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The fatty acid composition of muscle fat and subcutaneous adipose tissue of grazing heifers supplemented with plant oil-enriched concentrates¹

F. Noci^*,, P. French^*, F. J. Monahan and A. P. Moloney^*,2

^* Teagasc, Grange Beef Research Centre, Dunsany, Co. Meath, Ireland; and and School of Agriculture, Food Science and Veterinary Medicine, University College Dublin, Dublin 4, Ireland

	Abstract

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Our objective was to determine the effect of oil supplementation of pasture fed, beef cattle on the fatty acids, particularly CLA and PUFA, of muscle and s.c. adipose tissue. Forty-five Charolais crossbred heifers were blocked on BW and randomly assigned to 1 of 3 dietary regimens in a randomized complete block design (n = 15). The 3 treatments were: unsupplemented grazing (GO), restricted grazing plus a sunflower oil-enriched ration (SO), or restricted grazing plus a linseed oil-enriched ration (LO). Heifers were fed the experimental diets for approximately 158 d. Samples of LM muscle and s.c. adipose tissue were taken postmortem, the muscle fat was separated into neutral lipid and polar lipid (no separation was performed on the s.c. adipose tissue), and the fatty acid profile was determined by GLC. No effect of dietary treatment on carcass weight or total fatty acid concentration (mean 2,571 mg/100 g of muscle) in muscle fat was detected. Heifers offered SO had a greater (P < 0.001) proportion of CLA and C18:1trans-11 (1.90 and 9.35 vs. 1.35 and 6.89 g/100 g of fatty acids, respectively) in neutral lipid of muscle fat compared with those offered LO, which had a greater proportion of CLA and C18:1trans-11 than heifers offered GO (0.78 and 3.37 g/100 g of fatty acids, respectively). Similar effects were observed in the polar lipid and s.c. lipid. The PUFA:SFA ratio was greater in muscle fat and s.c. adipose tissue from supplemented heifers than in those offered GO (P < 0.001). Compared with LO, the PUFA:SFA ratio was greater (P < 0.05) in muscle fat of heifers offered SO, but there was no difference between SO and LO for this ratio in s.c. adipose tissue. The n-6:n-3 PUFA ratio was similar in muscle and s.c. adipose tissue for GO and LO, but it was greater (P < 0.05) for SO. It is concluded that supplementation of pasture-fed cattle with plant oil-enriched concentrates resulted in an increase in beef fat of some fatty acids considered to be of benefit to human health. Concentrates enriched with sunflower oil were more effective in increasing the CLA concentration, whereas linseed oil-enriched concentrates resulted in a more favorable n-6:n-3 PUFA ratio. The relevance to human health of the associated increase in C18:1trans-11 merits investigation.

Key Words: conjugated linoleic acid • grazing • plant oil • polyunsaturated fatty acid

	INTRODUCTION

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
There is increasing awareness of the importance of diet in the onset of human disease (Department of Health, 1994). Ruminant products are SFA-rich components of the human diet (Demeyer and Doreau, 1999), and consumption of SFA has been linked with coronary heart disease (Kromhout et al., 2002). However, ruminant fat is the main dietary source of CLA (Chin et al., 1992), which has a number of beneficial health effects (Ip et al., 1994; Belury, 1995; Pariza et al., 2001).

Ruminant products can also be a significant source of n-3 PUFA in the human diet (Scollan et al., 2001a) when the consumption of n-3 PUFA-rich foods such as fish is low. French et al. (2000) demonstrated that pasture finishing of cattle increased the concentration of CLA and n-3 PUFA in muscle compared with cereal-based concentrate finishing. Subsequently, Noci et al. (2005a) reported that this nutritional improvement in fatty acid composition was dependent on the duration of grazing. Conjugated linoleic acid is produced in the rumen by incomplete biohydrogenation of dietary C18:2n-6 but is also synthesized in adipose tissue and in the mammary gland by desaturation of C18:1trans-11 produced during ruminal biohydrogenation of C18:2n-6 and C18:3n-3 (Griinari et al., 2000).

