## 4 Discussion
One possible reason could be that only 5% of the fermentation products were added to the cell culture medium, thus the variation in final SCFA concentration may not be large enough to make a difference to barrier repair. Nevertheless, xylitol fermentation products showed better barrier repairing effect at the same low concentration level, suggesting the small difference in final SCFA concentrations may not be the main reason for the lack of effect. In our study, fermentation of SC, PDX, and lacittol produced mainly acetate, resulting in a constant SCFA proportion, while xylitol fermentation produced mainly butyrate, and led to much higher butyrate proportion. It seems that the better barrier repairing effects of xylitol fermentation products were attributed to butyrate, as also supported by the correlation analysis. Although all three major SCFAs were reported to improve intestinal barrier function, butyrate was likely to be the most potent [7; 8; 13]. butyrate is mostly utilized locally by colonocytes as a direct energy source, and provides 60-70% of the energy that is required for colonic cell proliferation and differentiation [14]. Conversely, acetate and propionate act more systemically in vivo, as the majority are rapidly absorbed and transported to the liver, and used as substrates for gluconeogenesis, lipogenesis, protein synthesis or cholesterol synthesis [3; 5]. Moreover, the discrepancy among acetate, propionate and butyrate was also assumed as partially related to their different ability to cross the cell membrane [7]. Nevertheless, in a previous study with the same cell model, no consistency between the cellular effects (including barrier function and gene expression) and the levels of butyrate in the fermentation products was observed [23]. Yet, correlation analysis was not conducted, and the concentration of acetate and propionate were not measured in that study. The missing information makes direct comparison with our study very difficult, as the beneficial effects on barrier function may also come from high butyrate proportion [31]. [PERSON] et al. found that SCFA mixtures with 20% and 50% butyrate repaired disrupted barrier function, and 50% butyrate further increased the TEER value [31]. In our study, fermentation products with 16-35% butyrate (SC, PDX and lacittol) improved barrier function to an equivalent level, while fermentation products with 60-70% butyrate ( xylitol) led to a better barrier repair. Our observation fits well with the findings of [PERSON] et al., suggesting that the proportion of butyrate may actually also be a key factor for barrier repair. Hence, the better reparative effects of xylitol fermentation products observed in our study are likely to be due to the high production of butyrate. It has been reported that butyrate facilitates cell differentiation, and enhances intestinal barrier function by reinforcing the tight junctions [3; 32; 33], as well as increasing mucin production [3; 34]. Our results imply that in colonic fermentation products, butyrate may play a major role in intestinal barrier repair. Of course, the benefits of acetate and propionate cannot be excluded [10; 11; 35]; there may even be synergistic effects with butyrate. Our results showed that xylitol may be more potent than PDX and lacittol in improving intestinal barrier function. However, it was found that low concentrations of SCFAs promote intestinal barrier function, while high concentrations have no beneficial effects and may even disrupt barrier function [23; 31]. Thus, fibers with slow but steady SCFA production like PDX could be beneficial in vivo compared to quickly fermented ones that cause localized high SCFA concentrations [17; 31]. In contrast, xylitol is readily fermented by colonic microbiota, but it is important to note that half of the consumed xylitol is absorbed in the small intestine in vivo, which leaves only 50% of the dietary xylitol available for colonic fermentation [18].