Nutrition and intramuscular fat: Effect of conjugated linoleic acid and vitamin A

The success of the genetic improvement programs of the pigs in favour of a better growth and feeding efficiency and against the backfat thickness of the carcass has led to the obtaining of extraordinarily lean carcasses with very little intramuscular fat (IMF<1%), giving as a result a clear worsening of the organoleptic parameters of the meat. The organoleptic traits (tenderness, juiciness, flavour, etc.) are directly related to the intramuscular fat content. The more intramuscular fat the better the organoleptic quality of the meat. In fact, according to the studies about the preferences of the consumers in Europe, the idea that pork should contain a 2.5-3% of IMF to improve its intrinsic organoleptic traits is becoming widespread.

The health and environmental conditions of the rearing, the genetic type or the gender, the live weight or the age at slaughter, the fattening degree or the lean tissue percentage of the carcass, the meat cut evaluated, as well as the nutrition and the feeding management are factors that have a direct bearing on the IMF content. From the nutritional viewpoint, several strategies, with a higher or lower success, have been suggested, and among them we would highlight the use of conjugated linoleic acid (CLA) in the compound feeds, the reduction of the vitamin A contents in the diet or the reduction of the protein and/or lysine levels and the arginine and leucine supplementation.

Conjugated linoleic acid (CLA)

CLA is a mix of geometric and positional isomers of linoleic acid (C 18:2). In the commercial preparations, the 10trans, 12 cis (C 18:2 10t-12c) isomer, responsible for the changes in the body fat; and the 9cis, 11trans (C 18:2 9c-11t) isomer, whose main effect would be the prevention of certain kinds of cancer due to an inhibiting effect on tumorigenesis, predominate. The first studies performed on pigs proved clearly that CLA would allow to reduce the fat deposition and to increase the lean tissue contents of the carcass. Nevertheless, in the studies carried out, the results seen showed a certain inconsistency, and this is associated with factors like the kind of CLA used, the fat or energy contents of the diet, the length of the feeding period or the kind of pig used. It seems that in those pigs with more than 23 mm of backfat, CLA could induce a reduction of the fattening, but this would not happen in pigs with a lower backfat thickness. For this same reason, CLA seems to be more effective in castrated pigs than in females or entire males. On the other hand, it also seems to be more effective when it is used in diets with a low energy content and/or without added fat. Equally, a certain inconsistency is seen in the influence of CLA on the intramuscular fat: while some experiments show a significant increase in its contents, in others no effect is seen. The causes for this lack of repeatability could be similar to those discussed previously. In the case of the trial performed by our team, 16 Landrace x Duroc females were used from approximately 73 kg live weight to 117 kg live weight (LW), when they were slaughtered in order to evaluate certain carcass and meat quality parameters. The animals were housed individually and fed ad libitum with a control diet with a 4% of sunflower seed oil or with a diet with a 4% of CLA for 54 days. In general, all the backfat thickness measurements, as well as the kidney area or the back area fat weight were reduced due to use of CLA. Nevertheless, the IMF content of the loin has not been affected by the kind of diet while the lean meat percentage has increased. Another aspect that has been seen has been the change in the fatty acids profile of the loin: the animals in the CLA group, as well as containing the CLA isomers used, showed a higher percentage of saturated fatty acids and a lower percentage of monounsaturated fatty acids. So, according to the results of this study, in which it is convenient to remember that a dose 2 to 4 times higher than usual has been used, CLA can help to improve the carcass quality in those pigs with a more conventional genetic and gender type, but it is difficult that it can simultaneously carry towards an increase of the IMF content.

Table 1. Effects of CLA on the production, carcass and meat quality parameters

IMF: intramuscular fat; SFAs: saturated fatty acids; MUFAs: monounsaturated fatty acids; CLA: sum of the linoleic acid isomers

Vitamin A

Another strategy considered is the reduction or the withdrawal of the content in vitamin A in the finishing compound feeds. Although the requirements, according to the NRC, are 1,300 IU/kg of compound feed, they are normally supplemented with 5,000-6,000 IU of vitamin A. According to an Australian study and other studies performed in cattle, the vitamin A withdrawal from the supplement would allow to increase, significantly, the intramuscular fat content in the meat. In a study carried out recently in Spain, similar results were also seen when reducing 10 times the vitamin A content in the finishing compound feeds for pigs. Nevertheless, this same result could not be confirmed when withdrawing completely vitamin A from the supplement in another experimental treatment. This same team had already seen an increase in the IMF when withdrawing vitamin A from the finishing compound feed in Duroc pigs, but not in Large-White pigs.

In our study 48 castrated 35 kg LW Landrace x Duroc pigs were used, and they were housed individually until they reached the slaughter LW of approximately 115 kg. The diets based on sorghum, cassava, wheat and soya 44% were formulated according to the requirements for the different growth stages, and 3 experimental treatments that contained 0; 1,250 or 5,000 IU/kg of compound feed were used. These diets were given ad libitum throughout the fattening of the pigs. In terms of the production results no differences were seen between the animals of the different experimental groups. In the same way differences were neither seen with respect to the fattening degree of the carcass, although the animals that did not receive vitamin A had leaner carcasses. Regarding the main goal of the study, not only no increase in the IMF content was seen, but the opposite: the animals that did not consume vitamin A had the lowest IMF content. Although the retinol levels registered in the liver correspond to the vitamin A gradient in the compound feeds, its presence in the case of the pigs that did not receive it in their compound feed seems to tell that they would be able to synthesize it from precursors probably present in the sorghum used in the compound feeds. So, the results of the withdrawal or the reduction of vitamin A with the goal of increasing the IMF content are debatable, and this strategy cannot be recommended as such, at least in similar conditions to those used in our experiment.

Table 2. Effects of the vitamin A levels in the compound feeds on the production and meat quality parameters.

IMF: intramuscular fat