This study investigated the effects of different crude protein (CP) levels on growth performance, carcass traits, meat quality, and fatty acid composition in Ningxiang finishing pigs. A total of 200 pigs (52.52 ± 0.41 kg) were assigned to five dietary treatments: high-protein (HP, 15.56%), moderate-high-protein (MHP, 13.99%), moderate-protein (MP, 12.94%), moderate-low-protein (MLP, 11.90%), and low-protein (LP, 10.31%). Feeding the MLP diet significantly improved average daily gain (ADG) compared to HP and LP diets (p < 0.05). Pigs fed the MP diet had higher lean meat percentage than those on the LP diet (p < 0.05), while both HP and MP diets reduced carcass fat percentage (p < 0.05). The LP diet significantly increased a*, pH45min, intramuscular fat (IMF), and C18:1n9c, while decreasing C23:0 and C20:3n6 in the longissimus dorsi muscle (p < 0.05). Liver metabolomics revealed that the LP diet inhibited nicotinate and nicotinamide metabolism (p < 0.05), accompanied by downregulation of genes related to the tryptophan–niacin metabolism and upregulation of genes involved with hepatic lipogenesis (p < 0.05). In muscle, the LP diet inhibited AMPK signaling via decreased p-AMPK expression, leading to increased IMF content (p < 0.05). In summary, the optimal growth performance of Ningxiang finishing pigs was achieved with a CP level of 11.90%. Additionally, the LP diet enhanced meat quality by modulating hepatic niacin metabolism and AMPK signaling.
Zeng et al. (Sat,) studied this question.