Dietary protein/carbohydrate ratio affects on fat deposition and gene expression of finishing pigs

Pork quality is one of the most important factors influencing the acceptability of pig meat. However, pork quality has been decreasing as a result of genetic selection for higher lean meat content and reduced backfat thickness, and the increasing trend towards the production of leaner meat has resulted not only in a decrease of subcutaneous fat, but also of intramuscular fat (IMF). Thus, producing pork with higher amount of IMF without an increase in subcutaneous fat is of importance to the pig industry. Several studies have shown that IMF content can be increased, without increasing subcutaneous fat deposition, by the feeding of low protein diets in the growing or finishing phase. The molecular mechanisms, underlying the effects of low protein diets or other components on IMF content, remain largely unknown. Expression of lipogenic genes in muscle may be associated with IMF content. The present study was conducted to investigate the effect of dietary protein/carbohydrate (CH₂O) ratio on fat deposition and expression of peroxisome proliferator activated receptor γ (PPARγ) and heart fatty acid-binding protein (H-FABP) genes in muscle and subcutaneous adipose tissue in pigs.
Twenty pigs (73.9 ± 1.2 kg BW) were used in a single factorial experiment, and allocated by BW and ultrasound backfat thickness to the two treatments: low protein/CH₂O ratio (LP; 11.2% CP and 68.2% CH₂O for phase I, and 10.1% CP and 69.3% CH₂O for phase II) diet, or high protein/CH₂O ratio (HP; 22.7% CP and 58.3% CH₂O) diet. Pigs were housed individually in pens, and had ad libitum access to feed and water. After 80-d feeding, 6 pigs from each treatment were selected and slaughtered. Dietary protein/CH2O ratio had no effect on growth performance; however, the IMF in longissimus muscle (LM) was increased (P < 0.01) when the LP diet was fed. Correspondingly, Warner–Bratzler sheer force of LM in the LP pigs was lower (P < 0.05) than the HP pigs. The LP increased mRNA levels of PPARγ (P < 0.05) and H-FABP (P = 0.09) in LM but not in subcutaneous fat, although the difference of H-FABP gene expression in LM was not statistically significant. The mRNA abundance of PPARγ in muscle correlated positively with IMF content (P < 0.05). The diet with low protein/CH2O ratio elevated IMF content with less effect on subcutaneous fat deposition.

The results indicated that an increase in IMF but not subcutaneous fat in pigs fed the LP diet is related to tissue-specific activation of PPARγ and H-FABP mRNA expression, especially the PPARγ gene.

X Guo, R Tang, W Wang, D Liu and K Wang, 2011. Livestock Science, 140: 111-116.