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High-Concentrate Diets and Polyunsaturated Oils Alter Trans and Conjugated Isomers in Bovine Rumen, Blood, and Milk

J. J. Loor^*, A. Ferlay, A. Ollier, K. Ueda, M. Doreau and Y. Chilliard

Herbivore Research Unit INRA-Theix, 63122 St-Genès Champanelle, France

Corresponding author: Yves Chilliard; e-mail: yves.chilliard{at}clermont.inra.fr.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGEMENTS REFERENCES

 
Three Holstein cows were fed a high-concentrate diet (65:35 concentrate to forage) supplemented with either 5% sunflower oil (SO), 5% linseed oil (LO), or 2.5% fish oil (FO) to examine effects on biohydrogenation and fatty acid profiles in rumen, blood plasma, and milk. Diets were fed in a 3 x 3 Latin square with 4-wk periods with grass hay as the forage. Milk yield, dry matter intake, and percentages of milk fat (2.64) and protein (3.22) did not differ. All diets resulted in incomplete hydrogenation of unsaturated fatty acids as indicated by the profiles of 18:1 isomers, conjugated 18:2 isomers, nonconjugated 18:2 isomers, and 18:0 in ruminal fluid. Percentages of 8:0–14:0 and 16:0 in milk fat were greater with FO. Percentage and yield of trans10,cis12-18:2 were small and greater in cows fed SO (0.14%, 0.57 g/d) than FO (0.03%, 0.15 g/d) or LO (0.04%, 0.12 g/d). Percentage and yield of trans10-18:1, however, increased with FO (6.16%) and SO (6.47%) compared with LO (1.65%). Dietary FO doubled percentage of cis11-18:1 in rumen, plasma, and milk fat. Despite a lack of difference in ruminal percentage of trans11-18:1 (10.5%), cows fed FO had greater plasma trans11-18:1 (116 vs. 61.5 µg/mL) but this response did not result in greater trans11-18:1 percentage in milk fat, which averaged 5.41% across diets. Percentage (2.2%) and yield (14.3 g/d) of cis9,trans11-18:2 in milk fat did not differ due to oils. Unique responses to feeding LO included greater than 2-fold increases in percentages of trans13+14-18:1, trans15-18:1, trans16-18:1, cis15-18:1, cis9,trans12-18:2 and trans11,cis15 -18:2 in umen, plasma, and milk, and cis9,trans13-18:2 in plasma and milk. Ruminal 18:0 percentage had the highest positive correlation with milk fat content (r = 0.82) across all diets. When compared with previous data with cows fed high-concentrate diets without oil supplementation, results suggest that greater production of trans10-18:1, cis11-18:1, and trans11,cis15-18:2 coupled with low production of 18:0 in the rumen may be associated with low milk fat content when feeding high-concentrate diets and fish oil. In contrast, SO or LO could lead to low milk fat content by increasing ruminal trans10-18:1 (SO) or trans11,cis15-18:2 and trans9,trans12-18:2 (LO) along with a reduction in mammary synthesis of 8:0-16:0. Simultaneous increases in ruminal trans11-18:1 with fish oil, at a fraction of sunflower oil supplementation, may represent an effective strategy to maintain cis9,trans11-18:2 synthesis in mammary while reducing milk fat output and sparing energy.

Key Words: high-concentrate diet • polyunsaturated oil • trans fatty acid • milk conjugated linoleic acid

Abbreviation key: CLA = conjugated linoleic acids, DHA = docosahexaenoic acid, EPA = eicosapentaenoic acid, FO = high-concentrate diet plus 2.5% fish oil, LO = high-concentrate diet plus 5% linseed oil, SO = high-concentrate diet plus 5% sunflower oil.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGEMENTS REFERENCES

 
Extensive literature over the past few decades suggests that milk fat depression is a multifactorial phenomenon (see review by Bauman and Griinari, 2003). Acetate and butyrate production (mol/d) are decreased by high dietary concentrate (Sutton et al., 2003), partly accounting for lower jugular plasma concentration and jugular-mammary venous difference for acetate and BHBA (Loor et al., 2005a). These changes may contribute to the effect of dietary concentrate level on milk fat yield. Others, however, have recently shown that despite an estimated 34% more acetate and 35% more butyrate produced in the rumen of cows fed high-concentrate diets there was a significant depression in milk fat yield (Oba and Allen, 2003). Recent studies clearly suggest that unique fatty acid intermediates produced by altered ruminal biohydrogenation are causally related to diet-induced milk fat depression (Bauman and Griinari, 2003).

