Dietary CLA supplementation and gender modify fatty acid composition of subcutaneous and intramuscular fat in Iberian × Duroc finishing heavy pigs

Introduction

There is a growing interest in the production of high quality dry-cured meat products for niche market, sometimes associated to the production of autochthonous pig breeds. This requires the production of fat pigs slaughtered at heavy weight (140-160 kg), where fat content and composition play a determinant role in the nutritive, sensory and technological quality of the meat (López Bote, 1998).

Fat pigs accumulate large amount of subcutaneous lipids and are extremely inefficient (Serrano et al., 2008). However, any strategy aimed to increase feed efficiency (reducing slaughter weight, crossing with lean pigs, etc.) has proven to reduce also intramuscular fat (IMF), and therefore have been discarded because of its negative effect on dry cured meat products (Carrapiso et al., 2003; Ventanas et al., 2007).

Several studies have reported that dietary CLA administration to growing-finishing pigs reduces subcutaneous fat and improves feed efficiency, without any effect on IMF concentration (Dugan et al., 1997; Joo et al., 2002). This has raised an interest in feeding CLA to pigs (Ostrowska et al., 1999; Thiel-Cooper et al., 2001).

Moreover, CLA has also beneficial effects such as anti-carcinogenic (Pariza and Hargraves, 1985), antiatherosclerothic (Lee et al., 1994) and enhance of immune system (Haro et al., 2006), and therefore dietary CLA administration to pigs has proven to be an effective way to concentrate active isomer in pig tissues, thus producing additional benefits to consumers (Pariza et al., 2001; Belury, 2002).

At the moment, most studies with CLA feeding to pigs have been performed on lean (Ostrowska et al., 1999; Thiel-Cooper et al., 2001; Martín et al., 2007; Larsen et al., 2009) or moderately fat pigs (Eggert et al., 2001; Fernández-Figares et al., 2008; Cordero et al., 2010) but there is lack of information of the possible effect of CLA on heavy fat pigs.

The objective of this research was to evaluate the effect of dietary CLA enrichment for finishing heavy fat pigs (castrated males and castrated females) on performance, carcass traits, backfat thickness, IMF content, and fatty acid composition including the concentration of CLA isomers in subcutaneous and IMF.

Material and methods

Animals and diets

The routine of animal care and experiment procedures used in this experiment was approved by the University Complutense of Madrid. Forty Iberian × Duroc pigs, half castrated males (CM) and half castrated females (CF), with 120.0 (± 2.83) kg live weight were used. The pigs were randomly distributed and located in eight pens of five animals (CM or CF). Pigs were fed with their respective experimental diets which were formulated to contain either no CLA (0%) or 1% CLA supplementation. The CLA oil supplementation (CLA-60, LoderstarTM, Belgium), contained approximately 60% of CLA isomers (30% c9,t11 and 30% t10,c12) (Table 1).

All the diets were formulated to provide the same protein and energy levels. Diets were provided ad libitum for 42 days. Before the beginning of the experiment all pigs were subjected to the same feeding and management. Pigs were slaughtered at a local slaughterhouse at 153.2 (± 3.0) kg live weight.

Ingredients, chemical composition and main fatty acids of experimental diets are shown in Table 1. Determination of the compositional analysis of feeds (in triplicate) was carried out according to AOAC (2005). Fatty acids of diets were extracted and quantified by the one-step procedure of Sukhija and Palmquist (1988) from lyophilised samples. Pentadecanoic acid (C15:0) (Sigma-Aldrich, Madrid, Spain) was used as internal standard. Fatty acid methyl esters were analysed by gas chromatography using a Hewlett Packard HP-6890 (Avondale, PA, USA) gas chromatograph equipped with a flame ionization detector and a capillary column (HP-Innowax, 100 m × 0.25 mm i.d. and 0.2 μm polyethylene glycol-film thickness) (Kramer et al., 1998).

