The protective role of a moderate protein diet in ETEC-infected piglets: Optimization of growth, immunity, and microbial balance.

Diarrhea is a common issue among weaned piglets and enterotoxigenic Escherichia coli (ETEC) is one of the causes. However, there is limited research on how dietary protein levels affect piglets in a diarrhea model. This study aimed to investigate the effects of dietary protein levels on growth performance, immunity and microbial balance of weaned piglets post-infection with ETEC. One hundred and ninety-two 24-d-old piglets (Duroc × Landrace × Yorkshire) with a body weight (BW) of 7.20 ± 0.12 kg were used in a 2 × 3 factorial treatment arrangement. The factors were ETEC challenge (oral 3 × 1011 colony-forming unit (CFU) ETEC O149:K19:K88 on experimental d 1, 2, and 3 or not), and three dietary crude protein (CP) levels (high protein [HP]-20.5% CP, medium protein [MP]-17.5% CP, and low protein [LP]-14.5% CP). On d 7, eight piglets from each treatment were sampled, and the remaining piglets were raised for another 22 d. The trial lasted 28 d. Results showed that ETEC infection increased the diarrhea rate, serum lipolyaccharide (LPS) level, and the levels of pro-inflammatory cytokines (interleukin-1β [IL-1β], interferon-γ [IFN-γ], and interleukin-6 [IL-6]), as well as the number of E. coli in the cecal digesta of piglets (P < 0.05). The ETEC infection reduced piglets' average daily gain (ADG):average daily feed intake (ADFI) (G:F) by 22.2% under the HP diet (P = 0.022) compared with the CHP group, but had no significant effect in MP and LP diets (P > 0.05). Moreover, the MP diet decreased serum diamine oxidase (DAO), D-lactic acid (D-LA), and IL-6 levels, compared with the LP diet on d 6 under ETEC infection, and the MP diet decreased gene expression of B-cell lymphoma 2 (Bcl-2) and Bcl-2-associated X protein (Bax) in ileal mucosa and the number of E. coli in the cecal digesta compared with the HP and LP diets on d 6 under ETEC infection (P < 0.05). Additionally, the G:F was increased with the increase of dietary protein levels under normal conditions from 1 to 6 d (P = 0.040). Furthermore, as dietary protein levels increased, growth performance (body weight on d 28 [BW28], ADG, and G:F) of piglets improved from 7 to 28 d (P < 0.05) and serum IFN-γ and LPS levels decreased on d 28 (P < 0.05) with or without ETEC infection. These findings indicated that a MP diet can enhance growth performance, reduce the expression of apoptosis-related genes, and regulate intestinal flora structure under ETEC infection. However, a HP diet remained the optimal choice for promoting growth performance under normal conditions or during an ETEC infection repair period.