Benzoic acid beneficially affects growth performance of weaned pigs which was associated with changes in gut bacterial populations, morphology indices and growth factor gene expression

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At weaning, piglets often suffer from a multitude of stressors, such as separation from dam and littermates and abrupt changes in diet and environment, which frequently lead to growth retardation, microecological imbalance, pathogen invasion and intestinal dysfunction, etc. (Hu et al., 2013a,b; McLamb et al., 2013; Jiao et al., 2014). Therefore, a substantial number of strategies have been developed to alleviate the negative effects of weaning stressors (Kim et al., 2012; Campbell et al., 2013), and for which feed additives are widely used in the pig industry worldwide, for example zinc oxide (Park et al., 2015), chito‐oligosaccharide (Jiao et al., 2014) and glutamine (Wang et al., 2015).
Benzoic acid (BA) is the simplest aromatic carboxylic acid, which is found in various species, including resins, fruits, milk and animal tissue, and is usually used in food industry due to its antimicrobial and preservative properties (Ning et al., 2010). Previous studies from our laboratory demonstrated that BA exerted a favourable effect on growth performance in piglets, which was related to decreased pH values of intestinal digesta, and improved intestinal morphology and intestinal microbiota composition (Diao et al., 2014, 2015; Chen et al., 2015). However, to the best of our knowledge, the underlying mechanisms responsible for the growth‐promoting effect of BA have not been completely elucidated.
It is commonly reported that weaning often results in intestinal atrophy (Hu et al., 2013a) and intestinal barrier dysfunction in piglets (Smith et al., 2010; Wijtten et al., 2011), which are responsible for the growth retardation. It is well known that some blood indexes can be used to evaluate intestinal permeability. D‐lactic acid, a special end product of bacteria in intestine, will release into blood when intestinal mucosa barrier is damaged. The concentration of D‐lactic acid in peripheral blood would increase because lacking of D‐lactate dehydrogenase in mammals (Ruh et al., 2000). Diamine oxidase (DAO), one kind of endocellular enzyme, only exists in villus cytoplasm of intestinal stratum supravasculare in mammals. When intestinal epithelial cells are injured, DAOs existing in epithelial cells are released into the blood circulation and finally cause an increase in the plasma DAO activity (Nieto et al., 2000). Therefore, the concentration of D‐lactic acid and the activity of DAO in blood could monitor the extent of intestinal barrier injury. Tight junctions are the important and pivotal components in the intestinal mucosal barrier (Ballard et al., 1995). Zonula occludens protein‐1 (ZO‐1) and occludin are the vital structural protein of the tight junction, which own important functions in maintaining the integrity of intestinal mucosal barrier and deciding the intestinal permeability (Blikslager et al., 2007). Meanwhile, previous studies have shown that several growth‐stimulating factors, such as epidermal growth factor (EGF), growth hormone receptor (GHR) and insulin‐like growth factor‐1 (IGF‐1), play a vital role in the development of the intestine of young animals (Schweiger et al., 2003; Theiss et al., 2004; Xiao et al., 2009). Li et al. (2006) reported that increasing the expression of IGF‐1 and IGF‐1R in the small intestine can protect weaned piglets from intestinal atrophy and dysfunction, thus improving growth performance. In view of the foregoing, it is plausible to hypothesize that dietary BA supplementation enhances the growth performance of weaned piglets by regulating the expression of genes related to intestinal mucosal tight junction and growth‐stimulating factors. This study was conducted to assess this hypothesis.
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