Sample collection and chemical analysis

At slaughter, carcass weight, carcass yield, hams, forelegs, loins, chops and bacon weight were recorded. Samples of subcutaneous fat and Longissimus dorsi at the level of the last rib were taken, weighed, vacuumpacked in low-oxygen permeable film, and kept frozen at –20°C for fatty acid analysis. Liver samples (approximately 30 g) were collected at slaughter, placed in liquid nitrogen and stored at –80°C for enzyme activity analyses.

Lipids from subcutaneous backfat and Longissimus dorsi muscle were extracted with a mixture of choloroform/ methanol (2:1 v/v) following the procedure described by Folch et al. (1957). Lipid extracts were methylated in the presence of sodium methoxide (Christie, 1982) and analyzed as described for dietary fatty acids.

Table 1. Ingredients, chemical composition and analyzed fatty acid composition of experimental diets

Dietary CLA supplementation and gender modify fatty acid composition of subcutaneous and intramuscular fat in Iberian × Duroc finishing heavy pigs - Image 1

Enzyme assays

Malic enzyme (ME) and glucose 6-phosphate dehydrogenase (G6PD) activities were determined in liver extracts as described by Álvarez et al. (2000). Weighed quantities of liver tissue sample were homogenised with an Ultra turrax T18basic (IKAw; Labortechnik, Staufen, Germany) in 3 vol. ice-cold buffer [20 mM Tris-HCl, 0.25 M sucrose, 2 mM ethylenediamine-tetraacetate EDTA], pH 7.4 and spun at 20,000 × g for 40 min. G6PD and ME activities were determined using continuous spectrophotometric assays following the NADPH formation at 340 nm. The soluble liver protein concentration was determined by the method of Bradford (1976) using bovine serum albumin as standard. All enzyme assays were conducted at 37°C. Care was taken to ensure that initial rates were being measured in all assays. Control experiments established that the enzyme was stable in the buffer used during the time period of the assay and at the temperature required (Álvarez et al., 1998). All enzyme assays were carried out in triplicate. The enzymatic activity units (IU) defined as μmol substrate converted to product per min at the assay temperature, were expressed per mg liver soluble protein (specific activity). All chemicals were supplied by Amersham Biosciences (Upsala, Sweden) and by Sigma-Aldrich (Madrid, Spain).

Statistical analysis

Data were analyzed using the GLM procedure of SAS (SAS, 2002). The effects of dietary CLA level, gender and the interaction CLA × Gender were studied. The experimental unit was the pen all the analyses done. The data are presented as the means and the standard errors of means. The initial weight was used as covariate for productive variables and the carcass weight (CW) for carcass characteristics. When the covariates were not significant (p > 0.05) they were removed from the model. All differences were considered significant at p < 0.05 and p values between 0.05 and 0.10 were considered a trend.

Results

Dietary CLA and gender had no influence (p > 0.05) on average daily gain (ADG), average daily feed intake (ADFI) and feed conversion efficiency (FCE). The ADG, ADFI and FCE values were 785.7 g, 3.59 kg and 4.57 kg kg^–1 and 795.2 g, 3.42 kg and 4.31 kg kg^–1 for treatment 0% CLA and 1% CLA respectively. The ADG, ADFI an FCE values were, 809.5 g, 3.61 kg and 4.46 kg kg^–1 for CM and 771.4 g, 3.40 kg and 4.41 kg kg^–1 for CF.

Dietary CLA had no effect on carcass characteristics (Table 2). Dietary CLA increased the sum of hams, forelegs and loins weights (H + F + L) (p = 0.10). The forelegs weight (FW) were higher (p = 0.03) in CM than in CF, while the bacon weight (BW) tended (p = 0.05) to be higher in CF than in CM. The interaction dietary CLA × gender showed not effect (p > 0.05) for all carcass traits studied.

The influence of dietary CLA and gender on fatty acid profile of subcutaneous backfat is presented in Table 3. Dietary CLA increased the C12:0, C14:0, C16:0, C18:0 fatty acid proportion. SFA increased from 40.02% to 44.48% in the pigs fed the diet containing 1% CLA when compared to those receiving the control diet. Concentration of C18:1 n-9, MUFA, C18:2 n-6 and PUFA fatty acids decreased in pigs fed the CLA diet. The CLA enhanced the proportion of the c9,t11- CLA and t10,c12-CLA isomers in subcutaneous backfat and reduced the MUFA/SFA, C18:1 n-9/C18:0 and C16:1 n-7/C16:0 ratios.