Some of the greatest reductions in milk fat yield have been observed with high-concentrate diets supplemented with vegetable or marine oils (Griinari et al., 1998; Piperova et al., 2000; Peterson et al., 2003; Loor et al., 2005a,b). Complete ruminal biohydrogenation of polyunsaturated fatty acids is reduced under these circumstances resulting in higher intestinal flows of trans-18:1 isomers, conjugated linoleic acids (CLA), and other nonconjugated 18:2 isomers (Piperova et al., 2002; Shingfield et al., 2003; Loor et al., 2004b). Vegetable oils rich in linoleic acid fed with a high-concentrate diet resulted in higher percentages of trans10-18:1 in milk fat (Griinari et al., 1998; Piperova et al., 2000; Peterson et al., 2003). Cis9,trans11-18:2, the primary CLA in milk fat, also increased along with small significant increases in trans7,cis9-18:2 (Piperova et al., 2000) and trans10,cis12-18:2 (Piperova et al., 2000; Peterson et al., 2003).

Cis9,trans11-CLA and vaccenic acid (its precursor in tissues) in meat and milk are examples of biohydrogenation intermediates that may have beneficial implications for human health (Corl et al., 2003). Alterations in biohydrogenation pathways due to diet are important because increased ruminal production of trans10-18:1 and trans10,cis12-18:2 have been associated with milk fat depression (Bauman and Griinari, 2003), and because a reduction in trans11-18:1 production in the rumen may decrease endogenous synthesis of cis9,-trans11-18:2 in tissues. Furthermore, trans11-18:1 in milk fat was transiently increased for 1 wk after addition of polyunsaturated oils to high-concentrate or corn silage-based diets, after which it decreased along with a simultaneous increase in trans10-18:1 (Chilliard and Ferlay, 2004; Roy et al., 2005).

Previous studies compared effects of TMR containing mixtures of fish oil and different sources of unsaturated fatty acids (AbuGhazaleh et al., 2003), or concentrate:forage ratio and linseed oil (Loor et al., 2005a) fed to dairy cows on blood plasma metabolites, milk composition, and profiles of trans-18:1 and CLA isomers in blood plasma and milk fat. Furthermore, previous results suggest that the nature of supplemental polyun-saturated fatty acids added to high-concentrate diets likely alters the profile and amount of hydrogenation intermediates available for secretion in milk. Thus, the primary objective of this study was to evaluate the effectiveness of high-concentrate diets in combination with fish oil [20 g/100 g of total fatty acids 20:5n-3 (eicosapentaenoic acid, EPA) and 7 g/100 g of 22:6n-3 (docosahexaenoic acid, DHA)], linseed oil (51 g/100 g 18:3n-3), or sunflower oil (69 g/100 g 18:2n-6) in modifying ruminal, blood plasma, and milk fat cis and trans isomers of 18:1, nonconjugated 18:2, and conjugated 18:2 isomers. In addition, we studied the correlations between these isomers in ruminal fluid and milk fat content.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGEMENTS REFERENCES

