## 1 Introduction  
The intestinal barrier is composed of gut microbiota, mucus, epithelial cells and some immune cells [1]. It is an important physical barrier that protects the underlying tissue from various pathogens and toxic substances [1]. Impaired intestinal barrier function has been associated with many chronic gastrointestinal diseases like coeliac disease, colorectal cancer, inflammatory bowel diseases, and also contributes to systemic diseases such as diabetes, allergic diseases and even chronic kidney disease [1; 2; 3]. Therefore, the maintenance of a healthy intestinal barrier, especially the repair of impaired barrier, is of great importance for the improvement of chronic diseases.  
Several studies have highlighted the role of short chain fatty acids (SCFAs) in such areas. They are known to serve as energy source for colonocytes, enhance intestinal barrier function, and help to repair wounded epithelium [4; 5; 6]. The three major SCFAs, acetate, propionate and butyrate, have demonstrated positive effects on intestinal barrier function in vitro [7; 8], ex vivo [9], and in vivo [10; 11; 12]. However, the level of individual SCFA required for improving intestinal barrier function seems to be different [7; 8; 13]. Moreover,SCFAs are naturally produced in the colon by microbial fermentation of undigested carbohydrates, but the composition of SCFAs in the fermentation products differs depending on the source, microbiota composition, site of fermentation, and host genotype [14; 15]. Thus, SCFA composition in different colonic fermentation products can vary significantly, which may affect intestinal barrier function differently. Unfortunately, there are few studies assessing the effects of mixed SCFAs or colonic fermentation products on intestinal barrier function.  
Polydextrose (PDX), lacittol and xylitol are ingredients widely used in THE food industry. They are non-digestible carbohydrates that can be fermented by colonic microbiota and have shown prebiotic properties [16; 17; 18]. The SCFA composition of fermentation products has been analyzed previously [19; 20]; however, the effects of these fermentation products on intestinal barrier function have not been evaluated. The EnteroMix in vitro model is a semi-continuous fermentation simulator that is used for studying colonic microbial fermentation, and has shown good correlations with in vivo studies [21]. Through four sequentially connected vessels that simulate ascending, transverse, descending and sigmoid colon, the EnteroMix model is able to achieve semi-continuous influx of nutrients and efflux of waste products, thereby providing a physicochemical condition that closely resembles the in vivo environment [21]. The HT29-MTX-E12 (E12) cell line is a subclone of human colorectal adenocarcinoma cell line HT29-MTX [22]. Because of its characterization on tight junction formation and adherent mucus production [22], E12 cells are highly suitable for studying intestinal barrier function. In the current study, the EnteroMix model and E12 cell model were combined. SCFA production following in vitro fermentation of PDX, lacittol and xylitol using EnteroMix model was analyzed, and the effects of the fermentation products on impaired colonic epithelial barrier function was investigated by an established HT29-MTX-E12 cell model [23]. The purpose of the study was to investigate the effects of the fermentation products of two widely used sweeteners, lacittol and xylitol, and one prebiotic, polydextrose, on impaired colonic epithelial barrier function. It was hypothesized that fermentation products of PDX, lacittol and xylitol have different beneficial effects on the repair of impaired intestinal epithelial barrier, and this discrepancy is due to different SCFA production.