**2. Effect of soybean meal inclusion in aquafeed to induced enteritis (SBMIE) in fish**

When studying enteritis in fish, it is important to consider all cell types involved in a correct function of the gastrointestinal tract (GIT). Like mammals, fish gut is critical for nutrient digestion and absorption, immunity, and interaction with the environment [24–26]. In fish, a simpler division of the GIT is described as compared to mammals: two different segments of the gut are distinguished: proximal or anterior intestine and distal or posterior intestine (**Figure 1a,b**). Of these, distal intestine is where most of the nutrient absorption occurs and is the object of study of SBMIE in fish [28–33].

The word enteritis refers to an inflammatory process happening in the gut, which can be caused by a diverse range of factors. The symptoms that define the condition are a shortening of the mucosal folds, a loss of the normal supranuclear vacuolization of the absorptive cells in the intestinal epithelium, a widening of the central stroma within the mucosal folding, with increased amounts of connective tissue, a profound infiltration of inflammatory cells in the lamina propria [4, 6, 19, 34–37], an increased presence of IgM [38], an increased amount of goblet cells in the epithelium, as well as a decreased height of the microvilli together with increased microvillar vesicle formation [36]. Baeverfjord and Krogdahl [6] also described this condition as "a non-infectious sub-acute inflammation of the distal intestine." Typically, FM replacement by SBM in fish diets is between 20 and 40% at which signs of enteritis are detected in a large variety of both marine and freshwater species including omnivores and

carnivores [39]. A reduction on feed intake and consequent decrease in weight gain is the first

**Figure 1.** (a) On top: Different section-planes through the digestive tract of the rainbow trout. Below: Sections through segment 1, 2, and 3 consisting of Liver (L), stomach (S), foregut (F) with pyloric appendices (P), hind gut/rectum (E) (Source: [27]). (b) (A) rainbow trout stomach showing well-developed gastric glands (G), submucosa (S), and the tunica muscularis (M). (B) Higher magnification of branched tubular gastric glands. Asterisk (\*) represents Shape and cellular morphology of mucous and (+) indicates pepsin and hydrochloric acid producing cell-type (oxyntopeptidic cells) (Source: [27]).

The Potential Impacts of Soy Protein on Fish Gut Health http://dx.doi.org/10.5772/intechopen.92695 93

There are several factors affecting the occurrence of SBMIE such as the inclusion levels, varieties, origins, and processing techniques of the different soybean products along with species variation and husbandry conditions (temperature, salinity, etc.). Nordrum et al. [40] who investigated the effect of salinity on the development of enteritis in salmonids. Urán [41] reported that with the increasing water temperature, the metabolic rate of Atlantic salmon increased which help to increase the severity of enteritis. Regarding the species variation effects on SBMIE, Nordrum et al. [40] also found that the effects of SBM on the intestinal morphology of rainbow trout were of less magnitude than for salmon. Similarly, Booman

indicator that a given diet is exerting a negative effect.

**Figure 1.** (a) On top: Different section-planes through the digestive tract of the rainbow trout. Below: Sections through segment 1, 2, and 3 consisting of Liver (L), stomach (S), foregut (F) with pyloric appendices (P), hind gut/rectum (E) (Source: [27]). (b) (A) rainbow trout stomach showing well-developed gastric glands (G), submucosa (S), and the tunica muscularis (M). (B) Higher magnification of branched tubular gastric glands. Asterisk (\*) represents Shape and cellular morphology of mucous and (+) indicates pepsin and hydrochloric acid producing cell-type (oxyntopeptidic cells) (Source: [27]).

carnivores [39]. A reduction on feed intake and consequent decrease in weight gain is the first indicator that a given diet is exerting a negative effect.