The subcutaneous backfat from CM had higher C18:0, SFA (p < 0.05) and lower, MUFA and MUFA/SFA and C18:1 n-9/C18:0 ratios than those from the CF, whereas C16:0 (p = 0.07) and C18:1 n-9 (p = 0.06) proportions tended to be higher and lower respectively in CM than in CF.

According to results showed in Table 4, in the IMF from Longissimus dorsi, dietary CLA inclusion increased C12:0, C14:0, C16:0, C16:1 n-7, SFA, c9,t11-CLA and t10,c12-CLA proportions, also reduced C18:1 n-9 and MUFA proportions and MUFA/SFA and C18:1 n-9/ C18:0 ratios, and produced no effect on C18:2 n-6 and PUFA concentration.

The gender had not effect (p > 0.05) on fatty acids profile of IMF (Table 4). However, CM tended (p < 0.10) to have higher C18:0, SFA and c9,t11-CLA proportions and lower MUFA proportion and MUFA/SFA and C18:1 n-9/C18:0 ratios than CF. The interaction dietary CLA level × gender showed not effect for all fatty acid proportion determined.

Dietary CLA, gender and interaction CLA × gender showed not effect on G6PD and ME activity in the liver (Table 5).

Table 2. Effect of dietary CLA and gender on carcass traits¹

Discussion

The productive results observed in the present experiment are in agreement with data from Serrano et al. (2008) obtained in Duroc × Iberian pigs. No effect of dietary CLA administration was observed on ADG, ADFI or FCE, according to Ramsay et al. (2001) and Tischendorf et al. (2002). However, some other experiments reported a beneficial effect of CLA on ADG (Thiel-Cooper et al., 2001), ADFI (Cook et al., 1998) and FCE in finishing pigs (Ostrowska et al., 1999). According to Dugan et al. (2001) heterogeneous response of pig to dietary CLA administration could be explained by the different dietary concentration of CLA isomers, the content of other dietary constituents, duration of the feeding trial and pig genotype.

Table 3. Fatty acids profile (%) of subcutaneous backfat according to CLA levels in the diet and gender

Table 4. Fatty acids profile (%) of intramuscular fat from Longissimus dorsi according to CLA levels in the diet and gender

Table 5. Influence of dietary CLA and gender on G6PD and ME activity (IU mg^-1 soluble protein) in the liver

No effect of dietary CLA administration was observed on carcass characteristics (Table 2), which is in disagreement with most previous research in which CLA was included at similar dietary concentration. Thiel- Cooper et al. (1998) in lean pigs fed a diet with 1% of CLA reported a 10% reduction in subcutaneous backfat. Ostrowska et al. (2003) reported a 23% reduction in subcutaneous backfat and an increase 10% of firmness (with 1% of CLA). Wiegand et al. (2001) with a dose of approximately 1% reported a reduction in subcutaneous backfat and an increase in marbling and firmness. Dunshea et al. (1998) and Ostrowska et al. (1999) reported up to a 25 to 30% reduction in subcutaneous backfat thickness at higher dietary CLA concentration. However, in other experiments no effect of 1% dietary CLA administration was observed on fat accumulation (Eggert et al., 2001). In this experiment, pigs were slaughtered at higher weights (> 120 kg), thus suggesting that the effectiveness of 1% dietary CLA is lower in fatty pigs than in lean genotypes. According to Ramsay et al. (2001), the influence of CLA on carcass fat and lean deposition seems to be affected by pig phase of growth and genotype, being different in lean than obese or fat pigs. In our experiment the CLA level in the diet (1%) did not affect subcutaneous fat deposition although tended to increase the combined weight of H + F + L.

In the present experiment a 1% dietary CLA inclusion did not affect IMF concentration (Table 2). This is contradictory to some previous research in which IMF increased due to dietary CLA administration with a slaughter weight around 105 kg (Joo et al., 2002; Wiegand et al., 2001) and 113 kg (Averette et al., 2002). However, no effect of 1% dietary CLA on IMF concentration have been found in pigs slaughtered at heavy weights (> 120 kg) (Eggert et al., 2001).