 
Animals and Diets
Three lactating multiparous Holstein cows, 84 ± 21 DIM, and cannulated in the rumen and duodenum were used in a 3 x 3 Latin square design during three 4-wk periods to evaluate responses to feeding diets with a high (65:35) concentrate to forage (grass hay) ratio supplemented (on a DM basis) with 2.5% fish oil (FO), 5.0% linseed oil (LO), or 5.0% sunflower oil (SO) (Loor et al., 2005c). The level of fish oil was chosen for the following reasons: 1) to avoid possible detrimental effects on DMI, which are proportional to the amount fed (Doreau and Chilliard, 1997; Donovan et al., 2000); 2) to avoid excessive decreases in milk fat yield (Chilliard et al., 2001); and 3) to allow intake of EPA plus DHA at less than 25% the amount of linoleic plus linolenic acid in cows receiving SO or LO, which in a previous in vitro study was as effective in elevating trans11-18:1 as linoleic acid (AbuGhazaleh and Jenkins, 2004). To allow cows to adapt to oil-supplemented diets, all cows were fed a diet supplemented with an oil mixture comprising sunflower oil, linseed oil, plus fish oil (40 g/d + 40 g/d + 20 g/d, respectively) for 3 wk before the first experimental period. Cows were housed in a tie-stall barn during the experiment. The concentrate mixture with oils (homogeneously mixed) was prepared daily and, along with the forage, was offered in equal amounts at 0900, 1330, and 1700 h for ad libitum consumption. Hay and concentrate were given separately. The desired forage:concentrate ratio was maintained by daily adjustment of forage and concentrate amounts offered, depending on the refusals of the previous day.

The concentrate mixture contained (DM basis) on average 28.2% beet pulp, 18.9% ground wheat, 18.9% barley, 14.2% rapeseed meal, 13.7% soybean meal, 2.0% wet beet molasses, 3.1% buffers, 0.5% binding agent, and 0.5% mineral-vitamin mix. Cows were milked at 0600 and 1700 h.

Sampling, Measurements, and Analyses
Ruminal fluid (300 mL) was collected during wk 4 by suction from the ventral sac via the ruminal cannula at 0900, 1100, 1300, 1500, 1700, 2000, and 0000 h (Loor et al., 2004b). Milk production and DMI were recorded daily throughout the experiment. Milk was sampled at each milking during the last 5 d of wk 4. One 50-mL aliquot from each of these milkings containing potassium bichromate (Merck, Fontenay-Sus-Bois, France) was stored at 4°C until analyzed for fat, protein, and lactose by infrared analysis with a 3-channel spectrophotometer (AOAC, 1997; CILAL, Theix, France). Additional 3-mL aliquots from 2 consecutive milkings on the last day of wk 4 were also collected and stored at –20°C before isolation of milk fat and fatty acid analysis. These samples were composited based on a.m. and p.m. milk production. Data on milk production and DMI were averaged over the last 5 d of wk 4 before statistical analysis.

For plasma total fatty acid analysis, blood samples (50 mL) were collected in EDTA-containing Vacutainer tubes (CML, Nemours, France) from the jugular vein at 0830 h on the last day of wk 4. For plasma metabolite analysis, an additional 10 mL of blood was collected from the jugular and abdominal mammary vein by venipuncture using EDTA-containing (0.47 mol/L) Vacutainers. Blood was centrifuged at 3000 x g for 15 min for harvesting plasma. Plasma was stored at –20°C until analyzed for fatty acids and metabolites. Plasma concentrations of metabolites were determined as described by Ferlay and Chilliard (1999) with an ELAN autoanalyzer using the same kits and procedures as in Loor et al. (2005a). Insulin and leptin were determined by radioimmunoassay as described in Delavaud et al. (2002).

Plasma total lipids were extracted with chloroform/methanol (2:1, vol/vol; Loor et al., 2005a). Fatty acids in plasma lipids were methylated with 2 mL of 0.5 N NaOCH₃ at 50°C for 30 min, followed by 2 mL of 14% boron trifluoride in methanol at 50°C for 30 min (Loor et al., 2005a). Tricosanoate (Sigma, Saint-Quentin Fallavier, France) was used as the internal standard. Fatty acids in milk fat were directly methylated with 1 mL of 2 N methanolic NaOCH3 at room temperature for 20 min, followed by 1 mL of 14% boron trifluoride in methanol at room temperature for 20 min (Loor et al., 2005a). Fatty acids in mixed ruminal fluid were methylated with the same protocol as for duodenal fatty acids (Loor et al., 2004b, 2005c). In all cases, fatty acid methyl esters were recovered in 1 mL of hexane. Samples were injected by autosampler into a Trace-GC 2000 Series gas chromatograph equipped with a flame-ionization detector (Thermo Finnigan, Les Ulis, France). Methyl esters from all samples were separated on a 100 m x 0.25 mm i.d. fused silica capillary column (CP-Sil 88, Chrompack, Middelburg, The Netherlands). Identification of 18:1, 18:2, CLA, and 18:3 isomers and odd and branched-chain fatty acids was as described in Loor et al. (2004b). A butter reference standard (BCR 164; Commission of the European Communities, Community Bureau of Reference, Brussels, Belgium) was used to estimate correction factors for short-chain (4:0–10:0) fatty acids.