The type of dietary lipid and the dietary forage to concentrate ratio have been shown to play a major role in determining the products of rumen metabolism (Harfoot and Hazelwood, 1988; Griinari et al., 1998) and to ultimately affect the fatty acid profile of meat or milk fat. Thus, inclusion of PUFA-rich plant oil or whole seeds in ruminant rations was shown in several studies (Scollan et al., 2001a; Mir et al., 2002; Noci et al., 2005b) to increase the concentration of CLA and PUFA in meat, despite the extensive biohydrogenation of dietary lipids within the rumen. Limited information is available on the efficacy of oil supplementation when the CLA and n-3 PUFA concentrations in tissue are already relatively high, as is the case in pasture-fed beef cattle (French et al., 2000; Engle and Spears, 2004; Noci et al., 2005a).

The objective of this study was to determine the effect of supplementation of grazing cattle with PUFA-rich plant oils on the fatty acid composition of intramuscular fat and s.c adipose tissue.

	MATERIALS AND METHODS

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
All procedures involving animals were carried out under license from the Irish Department of Health and Children and complied with all national regulations.

Forty-five Charolais crossbred heifers (mean initial BW 333 ± 39.90 kg) were blocked according to initial BW and, within block, randomly assigned to 1 of 3 dietary regimens (n = 15) in a randomized complete block design. The 3 groups grazed a predominantly perennial ryegrass (Lolium perenne) pasture for 158 d. Within each group, heifers were managed as 3 subgroups of 5 heifers each, formed according to BW block.

At the beginning of the experiment, heifers fed the unsupplemented pasture (GO) were assigned a daily pasture allowance of 8.8 kg of DM per animal. As the season progressed, pasture allowances were increased by increasing the land allocation. Heifers fed at pasture and supplemented with sunflower oil (SO, Trilby Trading, Drogheda, Ireland) or linseed oil (LO, Flood Horse Feeds, Newbridge, Ireland) received 1.6 kg daily of a concentrate that contained the test oil (Table 1) and grazed from smaller plots of similar botanical composition (daily pasture allowance 7 kg of DM per animal) to compensate for the energy supplied from the concentrates.

On the day of slaughter, the heifers were weighed without fasting, transported 120 km to a commercial abattoir (Meadow Meats, Rathdowney, Co. Laois, Ireland) and slaughtered within 4 h from removal from Grange Research Center. Carcass and perirenal fat weights were recorded postslaughter. Dressing percent was calculated as the weight of the cold carcass (hot carcass x 0.98) as a proportion of the preslaughter BW.

Postslaughter Measurements and Sampling Procedure
Carcasses were hung by the Achilles’ tendon and chilled for 24 h, after which the LM was excised from the right side of each carcass. Duplicate samples (25 mm in thickness) were cut from the region of the seventh rib of the LM, trimmed of all visible subcutaneous adipose tissue, vacuum-packed, and frozen at –30°C for fatty acid analysis. Samples of s.c. adipose tissue from the region of the ninth rib were vacuum-packed and stored at –20°C until analysis. The procedures for sample processing and for the fatty acid analysis of the neutral lipid (NL) and polar lipid (PL) fractions of muscle and total lipid (TL) in the s.c. adipose tissue were as described by Noci et al. (2005a). An index of fatty acid elongation was calculated as described by Pitchford et al. (2002):

Feed Chemical and Fatty Acid Analysis
Grass samples were semithawed, pooled within week, and bowl-chopped. One subsample was refrozen and freeze-dried before fatty acid analysis. A second sub-sample was stored frozen for general chemical analysis. Concentrate samples were pooled within month. One subsample was freeze-dried before fatty acid analysis, and a second was stored frozen for general chemical analysis. Feed DM concentration was measured as described by Moloney et al. (1996), Oil B (acid hydrolysis/ether extraction) as described in EC (1984) using an AOAC method (1990), and in vitro DM digestibility and ash concentration as described by O’Kiely et al. (1988). The fatty acid composition of the feedstuffs was determined using the procedure described by Sukhija and Palmquist (1988).

