Characteristics, lipogenic enzyme activity, and fatty acid composition of muscles in the Iberian pig: Effects of protein restriction and free-range feeding

Abstract

In Iberian pigs, protein restriction during the final fattening period does not affect productive parameters and carcase fatness but increases the intramuscular lipid content. The aim of this study was to investigate the effects of a low-crude protein diet on the muscle characteristics, metabolism, lipogenic enzyme activity, and fatty acid composition of Iberian pigs fed under confinement and free-range conditions. Twenty-four castrated male Iberian pigs with an average initial body weight (BW) of 116.0 kg were divided into three groups (n = 8); the first and second groups were fed isoenergetic (4100 kcal ME/kg dry matter) concentrated diets with standard protein (12.8%) (SP) and low protein (6.6%) (LP) contents, respectively, under outdoor confinement; the third group was fed on natural resources (acorn and grass) under free-range conditions (FR). The pigs were slaughtered at an average BW of 174.2 kg. Compared with pigs fed the SP diet, LP and FR pigs showed significantly high intramuscular fat (IMF) contents and low muscle and fiber areas. In the muscle, the activities of glucose-6-phosphate dehydrogenase (G6PDH) and malic enzyme (ME) were not affected by diet, but a slight increase in ME activity was observed in SP pigs compared to that in LP and FR pigs when the results were expressed as per g lipid. Plasma concentrations of glucose, creatinine and protein were not affected by the experimental diets, but plasma urea concentration was significantly lower in FR than in SP pigs, with intermediate values in LP. The effect of protein reduction in diets was more marked on neutral than on polar lipid contents, and higher saturated fatty acid and lower polyunsaturated fatty acid proportions were observed in LP pigs than in SP and FR pigs. So, a reduction in protein levels in the diets of Iberian pigs during the fattening period affected muscle lipid composition, despite the slight effect observed on lipogenic enzyme activity. Furthermore, the longissimus lumborum and serratus ventralis muscles showed similar responses to diets with different protein levels but differed in their chemical compositions and metabolic activities, which suggested that serratus ventralis, with a lower commercial value, could be adequate as the muscle of choice for analysis of meat composition and quality in the Iberian pig.