For fatty acid analysis (0.5 to 1 µL of methyl esters in hexane injected at a 50:1 split ratio), the injector temperature was maintained at 250°C and the detector temperature was maintained at 255°C. The oven temperature was held at 70°C for 1 min, increased at 5°C/min to 100°C (held for 2 min), then increased at 10°C/ min to 175°C (held for 40 min), and increased at 5°C/ min to a final temperature of 225°C (held for 15 min). Hydrogen was the carrier gas. Injector pressure was held constant at 158.6 kPa. For all samples, satisfactory separations of cis- and trans-18:1 and nonconjugated 18:2 isomers were obtained with a single chromatographic run (see Figure 1 in Loor et al., 2004b). Separation of CLA isomers in duodenal lipids was shown in Loor et al. (2005c). Diurnal profiles of trans10-18:1, trans11-18:1, cis9,trans11-18:2, trans10,cis12-18:2, trans11,cis15-18:2, and 18:0 in ruminal fluid were presented in Loor et al. (2004a). Intake and duodenal flows of fatty acids were presented in Loor et al. (2005c).

View larger version (29K): [in this window] [in a new window]   Figure 1. Correlation analysis among ruminal fluid concentration of selected fatty acids and milk fat content or among ruminal concentration of cis9-18:1 and trans10-18:1. Correlation values were –0.55 (P = 0.12), –0.61 (P < 0.05), –0.73 (P < 0.01), –0.70 (P < 0.01), +0.82 (P < 0.01), and +0.60 (P < 0.05) for panels A through F.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGEMENTS REFERENCES

 
Production Variables and Blood Plasma Metabolites
Average DMI and milk, fat, protein, and lactose concentration and yields did not differ in response to oil type (Table 1). In cows fed LO, triglyceride concentration in jugular plasma and jugular-mammary venous difference were higher compared with other treatments (Table 2). In contrast, feeding SO resulted in higher jugular concentrations of glucose. The jugular-mammary venous difference for insulin was greater with FO compared with other treatments. Across diets there appeared to be net production of leptin by the mammary gland (i.e., significantly negative jugular-mammary venous difference), in agreement with data on leptin expression in this tissue (Bonnet et al., 2002).

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGEMENTS REFERENCES

The marked reduction in milk stearic acid yield with fish oil was associated with an apparent reduction in biohydrogenation of trans-18:1 to 18:0 as shown by lower ruminal 18:0 percentage (Table 3) and duodenal flow (Loor et al., 2005c). Indeed, ruminal concentrations of trans-18:1 (Table 3), although numerically lower, did not differ significantly between FO (22.6%), LO (24.7%), or SO (25.6%), despite more than 2-fold lower intake of 18:2n-6 and 18:3n-3 in cows fed FO (Loor et al., 2005b). Intake of 18:0 across diets was low, and with FO was ~18 g/d lower (19 vs. 37 g/d; Loor et al., 2005b). Thus, it is likely that dietary 18:0 intakes had a modest contribution to its ruminal concentration. Reduced ruminal outflow of 18:0 derived from biohydrogenation of unsaturated fatty acids likely reduced availability of 18:0 (low plasma concentration; Table 4) for desaturation, resulting in lower milk yield of cis9-18:1. Based on extensive literature and current results, availability of 18:0 for endogenous cis9-18:1 production seems to be crucial for milk fat synthesis (see also Donovan et al., 2000; Chilliard et al., 2003; Loor et al., 2005b). Furthermore, the reduction in 18:0 availability seems to be the strongest link among nearly all previous studies dealing with milk fat depression induced by fish oil supplementation (see review by Chilliard et al., 2001; Donovan et al., 2000; Shingfield et al., 2003; Loor et al., 2005b).