There are several factors affecting the occurrence of SBMIE such as the inclusion levels, varieties, origins, and processing techniques of the different soybean products along with species variation and husbandry conditions (temperature, salinity, etc.). Nordrum et al. [40] who investigated the effect of salinity on the development of enteritis in salmonids. Urán [41] reported that with the increasing water temperature, the metabolic rate of Atlantic salmon increased which help to increase the severity of enteritis. Regarding the species variation effects on SBMIE, Nordrum et al. [40] also found that the effects of SBM on the intestinal morphology of rainbow trout were of less magnitude than for salmon. Similarly, Booman et al. [42] reported that soybean meal induced enteritis in Atlantic salmon (*Salmo salar*) and Chinook salmon (*Oncorhynchus tshawytscha*) but not in pink salmon (*O. gorbuscha*).

Some tight junction proteins that have been studied in fish with regards to SBMIE include transmembrane proteins like occludins and claudins and intracellular components like zonula occludens-1 (ZO-1) [26, 28, 31, 32]. Another intracellular component that interacts with the tight junction complex is myosin light chain kinase (MLCK), involved in cytoskeletal contraction, smooth muscle contraction, and, therefore, tight junction regulation and paracellular

The Potential Impacts of Soy Protein on Fish Gut Health http://dx.doi.org/10.5772/intechopen.92695 95

**3. Morphophysiological effects at fish gut of soy protein inclusion in** 

Although soy protein has widely been used in aquafeed as a cheap alternative protein source for FM; however, the presence of some ANFs in SBM restricts its level of inclusion in aquafeed. High inclusion of soybean ingredients causes several negative effects on palatability and intestinal

**Figure 3.** (a) Intestinal histology (posterior, intestine, 20 X) of rainbow trout (*O. mykiss*) fed FM-based control diet showing normal condition of intestine (Kumar et al., unpublished data). (b) Intestinal histology (posterior, intestine, 20 X) of rainbow trout (*O. mykiss*) fed low SBM (10.3%)-based diet showing inflamed muscularis, leukocyte infiltration of the lamina propria leading to swelling and mucosal fold fusion (bridging). Increased prevalence of globlet cells possibly to secrete more mucous to protect the epithelium. Asterisk denotes inflammation (Kumar et al., unpublished data). (c) Intestinal histology (posterior, intestine, 20 X) of rainbow trout (*O. mykiss*) fed high SBM (20.7%) based diet showing villi and lamina propria highly inflamed (leading to much wider mucosal folds), muscularis inflamed, villi shortened, disorganization of epithelium, reduction in supranuclear absorptive vacuoles, mucosal fold fusion (bridging) and some

structural disintegration. By far, this treatment led to the most changes (Kumar et al., unpublished data).

permeability [26, 43, 44].

**aquafeed**

To understand the mechanism of intestinal inflammation or enteritis, it is important to also understand the elements involved at the cellular level. Enterocytes are cuboidal shaped epithelial cells that are distinct on their apical surface than on their basal surface. The apical surface faces the intestinal lumen and shows the characteristic folding of intestinal cells, called microvilli. The basal side is connected to vasculature where absorbed nutrients are released. Nutrients can undergo transcellular transport on the apical and basolateral membranes of the cell; this can happen by diffusion or by active transport through transmembrane transporters like the glucose/Na+ cotransport system, or other amino acid transporters like glutamine transporter, and also through pumps and channels. Paracellular transport occurs in between epithelial cells, and only small molecules and ions, solutes, and fluids can reach the blood this way. Only small nutrients can diffuse this way, as, in healthy conditions, enterocytes are held together through important tight junctions that keep the intestinal integrity [31, 32]. Intercellular exchange between enterocytes is also possible though gap junctions and desmosomes. **Figure 2** showing the healthy and abnormal enterocyte condition in fish.

**Figure 2.** Figure showing the healthy and abnormal enterocyte condition in fish.

Some tight junction proteins that have been studied in fish with regards to SBMIE include transmembrane proteins like occludins and claudins and intracellular components like zonula occludens-1 (ZO-1) [26, 28, 31, 32]. Another intracellular component that interacts with the tight junction complex is myosin light chain kinase (MLCK), involved in cytoskeletal contraction, smooth muscle contraction, and, therefore, tight junction regulation and paracellular permeability [26, 43, 44].
