It is known that dry matter and various dietary nutrients can impact GEP secretions (Boisen & Moughan, 1996; Deglaire et al., 2008; Deglaire et al., 2006; Hodgkinson & Moughan, 2007; Miner-Williams et al., 2014a; Montagne et al., 2001; Ravindran et al., 2009). Several studies have examined the effects of various dietary components such as dietary protein, fibre and anti-nutritional factors such as lectins, tannins and enzyme inhibitors, on the flow of GEP (Butts et al., 1993b; Claustre et al., 2002; Cowieson et al., 2004; Deglaire et al., 2008; Deglaire et al., 2006; Gilani et al., 2005; Gilani & Sepehr, 2003; Ouellet et al., 2002). The effects of fibre, amino acids, peptides and proteins are discussed here.
The effect of dietary fibre intake on GEP secretion has been the subject of several reviews (Boisen & Moughan, 1996; Vickery, 1950). Morel et al. (2003) that have examined the effects of non-starch polysaccharides on growth and GEP secretions in
pigs. The study involved feeding pigs with either β-glucan, a highly degradable
polysaccharide or arabinoxylan, a polysaccharide with significant water-holding
properties. It was found that β-glucan increased mucin secretion and ileal
endogenous amino acid flows (Morel et al., 2003). Similar results have been noted in broiler chickens, where dietary cellulose levels in the diet substantially impacted the flow of crude protein and amino acids at the terminal ileum (Kluth & Rodehutscord, 2009). While the exact mechanism by which dietary fibre affects endogenous nitrogen losses (ENL) is not clear, several possible mechanisms have been suggested. These include 1) alteration of endogenous secretions due to the higher viscosity created by the fibre and 2) physical abrasion of the gastrointestinal lumen by dietary fibre (Leterme et al., 1992).
Different proteins and peptides from different protein sources can influence GEP secretions and composition differently. In growing rats, increasing the amount of zein (a protein devoid of lysine) in the diet increased the mean flow of endogenous lysine at the terminal ileum (Hodgkinson & Moughan, 2007). In a separate study using the same model, Deglaire et al. (2008) found that alimentation with diets containing intact casein led to a greater ileal endogenous protein flow in comparison to diets based on hydrolysed casein. However, when compared to free amino acids as the sole nitrogen source in diets, hydrolysed casein also increased the endogenous protein flows at the terminal ileum (Deglaire et al., 2007). Similar trends were reported by Han et al. (2008) for rats, by (Butts et al., 1993a) for pigs and by Moughan et al. (2005) for human subjects, wherein the ENL at the terminal ileum increased when hydrolysed casein was included in the diet.
From the above studies, it is clear that the form of bovine casein (unhydrolysed or hydrolysed) can impact the ENL. In general, the ingestion of bovine casein hydrolysates led to an increase in the ENL when compared with the flows obtained with a protein-free diet. This increase was more than that observed for diets containing amino acids but less than the increase brought about by the intact casein. Furthermore, two independent studies, Ravindran et al. (2009) and Hodgkinson et al. (2000) have reported that feeding increasing concentrations of enzyme hydrolysed casein to male broiler chickens and growing pigs respectively, increased the GEP flows in a dose-dependent manner.
A recent comprehensive study (Rutherfurd et al., 2015a) sheds light on how different protein sources may impact the GEP flows. The authors investigated the effect of feeding protein hydrolysates from five different protein sources including
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gelatin, beef, casein, soy protein isolate and lactalbumin to male rats. It was found that basal endogenous ileal amino acid flows were different for the peptide hydrolysates derived from the different protein sources. The authors suggested that the differences may arise from the effects of potentially different bioactive peptides arising from digestion of the different protein sources used in the study (Rutherfurd et al., 2015a).
The full extent to which the dietary bioactive peptides can influence GEP secretion and therefore elicit GEP-derived bioactive peptides is presently not well understood. However, it is well-known that peptides released from dietary proteins in the GIT can stimulate secretion of GEP (Moughan et al., 2007). Zoghbi et al. (2006) reported
that based on a mucin-producing rat colon adenocarcinoma cell line study, β-
casomorphin-7 (a bovine milk β-casein derived bioactive peptide), increased the
secretion of mucin by directly affecting the secretory activity of the goblet cells,
inducing the expression of Mucin-5AC gene and by activating the μ-opioid
receptors (Zoghbi et al., 2006). These findings are in agreement with the previously reported findings of Claustre et al. (2002) who reported that enzymatic hydrolysates
of casein and lactalbumin and purified β-casomorphin-7 caused mucin release in
isolated vascularly perfused rat jejunum. Han et al. (2008) investigated the effect of hydrolysed casein on the mRNA expression of mucin genes in the small intestine of rats and reported that the gene expression of mucin-3 was significantly increased by the hydrolysed casein diet in comparison to protein-free or synthetic amino acid diets. Several other studies have also reported that bioactive peptides can bind to specific receptors in the gut and thereby modulate gut related biological processes, for example, gut motility and satiety (Moller et al., 2008; Moughan et al., 2007;
Shahidi & Zhong, 2008). It is evident that bioactive peptides present within food proteins and released during digestion in the GIT can have an effect on gut physiology. The possibility that GEP may act as a source of bioactive peptides, and that dietary bioactive peptides can in turn modulate the secretion of the GEP offers a unique opportunity to manipulate gut modulatory processes by fine-tuning either of the two elements. Therefore, it is clearly important that the GEP are studied within the context of being a source of bioactive peptides. This apparent interdependency between food and GEP-derived peptides can then be exploited to obtain an optimal supply of endogenous and exogenously derived bioactive peptides in the gut.
In conclusion, the above discussion indicates that GEP may act as a potential source of bioactive peptides when subjected to digestion in the GIT, and hence the GEP could be considered as a gut cryptome. The GEP and food peptidomes may together constitute the population of ExBP in the GIT. The study of the gut cryptome as a source of bioactive peptides, therefore, emerges as an important research area that merits further investigation.
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