Interestingly, we observed a significant negative correlation (r = –0.73, P = 0.02) between ruminal fluid cis9-18:1 and milk fat content (Figure 1C). This negative relationship could be due in part to the fact that oleic acid in the rumen can be isomerized to trans10-18:1 (Mosley et al., 2002) [which tended (P = 0.12) to be negatively correlated (r = –0.55) with milk fat content], and is supported by a positive correlation (r = 0.60, P = 0.08) between both fatty acids in the rumen (Figure 1F). Ruminal oleic acid also was positively correlated (r = 0.63, P = 0.07) with ruminal trans9,trans12-18:2, which, based on our previous analysis (Loor et al., 2005a) and current data (Figure 1D), is negatively associated with milk fat content. Intake of oleic acid in this study was 124 g/d lower in cows fed FO compared with SO and LO (Loor et al., 2005c), but there were no differences in ruminal fluid cis9-18:1 percentage across diets (Table 2), which raises the possibility that, in cows fed FO, other 18:1 and(or) 18:2 intermediates gave rise to oleic acid. In this context, the negative correlation (r = (0.71, P < 0.05) between ruminal oleic acid and trans11-18:1 suggests the latter could be isomerized to cis9-18:1 (Mosley et al., 2002). Despite the negative correlation between ruminal oleic acid and milk fat content, data do not lend support to this fatty acid as being involved in the low-fat milk syndrome because 1) there was no correlation (r = –0.08) between milk fat cis9-18:1 and milk fat content and 2) ruminal cis9-18:1 was not significantly correlated with its percentage in milk fat (r = 0.20).

Data from this study suggest that feeding high-concentrate diets with linseed oil and sunflower oil may result in a more pronounced decrease in de novo fatty acid synthesis than fish oil, which would partly account for the observation that the 3 oils resulted in milk with low fat content. Thus, a decrease in synthesis of 8:0–16:0 (resulting in their lower percentage in milk fat with both LO and SO, Table 7) could be a more important cause of milk fat depression with either 18:3-(Loor et al., 2005a) or 18:2-rich oil supplementation, than with FO supplementation.

Trans10-18:1, Cis11-18:1, Nonconjugated Isomers, and Milk Fat Depression
We previously compiled literature data confirming (Griinari et al., 1998; Bauman and Griinari, 2003) that increased percentage of trans10-18:1 in milk fat is positively correlated with milk fat depression in cows fed high-concentrate diets with or without unsaturated oils, or mixed diets with various levels of fish oil (Loor et al., 2005a). In the present study, we found that FO and SO compared with LO, resulted in 2.7-fold increase in trans10-18:1 in rumen, a 3-fold increase in plasma, and a 4-fold increase in milk. Despite the similar low level of milk fat content and yield among the 3 diets, ruminal trans10-18:1 tended (P = 0.12) to be negatively correlated with milk fat content. Our data indicate that low milk fat content when feeding FO is associated with appreciable increases in cis11-18:1 in ruminal fluid (2.1-fold), plasma (2.1-fold), and milk fat (2- to 3-fold). We had previously observed a 2-fold increase in cis11-18:1 percentage in milk fat along with a 28% reduction in milk fat yield in cows receiving a 300-mL infusion of fish oil into the rumen compared with controls (Loor et al., 2005b). Data from Offer et al. (1999), Donovan et al. (2000), Whitlock et al. (2002), and Shingfield et al. (2003) provide other evidence for a link between cis11-18:1 and FO-induced milk fat depression. Dietary FO resulted in a 7-fold increase in percentage of trans9,cis11-18:2 in rumen (Table 3), suggesting this fatty acid may be a ruminal precursor of cis11-18:1. Interestingly, concentration of trans9,cis11-18:2 in plasma was 12-fold greater with FO (Table 5) compared with LO or SO, but its concentration in milk fat did not differ (Table 8). The relatively high concentrations of trans11,cis15-18:2 in rumen, plasma, and milk with FO could also contribute to low milk fat yield and content (see Loor et al., 2005a).