The effects of dietary CLA administration on subcutaneous and intramuscular fatty acids are shown in Table 3 and 4 respectively. Dietary CLA increased c9,t11-CLA and t10,c12-CLA, which agrees with the findings of Kramer et al. (1998), Eggert et al. (2001) and Martín et al. (2007). On the other hand, the accumulation of c9,t11-CLA was higher than t10,c12-CLA in subcutaneous and intramuscular fat. This result is also in agreement with data from Eggert et al. (2001), Smith et al. (2002) and Martín et al. (2007) and might suggest that the t10,c12 isomer would be either less efficiently incorporated or have a different metabolism. On the other hand, Larsen et al. (2009) found in lean pigs 1.19 and 0.93% of c9,t11-CLA and t10,c12-CLA respectively in bacon from pigs fed 1% CLA enriched diets for 56 days. However, it is noticeable that despite similar experimental conditions in our study the concentration of both isomers were considerably lower (0.33 and 0.18% for c9,t11-CLA and t10,c12-CLA respectively), than in the study of Larsen et al. (2009). The main distinctive characteristic between these two experiments is the marked adipogenicity of Iberian pigs (López-Bote, 1998).

A possible explanation of the effect of dietary CLA on CLA isomers accumulation observed in our experiment could be the higher carcass fat concentration in Iberian × Duroc heavy pigs compared to lean genotypes. It has been recently reported lack of fat mobilization induced by CLA in obese humans (Desroches et al., 2005), while a similar dose was shown to reduce body fatness either in healthy exercising subjects (Thom et al., 2001) or in postmenopausal healthy women (Raff et al., 2009).

Previous experiments have shown an increase in C14:0, C16:0, C18:0, SFA proportion and a reduction of C18:1 n-9 proportion and MUFA fatty acids in subcutaneous backfat due to dietary CLA inclusion (Bee, 2001; Eggert et al., 2001; Ramsay et al., 2001; Averette et al., 2002; Smith et al., 2002; Martín et al., 2007). In the present study subcutaneous backfat SFA increased from 40.0% to 44.5% during 42 days of experiment, but Larsen et al. (2009) found that SFA increased from 38.5% to 46% during 56 days, thus suggesting that in finishing heavy pigs 42 days supplementation with CLA is enough to modify backfat SFA. The lowering PUFA concentration of CLA in subcutaneous fat is also a subject of potential interest, since there is a negative relationship between PUFA content and fat consistency. An excessive accumulation of PUFA in pig fat leads to an extension of the processing time of carcass joints due to a low fat consistency and therefore, impaired water migration (López-Bote, 1998), which is of importance in production of quality meat products.

The Δ-9 desaturase index, assessed by the MUFA/SFA, C18:1 n-9/C18:0 and C16:1 n-7/C16:0, decreased with the CLA administration, which is in agreement with previous research in which stearoyl coenzyme A desaturase activity in porcine subcutaneous adipose tissue was either measured (Smith et al., 2002) or estimated (Eggert et al., 2001; Martín et al., 2007). According to Park et al. (2000), the t10,c12-CLA isomer of CLA has an inhibitory effect on Δ-9 desaturase activity compared to others CLA isomers, whereas c9,t11-CLA isomer seems to have not effect on fatty acid desaturation.

Regarding the influence of gender on fatty acid composition of subcutaneous backfat, Pieszka et al. (2006) observed that in gilts fed with CLA diets, the SFA content in fat was lower and the content of PUFA, MUFA and PUFA/SFA ratio were higher than in barrows. In agreement with our results, some other experiments also reported that subcutaneous backfat from males is more saturated than that from females (Courboulay and Massabie, 1996; Smithard et al., 1980), although, Serrano et al. (2008) did not observe influence of gender on fatty acid composition of subcutaneous backfat from Duroc × Iberian heavy pigs.

