bacteria in conjunction but not in isolation can hydrogenate linoleic and linolenic acid to stearic acid.B. fi brisolvens hydrogenates the PUFA to cis 9 trans 11 CLA and TVA and Fusocillus completes the hydrogenation of TVA to stearic acid.It may be that the long chain PUFA in fi sh oil inhibits Fusocillus resulting in a signifi cant elevation in TVA and consequently an increase in milk CLA through TVAs bio-conversion in the mammary gland (Shingfi eld et al., 2003).

Table 1 .
Formulation of the experimental concentrates

Table 2 .
Dry matter and major fatty acid intake and duodenal fl ow