## 4 Discussion
To the best of our knowledge, this is the first study that reports the effect of fermented onion on mucus production. The short-chain fatty acids (SCFAs) contents including acetate, butyrate, and propionate were analyzed (Figure 7). Among the DSS-induced groups, butyrate only increased in the LO and HO groups. In contrast, acetate and propionate were increased in all DSS-induced groups, especially in the HO group. These results are consistent with a previous study, which showed that the DSS significantly increased acetate and propionate concentrations while significantly decreasing butyrate [36]. Further research is required to fully understand this mechanism. The major immunoglobulins that constitute most humoral and mucosal immunity are immunoglobulin A (IgA) and immunoglobulin G (IgGG). IgA is the main antibody generated within the intestines, while IgG is the most prevalent antibody in the peripheral blood [37]. Previous research has shown that patients with IBD had higher levels of serum IgG in the intestinal lumen [38; 39]. Our results showed that IgG levels were significantly increased in the DSS-induced groups, especially in the FO group, and dramatically decreased in the HO group. A recent study suggests that IgG might play a pathogenic role in intestinal inflammation via Fc gamma receptor (FcyR) activation by IgG, which leads to the production of IL-1\(\beta\), type 17 immunity, and the exacerbation of intestinal inflammation [40]. Similar results were shown in IgA, which increased in the DSS-induced groups, except in the HO group, in which IgA decreased. DSS causes impaired mucosal barriers, which result in a leaky gut, allowing substances to easily pass through the gut. These substances have the ability to stimulate IgA secretion. Furthermore, several studies have shown that IBD patients have higher IgA levels than healthy individuals [22; 41]. Moreover, we found that the FO groups had significantly higher IL-6 and a slight increase in TNF-\(\alpha\) and IL-1\(\beta\) protein expression. In fermented onion, on the other hand, improvement in these inflammation factors was shown. These findings are consistent with all data previously mentioned, suggesting that in DSS-induced IBD rats, fermented onions had better beneficial impacts on gut health than fresh onions, considering that fermentation may reduce FODMAP properties and induce anti-inflammation. The differences in effects between fermented and fresh onions on gut health might be due to the degradation of FODMAP compounds by fermentation, which results in an increase in butyrate in the fermented onion groups, as butyrate has been shown to control oxidative stress in numerous studies [42]. The differences in the chemical compounds of fresh and fermented onions may be another factor that might play role in the different effects of these two types of onions. In a prior study, chemical analysis revealed that pickled onions had much higher levels of allyl methyl trisulfide, 3,5-diethyl-1,2,4-trifluoridene, and 2-hexyl-5-methyl-3(2H)-furanone than fresh onions did [43]. These compounds have also been shown in several studies to have a positive effect on IBD in rats. This may lead to a future study in which we may extensively explore the relationship between the different components of fresh and fermented onion and gut health.