In IMF from Longissimus dorsi, Bee (2001), in pigs slaughtered at 105.2 kg live weight, and Cordero et al. (2010), in heavy pigs, observed an increase of C16:0 and SFA and a reduction of C18:1 n-9 and MUFA with dietary CLA, whereas on C18:2 n-6, no effect was observed. Eggert et al. (2001) also observed that dietary CLA increased C16:0 and SFA concentration, but did not find effect on C18:2 n-6 and PUFA. It is interesting to note that increase in the C16:0 proportion in IMF is similar or a little bit lower than in subcutaneous backfat. The decreasing effect of CLA on C18:1 n-9, MUFA proportions and Δ-9 desaturase index was of lower magnitude in IMF than in subcutaneous backfat, which suggests lower regulatory effect of Δ-9 desaturase in muscle tissue. In the current experiment this result was likely due to the lower t10,c12-CLA isomer concentration in IMF than in subcutaneous fat.

In this study the relationships found between IMF percentage and C14:0, C16:0 and C18:2 n-6 proportions in IMF is of interest. Previous works have also shown a relationship between SFA and intramuscular fat content (Bee, 2001; Eggert et al., 2001; Ramsay et al., 2001), thus suggesting an indirect mechanism of dietary CLA on IMF content. Future research is needed in this topic.

G6PD and ME are the main enzymes involved in supplying NADPH for the reductive biosynthesis of fatty acids (Mourot et al., 1995), but they also contribute to metabolic pathways other than lipogenesis. Both enzyme activities were unaltered by dietary CLA supplementation. The G6PD activity was higher than ME and seemed to be the main producer of NADPH. This observation agrees with comparative studies in newborns (Le Dividich et al., 1994) and growing pigs (Mourot et al., 1995). Bee (2001) did not observe effect of dietary CLA on malic and fatty acid synthesis enzymes activity in subcutaneous backfat and omental fat in finishing pigs, but the same author (Bee, 2000) found a positive effect of CLA on G6PD and ME activity in piglet adipose tissues. According to Daza et al. (2007), the effect of these enzymes is more marked in young animals or when animals receive different levels of energy in the diet. We did not find any effect of CLA on lipogenic enzymes and this confirm that the increase of SFA and the decrease of MUFA concentrations in both tissues studied is not due to an increased synthesis, but more likely to a down-regulation of Δ-9 desaturase activity. This is in agreement with Bretillon et al. (1999) who reported that both the activity and gene expression of hepatic Δ9-desaturase were inhibited by dietary CLA.

It is concluded that a 1% dietary CLA inclusion produced no effect on carcass traits and IMF content, but affects fatty acids profile of subcutaneous backfat and IMF. Levels of CLA isomers in subcutaneous and IMF were lower than in lean pigs, which suggest that a higher dietary CLA supplementation is needed to achieve the same levels in tissues for obese heavy pig genotypes, thus indicating that a correct combination of CLA concentration and administration time should be defined in each productive circumstance according to pig fatness.

The accumulation of the CLA isomers was lower in heavy pigs compared to the lean genotype probably as a consequence of the higher carcass fat concentration in the former. More studies are needed to understand the underlying mechanism of CLA level in the diet on carcass and fat quality in obese pigs.

Acknowledgements

This research was funding and supported by MOLIMEN S.L (Sant Feliu de Llobregat), AGL 2007-63655 and ADE (04/05/SG/0008) from the Regional Government in Castilla-León, and the PROFIT program (FIT- 060000-2006-43 and FIT-060000-2007-154) from the Spanish Government.

References

ÁLVAREZ M.J., LÓPEZ-BOTE C.J., DÍEZ A., CORRAZE G., ARZEL J., DIAS J., KAUSHIK S.J., BAUTISTA J.M., 1998. Dietary fish oil and digestible protein modify susceptibility to lipid peroxidation in the muscle of rainbow trout (Oncorhynchus mykiss) and sea bass (Dicentrarchus labrax). Brit J Nutr 80, 281-289.