Statistical Analysis
Data were subjected to ANOVA according to a randomized complete block design. The model used had block and dietary treatment as main effects. Subsequent to a significant F-test, means were separated using the LSD procedure. Because the concentrates were offered individually, animal was considered the experimental unit in the statistical analysis. The statistical analyses of the data were performed using Genstat 5.0 (VSN Int. Ltd., Oxford, UK).

	RESULTS

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Feed Chemical Composition
The chemical composition of the feeds is summarized in Table 2. The fatty acid composition of the oils used in the experiment is shown in Table 3. Grass had a greater proportion of C16:0 and SFA than the concentrates. The LO concentrate had a similar proportion of C18:3n-3 to grass, but C18:3n-3 was almost absent from the SO concentrate. The concentrates had more C18:1cis-9, than the grass, and this accounted for the high proportion of MUFA in the concentrates. The total proportion of PUFA was influenced greatly by the proportion of C18:3n-3 for grass and the LO concentrate, and by the proportion of C18:2n-6 for the SO concentrate.

Neutral Lipids.
The proportions of fatty acids in the NL fraction of muscle fat are shown in Table 5. The SO cattle had a greater proportion of C18:1trans-9, C18:1trans-11, C18:2n-6, CLAcis-9,trans-11, C20:1, C20:2n-6, C20:3n-3, C20:3n-6, MUFA, and PUFA but a lower proportion of C14:0, C16:0, C17:0, C18:0, C18:1cis-11, C18:3n-3, C20:0, C22:5n-3, and total SFA than the GO cattle. The PUFA:SFA and the n-6:n-3 PUFA ratios were greater in SO cattle than in GO cattle. The LO cattle had a greater proportion of C18:1trans-9, C18:1trans-11, CLAcis-9,trans-11, C20:1, C20:2n-6, C20:3n-3, total PUFA, and PUFA:SFA ratio, but a lower proportion of C16:0, C17:0, C18:0, C20:0, total SFA, C22:5n-3, and C22:6n-3 than the GO cattle. The n-6:n-3 PUFA ratio did not differ between SO and LO cattle. The SO cattle had a greater proportion of C18:1trans-11, C18:2n-6, CLAcis-9,trans-11, total MUFA, and PUFA:SFA ratio, but a lower proportion of C14:0, C16:0, C17:0, C18:3n-3, C20:2n-6, and total SFA than the LO cattle.

	DISCUSSION

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In the present experiment, the management strategy imposed was successful in ensuring similar mean carcass weights and muscle fatty acid concentrations across the treatments as intended. Interpretation of the effects of diet on fatty acid composition is therefore not confounded by differences in carcass fatness (Leat, 1978). Heifers offered the oil-enriched supplements had greater fat intake (453, 453, and 224 g/d for SO, LO, and GO cattle, respectively). The difference in intake did not cause differences in total fatty acid concentration in muscle fat and s.c adipose tissue because total energy consumption was balanced by the greater intake of grass by the GO cattle.

