**2. Results**

#### *2.1. Intestinal Mucosal Morphology*

Figure 1 and Table 1 show intestinal morphology indices. Compared with the CON group, the villus height (VH) and VH to cryp<sup>t</sup> depth (CD) ratio of the small intestine were decreased (*p* < 0.05) in the AS group (basal diet + 0.1% H2O2 in drinking water). Compared with the AS group, COS increased the VH of the small intestine, and the VH:CD of the duodenum and jejunum.

**Figure 1.** Effects of chitosan oligosaccharides on intestinal morphology when challenged with hydrogen peroxide. CON group, basal diet; AS group, basal diet + 0.1% hydrogen peroxide (H2O2) in drinking water; ASC group, basal diet + 200 mg/kg COS + 0.1% H2O2 in drinking water. Scale label: 50 μm.


**Table 1.** Effects of chitosan oligosaccharides on intestinal mucosal morphology when challenged with hydrogen peroxide.

1 COS, chitosan oligosaccharide; CON group, basal diet; AS group, basal diet + 0.1% hydrogen peroxide (H2O2) in drinking water; ASC group, basal diet + 200 mg/kg COS + 0.1% H2O2 in drinking water; VH, villus height; CD, cryp<sup>t</sup> depth. 2 SEM, standard error of mean. a,b,c Two different superscripts indicate a significant difference in the same row (*p* < 0.05).

## *2.2. Intestinal Permeability*

Compared with the CON group, serum diamine oxidase (DAO) activity and D-lactate acid (D-LA) content were upregulated (*p* < 0.05) in the AS group (Figure 2). Compared with the AS group, COS downregulated (*p* < 0.05) serum DAO activity and D-LA content.

**Figure 2.** Effects of chitosan oligosaccharides on serum DAO and D-LA after hydrogen peroxide challenge. CON group, basal diet; AS group, basal diet + 0.1% H2O2 in drinking water; ASC, basal diet + 200 mg/kg COS + 0.1% H2O2 in drinking water. (**A**) DAO content; (**B**) D-LA activity. Values are means ± standard errors. Columns that have different numbers above them statistically differ (*p* < 0.05). a,b,c Columns that have different letter statistically differ (*p* < 0.05).