ÁLVAREZ M.J., DÍEZ A., LÓPEZ-BOTE C.J., GALLEGO M., BAUTISTA J.M., 2000. Short-term modulation of lipogenesis by macronutrients in rainbow trout (Oncorhynchus mykiss) hepatocytes. Brit J Nutr 84, 619-628. AOAC, 2005. Official Methods of Analysis, 18th ed. Arlington, VA, USA.

AVERETTE GATLIN L., SEE M.T., LARICK D.K., LIN X., ODLE J., 2002. Conjugated linoleic acid in combination with supplemental dietary fat alters pork fat quality. J Nutr 132, 3105-3112.

BEE G., 2000. Dietary conjugated linoleic acid consumption during pregnancy and lactation influences growth and tissue composition in weaned pigs. J Nutr 130, 2981-2989.

BEE G., 2001. Dietary conjugated linoleic acids affect tissue lipid composition but not de novo lipogenesis in finishing pigs. Anim Res 50, 383-399. BELURY M.A., 2002. Dietary conjugated linoleic acid in health: physiological effects and mechanisms of action. Annu Rev Nutr 22, 505-531.

BRADFORD M.M., 1976. Rapid and sensitive method for quantitation of microgram quantities of protein utilizing principle of protein-dye binding. Anal Biochem 72, 248-254.

BRETILLON L., CHARDIGNY J.M., GREGIORE S., BERDEAUX O., SEBEDIO J.L., 1999. Effects of conjugated linoleic acid isomers on the hepatic microsomal desaturation activities in vitro. Lipids 34, 965-969.

CARRAPISO A.I., BONILLA F., GARCÍA C., 2003. Effect of crossbreeding and rearing system on sensory characteristics of Iberian ham. Meat Sci 65, 623-629.

CHRISTIE W.W., 1982. Lipid analysis, 2nd ed. Pergamon Press, Oxford, UK. pp. 53-54.

COOK M.E., JEROME D.L., CRENSHAW T.D., BUEGE D.R., PARIZA M.W., ALBRIGHT K.J., SCHMIDT S.P., SCIMERA J.A., LOFGREN P.A., HENTGES E.J., 1998. Feeding conjugated linoleic acid improves feed efficiency and reduces carcass fat in pigs. Faseb J 12, A836.

CORDERO G., ISABEL B., MENOYO D., DAZA A., MORALES J., PIÑEIRO C., LÓPEZ-BOTE C.J., 2010. Dietary CLA alters intramuscular fat and fatty acid composition of pig skeletal muscle and subcutaneous backfat. Meat Sci 85, 235-239.

COURBOULAY V., MASSABIE P., 1996. Répercussion de la durée d'utilisation d'un aliment riche en acide linoléique sur la qualité des gras du porc. Journées de la Recherche Porcine en France 28, 157-162. [In French].

DAZA A., REY A.I., MENOYO D., BAUTISTA J.M., OLIVARES A., LÓPEZ-BOTE C.J., 2007. Effect of level of feed restriction during growth and/or fattening on fatty acid composition and lipogenic enzyme activity in heavy pigs. Anim Feed Sci Tech 138, 61-74.

DESROCHES S., CHOUINARD P.Y., GALIBOIS I., CORNEAU L., DELISLE J., LAMARCHE B., COUTURE P., BERGERON N., 2005. Lack of effect of dietary conjugated linoleic acids naturally incorporated into butter on the lipid profile and body composition of overweight and obese men. Am J Clin Nutr 82, 309-319.

DUGAN M.E.R., AALHUS J.L., SCHAEFER J.A.L., KRAMER J.K.G., 1997. The effect of conjugated linoleic acid on fat to lean repartitioning and feed conversion in pigs. Can J Anim Sci 77, 723-725.

DUGAN M.E.R., AALHUS J.L., LIEN K.A., SCHAEFER J.A.L., KRAMER J.K.G., 2001. Effects of feeding different levels of conjugated linoleic acid and total oil to pigs on live animal performance and carcass composition. Can J Anim Sci 81, 505-510.

DUNSHEA F.R., OSTROWSKA E., MURALITHARAN M., CROSS R., BAUMAN D.E., PARIZA M.W., SKARIE C., 1998. Dietary conjugated linoleic acid decreases back fat in finisher gilts. J Anim Sci 76(Suppl 1), 131 (Abstract).