Despite the high degree of biohydrogenation of dietary PUFA reported by Scollan et al. (2001b) and by Doreau and Ferlay (1994), supplementation with PUFA-rich rations in the present experiment resulted in a decrease in the SFA and an increase in the PUFA proportion in the muscle and in s.c. adipose tissue. This decrease in SFA suggests an increase in the incorporation of PUFA in muscle and s.c. adipose tissue at the expense of SFA, due to the different proportions of fatty acids in the unsupplemented and supplemented diets. An increase in the dietary supply of rapeseed oil had a similar effect of decreasing SFA and increasing the unsaturated fatty acids in milk from grazing dairy cows (Fearon et al., 2004). The decrease in SFA seems to be mainly due to a decrease in the proportion of palmitic acid. Cattle in the current study had a lower proportion of palmitic acid in muscle fat than that reported by Engle and Spears (2004), but just marginally lower than that reported by French et al. (2000) for grazing cattle. Oil supplementation decreased further the proportion of palmitic acid as a reflection of the greater level of PUFA in the diet. This represents a nutritional improvement compared with the tissues from animals fed exclusively on pasture. Mir et al. (2002) reported an increase in PUFA and a decrease in SFA such that the PUFA:SFA ratio in muscle increased from 0.052 to 0.072 when steers were supplemented with 6% sun-flower oil on a DM basis. Similarly, Madron et al. (2002) increased the PUFA:SFA ratio in muscle by feeding extruded soybeans, also rich in C18:2n-6. The PUFA:SFA ratio in muscle achieved in this experiment with the SO treatment (0.21), although lower than current recommendations (> 0.45; Department of Health, 1994) is among the greatest reported for cattle fed unprotected fat sources. Realini et al. (2004) and Duckett et al. (1993) similarly reported high PUFA:SFA ratios in pasture-fed cattle (0.2 in muscle containing 1.68% fat and 0.26 in muscle containing 2.09% fat, respectively). It is recognized that the fat concentration of muscle has a major influence on the PUFA:SFA ratio because PUFA are mainly found in the PL fraction, which is diluted by the growth in NL fraction as animals accrete lipid. Scollan et al. (2003) proposed a negative exponential relationship between the amount of intramuscular fat and the PUFA:SFA ratio. Comparisons of the PUFA:SFA ratio across studies should therefore be made with caution because lean animals will have a greater PUFA:SFA ratio irrespective of ration composition (Raes et al., 2003).

Compared with GO cattle, increasing the supply of C18:2n-6 through the SO concentrate was more effective in raising the total PUFA (predominantly through an increase in C18:2) and decreasing the SFA proportions of muscle and s.c. adipose tissue than increasing the supply of C18:3n-3 through the LO concentrate. Because the SO and LO concentrates supplied a similar quantity of PUFA (283 g of PUFA/d), this may reflect a lower level of biohydrogenation of C18:2n-6 because C18:2n-6 seems to have an inhibitory effect on its own biohydrogenation (Harfoot et al., 1973). Such an effect is not documented for C18:3n-3. In support of the above suggestion, Chow et al. (2004) demonstrated that in vitro biohydrogenation of C18:2 from sunflower oil or linseed oil averaged 75%, whereas biohydrogenation of C18:3 from linseed oil averaged 84% under the same conditions. The increase in the proportion of C18:2 in the PL fraction of fat from SO cattle indicates that a proportion escapes from the rumen. Mir et al. (2003) also found a significant increase in the total PUFA in muscle due to the increased incorporation of C18:2n-6 in the muscle for cattle supplemented with sunflower oil.

Increasing the incorporation of n-6 PUFA in muscle is an undesirable effect of supplementation with sunflower oil. In the current experiment, feeding grass contributed to a high daily intake of C18:3n-3, which, despite extensive ruminal biohydrogenation, maintained the n-6:n-3 PUFA ratio largely within the recommended values (< 2:1) for total muscle fat and s.c. adipose tissue (Department of Health, 1994). A similar effect of feeding animals at pasture was observed by Realini et al. (2004) and French et al. (2000). The lipid classes were separated to allow for the assessment of dietary effects on NL and PL because the incorporation of C18:3n-3 and PUFA in general is greater in PL than in NL. However, the 2-fold increase in the supply of C18:3n-3 by supplementation of linseed oil (221 vs. 106 g/d) did not increase the proportion of n-3 PUFA in PL suggesting greater biohydrogenation of the supplementary C18:3n-3 than that supplied from grass or saturation of the capacity of tissue to accrete n-3 PUFA. These findings indicate that supplementing grazing cattle with additional C18:3n-3 in an oil is not an effective strategy for increasing tissue concentrations. Because French et al. (2000) reported a linear increase in the proportion of C18:3 in the muscle fat as the level of grazed grass in the diet increased, the form of the additional C18:3n-3 seems to be important. Supplementation of grass with LO maintained to a large extent the fatty acid profile found in muscle and s.c. adipose tissue in GO cattle. The supply of C18:2n-6 in linseed oil (51 g/d) is the likely explanation for the increased proportion of this fatty acid in LO cattle compared with GO cattle. Despite the greater n-6:n-3 PUFA ratio in muscle and s.c. adipose tissue in LO cattle compared with GO cattle, both groups had an n-6:n-3 PUFA ratio lower than 2:1, within recommended values. A further difference between fat from GO and LO cattle was the greater proportion of long chain n-3 PUFA in PL in GO cattle. The 2 groups did not differ in the elongation index described by Pitchford et al. (2002), and this could suggest a negative influence of oil-feeding on ⁵ and ⁶ desaturase enzymes, which are involved in the conversion of C18 PUFA to their long chain derivatives.