Ruminal percentage of trans9,trans12-18:2 and trans9,cis12-18:2 were negatively correlated with milk fat content, which agrees with our previous study with LO supplementation (Loor et al., 2005a). Although their concentrations in plasma and percentages in milk fat did not differ due to oils in the present study, both isomers increased in ruminal fluid with LO (Table 2). The negative relationship between rumen percentage and milk fat content (Figure 1D) for trans9,trans12-18:2 (r = –0.69; P = 0.05) suggests that this isomer might be a component of the mechanisms whereby 18:3- rich oils cause milk fat depression. Responses to feeding LO were marked also for percentages of several other 18:3n-3 biohydrogenation intermediates: trans13+14-18:1, trans15-18:1, trans16-18:1, cis15-18:1, cis9,-trans12-18:2, and trans11,cis15-18:2 in rumen, plasma, and milk fat, and cis9,trans13-18:2 in plasma and milk (likely arising from desaturation of trans13-18:1). In a previous study, milk fat cis15-18:1, cis9,trans12-18:2, cis9,trans13-18:2, and trans11,cis15-18:2 were negatively correlated with milk fat 4:0-16:0 percentage (Loor et al., 2005a), and are other candidates to explain the decrease in de novo lipogenesis with LO.

Although abomasal infusions of trans10,cis12-18:2 or dietary rumen-inert mixtures of CLA have caused milk fat depression (Baumgard et al., 2000; Loor and Herbein, 2003; Perfield et al., 2004), feeding studies are inconclusive on the putative role of nondietary, endogenously synthesized trans10,cis12-18:2. Increases in milk fat trans10,cis12-18:2 in cows with diet-induced milk fat depression were very modest (less than 0.10 to 0.15% of total milk fatty acids; Piperova et al., 2000; Whitlock et al., 2002; Peterson et al., 2003; present study). Even with polyunsaturated fatty acid supplementation, results show discrepancies between trans10,cis12-18:2 and milk fat yield (AbuGhazaleh et al., 2003; Shingfield et al., 2003; Loor et al., 2005a,b) or even no correlation between milk trans10,cis12-18:2 and milk fat content (Precht et al., 2002; Bradford and Allen, 2004; Loor et al., 2005a; present study). It appears likely that other biohydrogenation intermediates in addition to trans10,cis12-18:2 are involved in diet-induced milk fat depression (Bauman and Griinari, 2003; Peterson et al., 2003; Loor et al., 2005a,b).

Biohydrogenation Intermediates with Practical Applications
Responses in the percentage and yield of certain fatty acids with potential applications in human health and energy sparing in the cow (Bauman and Griinari, 2003) differed according to polyunsaturated fatty acid supply in the diet. Although we did not include a control diet for comparison, all oils resulted in high trans11-18:1 and cis9,trans11-18:2 in milk fat, which are far above typical values in milk from cows receiving nonlipid-supplemented diets. Comparison of our measurements of duodenal flow of trans11-18:1 and cis9,trans11-18:2 (Loor et al., 2004b; 2005c) with their yield in milk fat across diets confirmed (Piperova et al., 2002; Shingfield et al., 2003) that endogenous synthesis is the primary source of cis9,trans11-18:2. Even if all of the duodenal cis9,trans11-18:2 was secreted in milk fat, we observed that when feeding FO, LO, or SO, at least 97, 90, or 82%, respectively, of milk cis9,trans11-18:2 was synthesized via trans11-18:1 desaturation. These estimates are lower than previous ones (Piperova et al., 2002) and suggest for the first time that endogenous synthesis of cis9,trans11-18:2 may decrease as duodenal flow of this isomer increases. More importantly, we present evidence that supplementation of a high-concentrate diet with fish oil providing <25% EPA and DHA relative to linoleic acid from sunflower oil was effective in enhancing flow of trans11-18:1 to maintain synthesis of cis9,-trans11-18:1 in the mammary gland. This result confirms in vitro data showing similar accumulation of trans-18:1 due to DHA compared with soybean oil at 6 times the level of DHA supplementation (AbuGhazaleh and Jenkins, 2004).

