Abstract
Manipulating dietary nutrients, especially protein fractions, holds significance in enhancing the antioxidant capacity and immunity function of ruminants. This study investigated the impact of dietary adjustments in soluble protein (SP) levels, in conjunction with a reduction in crude protein (CP) content, on the antioxidant capacity, inflammatory response, carcass characteristics, and meat quality of sheep. This study had four dietary treatments, including a control diet (CON) adhering to NRC standards with a CP content of 16.7% on a dry matter basis and three diets with an approximately 10% reduction in CP content compared to CON with SP levels (% of CP) of 21.2 (SPA), 25.9 (SPB) and 29.4% (SPC), respectively. Thirty-two healthy male Hu sheep, with an initial live weight of 40.37 ± 1.18 kg and age of 6 months, were randomly divided into four groups to receive these respective diets. Our data revealed no significant differences in slaughter performance among treatments (p > 0.05), although low-protein treatments decreased the stomachus compositus index (p < 0.05). Compared with CON, as SP was adjusted to 21.2%, total antioxidant capacity (T-AOC) and catalase (CAT) concentrations were decreased in the serum (p < 0.05), glutathione peroxidase (GSH-Px) content was decreased in jejunum and ileum (p < 0.05), superoxide dismutase (SOD) concentration was reduced in the duodenum (p < 0.05), and malondialdehyde (MDA) content was increased in spleen and ileum (p < 0.05). On the other hand, pro-inflammatory cytokine (IL-1β, IL-6 and IL-8) contents were upregulated in the serum (p < 0.05), while immunoglobulin (IgA and IgM) contents were reduced in the duodenum (p < 0.05) with SP adjustments. Additionally, the SPB and SPC diets reduced the content of saturated fatty acids and increased the content of polyunsaturated fatty acids compared with CON (p < 0.05), along with retention in the tenderness and water-holding capacity of the longissimus lumborum muscle. In summary, reducing CP by 10% with an SP proportion of ~25-30% improved meat quality without compromising antioxidant capacity and immunity function, while lower SP levels had adverse effects.