Feeding LO, however, increased the n-3 PUFA proportion in muscle and s.c. adipose tissue compared with SO. This result was not surprising because a diet rich in C18:3n-3 (crushed or extruded linseed) was shown by Raes et al. (2004) to decrease the n-6:n-3 PUFA ratio from 6:1 to less than 4:1 in Belgian Blue bulls, and similar results were obtained by Scollan et al. (2001a) by feeding a 60:40 silage to concentrate ratio with a whole linseed-based ration. The n-6:n-3 PUFA ratio of beef is of relevance in its contribution to the whole diet of humans. Despite medical advice, n-6 PUFA consumption by humans is excessive resulting in dietary n-6:n-3 PUFA ratios of 7.2:1 and 7.4:1 for men and women in the UK, respectively (Gregory et al., 1990). It is recognized that whereas an increase in the concentration of n-3 PUFA was achieved in the current study, the absolute contribution of such beef to the recommended consumption of C18:3n-3 is small (1.6 g/d for men and 1.1 g/d for women; Institute of Medicine of the National Academies, 2002). Thus, 200 g of fresh muscle produced from grazing cattle supplemented with linseed oil in the current study would provide approximately 69 mg of C18:3n-3. A similar quantity of muscle from grazing cattle supplemented with sunflower oil would supply 160 mg of C18:2n-6, compared with a recommended intake of 17 and 12 g/d for men and women, respectively (Institute of Medicine of the National Academies, 2002).

The beneficial effect of forage feeding on the proportion of CLA in muscle has been demonstrated previously (French et al., 2000; Steen and Porter, 2003; Engle and Spears, 2004). Conjugated linoleic acid is produced in the rumen as an intermediate of biohydrogenation of C18:2n-6 (Harfoot and Hazelwood, 1988). Enriching concentrate rations with sunflower oil or other oil sources rich in C18:2n-6 has been shown to increase the concentration of CLAcis-9,trans-11 in muscle fat (Madron et al., 2002; Mir et al., 2002, 2003; Noci et al., 2005b). In dairy cows, the concentration of CLA in milk was increased from 1.4 to 1.9% when grazing animals were supplemented with increasing concentrations of rapeseed oil (Fearon et al., 2004). In addition, the concentrations of C18:1 trans fatty acids (as no further separation is reported) in the milk were also similar to the figures obtained in the current study for C18:1 trans-11 (6.78% when rapeseed oil was included at a level of 600 g/d per cow compared with 8.6 and 6.3% for grazing cattle supplemented with sunflower oil and linseed oil in this study, respectively). Engle and Spears (2004) also reported 6.1% for total C18:1 trans in muscle fat when feeding a high oil corn to finishing steers.

The large amount of C18:2n-6 supplied by the SO diet likely contributes to the increase in CLA noted in this study, i.e., incomplete biohydrogenation of dietary C18:2n-6 and accumulation of CLA in the rumen. The pathway of biohydrogenation of C18:2n-6 also includes C18:1trans-11 as an intermediate (Harfoot and Hazelwood, 1988). Because Griinari et al. (2000) demonstrated that most of the CLA found in milk fat is the result of desaturation of C18:1trans-11 by the action of ⁹-desaturase, an enzyme that has high activity also in adipose tissue (St John et al., 1991), the increase in tissue CLA concentration likely reflects an increase in C18:1trans-11 accumulation in the rumen and subsequent tissue desaturation.