Previous studies have observed increases in milk trans10-18:1 with fish oil supplementation to TMR (Donovan et al., 2000; Whitlock et al., 2002; Shingfield et al., 2003) or fish oil infusion into the rumen in cows fed corn silage diets (Loor et al., 2005b). Our data suggest that it is feasible to enhance ruminal outflow of trans11-18:1 and simultaneously trans10-18:1, cis11-18:1, or trans11,cis15-18:2 by supplementing fish oil at a fraction of linoleic acid-rich oils when fed in combination with high-concentrate diets. The net result is greater trans11-18:1 and cis9,trans11-18:2 in milk and potentially more trans10-18:1, cis11-18:1, or trans11,cis15-18:2 to reduce milk fat synthesis. This could be an effective strategy to reduce milk fat output and spare energy, which may be useful in certain situations in high-yielding dairy cows (Bauman and Griinari, 2003), while enhancing concentration of the anticarcinogenic cis9,trans11-18:2 (Corl et al., 2003). However, there is a need to evaluate how the increase in several other trans isomers of 18:1, 18:2, or 18:3 (Tables 8 and 9) could positively or negatively affect the nutritional value of milk fat.

In summary, milk fat content did not differ among oil treatments but was clearly lower when compared with feeding high- or low-concentrate diets alone (Loor et al., 2005a). Trans10,cis12-18:2 increased in rumen or milk fat with sunflower oil, but percentage and yield of trans10-18:1 was similar among fish oil and sunflower oil. Only feeding SO led to changes typical for classical diet-induced milk fat depression, i.e., increased trans10-18:1 and trans10,cis12-18:2 and decreased de novo fatty acid synthesis. Fish oil resulted in a marked increase in percentage of cis11-18:1 in rumen, plasma, and milk fat. The effects of FO, compared with LO and SO, on milk fat were not mediated via inhibition of de novo fatty acid synthesis. Results with LO confirmed previous data (Loor et al., 2005a) for higher trans13+14-18:1, trans15-18:1, trans16-18:1, cis15-18:1, cis9,trans12-18:2, cis9,trans13-18:2 and trans11,cis15-18:2. Although persistent, increases in cis11-18:1, trans11,cis15-18:2, and trans9,trans12-18:2 with both LO and FO occurred along with the increase in other isomers in ruminal fluid and milk fat, making it difficult to differentiate their respective roles in diet-induced milk fat depression. Taken together, our data indicates that milk fat depression typically observed with fish oil supplementation could be linked to reduced availability of stearic acid and a potential antilipogenic effect of trans10-18:1, cis11-18:1, or trans11,cis15-18:2. Simultaneous increases in ruminal trans11-18:1, with fish oil at a fraction of sunflower oil supplementation, may represent an effective strategy to maintain cis9,-trans11-18:2 synthesis in mammary while reducing milk fat output and sparing energy. Feeding studies are still inconclusive on the role of ruminally-derived trans10,cis12-18:2 and milk fat depression but highlight the possibility that other biohydrogenation intermediates may also play a role in this phenomenon.

	ACKNOWLEDGEMENTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGEMENTS REFERENCES

 
Authors acknowledge the assistance of J. P. Pezant and the team of Les Cèdres Experimental Unit for feeding, milking, blood sampling, and caring for cows, as well as M. Tourret, and C. Delavaud for help in laboratory analyses. This study was partly funded by the INRA Programme "Micronutrients in animal products, and their health value for consumers."

	FOOTNOTES

 
^* Current address: Department of Animal Sciences, University of Illinois, 206 ERML, Urbana, IL, 61801 (E-mail: jloor{at}uiuc.edu).

Current address: Graduate School of Agriculture, Hokkaido University, Sapporo, 060-8589, Japan (E-mail: ueko{at}anim.agr.hoku-dai.ac.jp).

Received for publication May 4, 2005. Accepted for publication July 14, 2005.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGEMENTS REFERENCES

 


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