RESUMEN
Dietary CLA has been reported to decrease backfat and increase marbling in pigs. Our objective was to determine whether the increase in marbling involved changes in intramuscular adipocyte number or size or both. Twenty barrows (53 kg) were penned in pairs and pens were randomly assigned to receive diets containing either 1% soybean oil (SBO) or CLA (60% CLA isomers) for 6 wk. Body weight and feed intake were determined weekly. At slaughter, loin samples were obtained and flash frozen for RNA extraction and real-time reverse-transcription PCR analysis of gene expression. After a 24-h chill, loin eye area and backfat depth were measured and subjective marbling and color scores were assigned. Loin, backfat, and belly fat samples were obtained for fatty acid analysis by gas chromatography. Loin samples were also frozen in ice-cold isopentane for histological analysis of intramuscular adipocytes. Dietary CLA did not affect BW or feed intake at any point (P > 0.10), nor did treatment groups differ in HCW (P = 0.417) or loin color (P = 0.500). The CLA-fed pigs did have less (P = 0.018) backfat and smaller (P = 0.047) loin eye area than SBO-fed pigs and had a trend for an increase (P = 0.069) in marbling score. Relative gene expression for markers of preadipocytes (preadipocyte factor 1; Pref-1), differentiating adipocytes (PPARγ), and mature adipocytes [fatty acid binding protein 4 (FABP4) and perilipin (PLIN)] were determined and normalized to the expression of acidic ribosomal phosphoprotein. No significant differences were detected, but the expression of PPARγ (P = 0.265), PLIN (P = 0.265), and FABP4 (P = 0.148) was numerically greater in CLA-fed pigs than in SBO-fed pigs. Loin samples were stained with Oil Red O to identify intramuscular adipocytes. The average cell area was increased (P = 0.030) in CLA-fed pigs. The cis-9,trans-11 and trans-10,cis-12 CLA isomers were incorporated (P = 0.006) into backfat and belly fat, but only trans-10,cis-12 CLA was increased in the loin (P = 0.004) of CLA-fed pigs. The proportion of SFA was increased (P = 0.006) by CLA in all tissues. These results indicate that the increase in marbling in pigs fed CLA may be related to increased intramuscular adipocyte size, and the combination of increased marbling and degree of saturation could improve the eating quality of CLA-fed pork.