An increase in muscle CLA due to consumption of a supplement rich in C18:3n-3 was reported by Enser et al. (1999), Stasiniewicz et al. (2000), and Raes et al. (2003). This was also seen in the current study despite a high consumption of C18:3n-3 from grazed grass. This observation supports the role of C18:1 trans-11 in tissue synthesis of CLA because CLA is not produced during ruminal biohydrogenation of C18:3n-3 (Lock and Garnsworthy, 2002). The combination of C18:2n-6 and C18:3n-3 in the SO diet could explain the greater concentration of CLA found in muscle and SAT compared with LO, as a contribution of both pathways. However, the relative amounts of C18:3n-3 and C18:2n-6 consumed would seem to be important because LO cattle also consumed C18:2n-6. Clarification of the dietary ratio of C18:3n-3 to C18:2n-6 to maximize CLA synthesis merits attention. The presence of high concentrations of C18:2n-6 in the rumen has been reported to inhibit the biohydrogenation of C18:2n-6, possibly affecting the hydrogenation step from C18:1trans-11 to C18:0 (Harfoot et al., 1973). Dietary inclusion of PUFA at the level used in this experiment did not seem to negatively affect the activity of ⁹–desaturase using the activity index reported by Malau-Aduli et al. (1997), consistent with Beaulieu et al. (2002), who observed no inhibitory effect of 5% soybean oil addition on the activity of this enzyme.

In view of the myriad beneficial effects of CLA in animal models of human disease, the increase in muscle CLA concentration due to oil supplementation can be considered a positive finding. Thus, 200 g of fresh muscle produced from grazing cattle supplemented with sunflower oil in the current study would provide approximately 90 mg of CLA, which is greater than the dose of CLA indicated by Knekt et al. (1996) to reduce the incidence of breast cancer in women. Moreover, because C18:1trans-11 is also converted to CLA in humans (Kuhnt et al., 2006), the increase in the concentration of this fatty acid would augment the CLA potential of beef from oil-supplemented cattle. However, the findings from studies of the efficacy of CLA in human studies have to date been equivocal (reviewed by Yaqoob et al., 2006). This may reflect the use of mixtures of CLA isomers (e.g., CLA trans-10, cis-12 appears to have a negative effect on blood lipids, whereas the cis-9, trans-11 isomer does not), the short duration of many of the reported studies, and insufficient statistical power to detect differences of biological significance. Considerably more information is required in this regard.

Epidemiological associations between the risk of coronary heart disease and the consumption of trans PUFA have been reported. The trans fatty acid profile in ruminant fat tends be enriched with the trans-11 isomer of C18:1 as was seen in the current study. Industrially derived oils such as hydrogenated vegetable oils, in contrast, have a broader spectrum of C18:1 trans isomers and have a considerably greater concentration of C18:1 trans-9 and trans-10 isomers (Scollan et al., 2006). The relative risk to human health of consuming the individual isomers remains to be elucidated, but current epidemiological evidence suggests that consumption of ruminally derived trans PUFA is not a risk factor for heart disease (Jakobsen, 2006).

In conclusion, this study demonstrated that supplementation of grazing heifers with plant oils rich in poly-unsaturated fatty acids led to a substantial increase in conjugated linoleic acid compared with grazing alone. The high concentration of C18:1trans-11 observed in beef from the supplemented cattle enhances the value of this beef still further because C18:1trans-11 can be converted to conjugated linoleic acid in human tissue. The choice of plant oils employed will depend on the desired outcome, i.e., maintenance of the n-6:n-3 poly-unsaturated fatty acid ratio or maximizing the concentration of conjugated linoleic acid. The influence of pre-grazing management and level and blend of supplementary oils on muscle conjugated linoleic acid and C18:1trans-11 remains to be identified. In general, these changes can be considered beneficial to the health of beef consumers, but the implications of the increase in trans fatty acids per se remain to be elucidated.

	Footnotes

 
¹ This research was supported through the Food Institutional Research Measure of the Department of Agriculture and Food of Ireland. The technical assistance of V. McHugh, A. McArthur, and N. Blount, and the cooperation of Meadow Meats, Rathdowney, Ireland are gratefully acknowledged.

² Corresponding author: aidan.moloney{at}teagasc.ie

Received for publication February 24, 2006. Accepted for publication November 6, 2006.

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