EGGERT J.M., BELURY M.A., KEMPA-STECZKO A., MILLS S.E., SCHINCKEL A.P., 2001. Effects of conjugated linoleic acid on the belly firmness and fatty acid composition of genetically lean pigs. J Anim Sci 79, 2866-2872.

FERNÁNDEZ-FIGARES I., CONDE-AGUILERA J.A., NIETO R., LACHICA M., AGUILERA J.F., 2008. Synergistic effects of betaine and conjugated linoleic acid on the growth and carcass composition of growing Iberian pigs. J Anim Sci 86, 102-111.

FOLCH J., LEES M., STANLEY G.H.S., 1957. A simple method for isolation and purification of total lipids from animal tissues. J Biol Chem 226, 497-508.

HARO A.M., ARTACHO R., CABRERA-VIQUE C., 2006. Linoleic conjugated acid current interest in human nutrition. Med Clin 127, 508-515.

JOO S.T., LEE J.L., HA Y.L., PARK G.B., 2002. Effects of dietary conjugated linoleic acid on fatty acid composition, lipid oxidation, color, and water- holding capacity of pork loin. J Anim Sci 80, 108-112.

KRAMER J.K.G., SEHAT N., DUGAN M.E.R., MOSSOBA M.M., YURAWECZ M.P., ROACH J.A.G., EULITZ K., AALHUS J.L., SCHAEFER A.L., KU Y., 1998. Distributions of conjugated linoleic acid (CLA) isomers in tissue lipid classes of pigs fed a commercial CLA mixture determined by gas chromatography and silver ion high-performance liquid chromatography. Lipids 33, 549-558.

LARSEN S.T., WIEGAND B.R., PARRISH F.C. Jr., SWAN J.E., SPARKS J.C., 2009. Dietary conjugated linoleic acid changes belly and bacon quality from pigs fed varied lipid sources. J Anim Sci 87, 285-295.

LE DIVIDICH J., HERPIN P., MOUROT J., COLIN A.P., 1994. Effect of low-fat colostrum on fat accretion and lipogenic enzyme activities in adipose tissue in the 1-dayold pig. Comp Biochem Phys 108, 663-671.

LEE K.N., KRITCHEVSKY D., PARIZA M.W., 1994. Conjugated linoleic acid and atherosclerosis in rabbits. Atherosclerosis 108, 19-25.

LÓPEZ-BOTE C.J., 1998. Sustained utilization of the Iberian pig breed. Meat Sci 49(Suppl), 17-27.

MARTÍN D., ANTEQUERA T., GONZÁLEZ E., LÓPEZBOTE C.J., RUIZ J., 2007. Changes in the fatty acid profile of the subcutaneous fat of swine throughout fattening as affected by dietary conjugated linoleic acid and monounsaturated fatty acids. J Agr Food Chem 55, 10820- 10826.

MOUROT J., KOUBA M., PEINIAU P., 1995. Comparative study of in vitro lipogenesis in various adipose tissues in the growing domestic pig (Sus domesticus). Comp Biochem Phys B 111, 379-384.

OSTROWSKA E., MURALITHARAN M., CROSS R.F., BAUMAN D.E., DUNSHEA F.R., 1999. Dietary conjugated linoleic acid increase lean tissue and decrease fat deposition in growing pigs. J Nutr 12, 2037-2043.

OSTROWSKA E., CROSS R.F., MURALITHARAN M., BAUMAN D.E., DUNSHEA F.R., 2003. Dietary conjugated linoleic acid differentially alters fatty acid composition and increases conjugated linoleic acid content in porcine adipose tissue. Brit J Nutr 90, 915-928.

PARIZA M.W., HARGRAVES W.A., 1985. A beef-derived mutagenesis modulator inhibits initiation of mouse epidermal tumors by 7,12-dymethylbenz(a)anthracene. Carcinogenesis 6, 591-593.

PARIZA M.W., PARK Y., COOK M.E., 2001. The biologically active isomers of conjugated linoleic acid. Prog Lipid Res 40, 283-298.

PARK Y., STORKSON J.M., NTAMBI J.M., COOK M.E., SIH C.J., PARIZA M.W., 2000. Inhibition of hepatic stearoyl-CoA desaturase activity by trans-10,cis-12 conjugated linoleic acid and its derivatives. BBA-Mol Cell Biol L 1486, 285-292.

PIESZKA M., PASCIAK P., JANIK A., BAROWICZ T., WOJTYSIAK D., MIGDAL W., 2006. The effect of sex and dietary antioxidants β-carotene, vitamins C and E in a CLA enriched diet on the lipid profile and oxidative stability of pork meat. J Anim Feed Sci 15, 37-45.

RAFF M., THOLSTRUP T., TOUBRO S., BRUUN J.M., LUND P., STRAARUP E.M., CHRISTENSEN R., SANDBERG M.B., MANDRUP S., 2009. Conjugated linoleic acids reduce body fat in healthy postmenopausal women. J Nutr 139, 1347-1352.

RAMSAY T.G., EVOCK-CLOVER C.M., STEELE N.C., AZAIN M.J., 2001. Dietary conjugated linoleic acid alters fatty acid composition of pig skeletal muscle and fat. J Anim Sci 79, 2152-2161.

SAS, 2002. User's guide. SAS Institute, Cary, N C.

SERRANO M.P., VALENCIA D.G., FUENTETAJA A., LÁZARO R., MATEOS G.G., 2008. Effect of gender and castration of females and slaughter weight on performance and carcass and meat quality of Iberian pigs reared under intensive management systems. Meat Sci 80, 1122-1128.

SMITH S.B., HIVELY T.S., CORTESE G.M., HAN J.J., CHUNG K.Y., CASTEÑADA P., GILBERT C.D., ADAMS V.L., MERSMANN H.J., 2002. Conjugated linoleic acid depresses the Δ9 desaturase index and stearoyl coenzyme A desaturase enzyme activity in porcine subcutaneous adipose tissue. J Anim Sci 80, 2110-2115.

SMITHARD R.R., SMITH W.C., ELLIS M., 1980. A note on the fatty acid composition of backfat from boars in comparison with gilts and barrows. Anim Prod 31, 217- 219.

SUKHIJA P.S., PALMQUIST D.L., 1988. Rapid method for determination of total fatty acid content and composition of feedstuffs and feces. J Agric Food Chem 36, 1202- 1206.

THIEL-COOPER R.L., PARRISH F.C.JR., WIEGAND B.R., LOVE J.A., 1998. Body composition and sensory characteristics of pork from CLA-fed pigs. ISU Swine Res. Rep., Iowa State Univ [on line]. Available in http//www.exnet. iastate.edu/page/ansci/swinereports/ Meat98.html. [21 February, 2001].

THIEL-COOPER R.L., PARRISH F.C.J.R., SPARKS J.C., WIEGAND B.R., EWAN R.C., 2001. Conjugated linoleic acid changes swine performance and carcass composition. J Anim Sci 79, 1821-1828.

THOM E., WADSTEIN J., GUDMUNDSEN O., 2001. Conjugated linoleic acid reduces body fat in healthy exercising humans. J Int Med Res 29, 392-396.

TISCHENDORF F., SCHÖNE F., KIRCHHEIM U., JAHREIS G., 2002. Influence of conjugated linoleic acid mixture on growth, organ weights, carcass traits and meat quality in growing pigs. J Anim Physiol An N 86, 117-128.

VENTANAS S., VENTANAS J., RUIZ J., 2007. Sensory characteristics of Iberian dry-cured loins. Influence of crossbreeding and rearing system. Meat Sci 75, 211-219.

WIEGAND B.R., PARRISH J.R. F.C., SWAN J.E., LARSEN S.T., BAAS T.J., 2001. Conjugated linoleic acid improves feed efficiency, decreases subcutaneous fat and improves certain aspects of meat quality in Stress-Genotype pigs. J Anim Sci 79, 2187-2195.

This article was originally published in the Spanish Journal of Agricultural Research 2010 8 (4),962-970. Engormix.com thanks the author for this huge contribution.