Ph D Thesis Amelie Deglaire - TEL

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which depends on extracellular glycosidases and sulphatases of enteric microorganisms
(Neutra & Forstner, 1987). Because of this high proteolytic resistance, especially in the
small intestine, mucins are an important component of ileal endogenous protein losses
(Montagne et al., 2004). Lien et al. (1997b) estimated the daily ileal output of mucins to
be 5.3−5.6 g/d in 55-kg pigs fed a protein-free or synthetic AA diet, which contributed to
11% of ileal endogenous protein. Threonine, proline and serine are predominant AA in
mucins and reached 35, 24 and 16% of the ileal endogenous AA recovered at the terminal
ileum (Lien et al., 1997b). In particular, gut threonine loss makes a significant
contribution to the metabolic requirement for threonine (Gaudichon et al., 2002).
Dietary components have been reported to modulate mucin synthesis quantitatively and
qualitatively (Montagne et al., 2004). Dietary fibres were shown to increase mucin
excretion at the terminal ileum (Mariscal-Landin et al., 1995; Lien et al., 2001; Morel et
al., 2003), with effects depending on the solubility and the nature of the fibres
influencing mucus erosion and proteolytic degradation, respectively. The specific effect
of bulk- forming (e.g. fibre swelling due to water absorbtion in the gut) was previously
shown to increase mucin secretion in the small and large intestine of rats, whereas
fermentable components increased mucin secretion only in the caecum (Tanabe et al.,
2006). Dietary protein, although less studied than fibre, has been shown to modify the
recovery of mucin in endogenous protein. In calves fed diets containing 14 and 28 g
crude protein/kg dry matter intake (DMI), the duodenal flow of mucin protein increased
from 1.1 to 2.4 g/kg DMI, respectively, but no differences were observed for endogenous
protein recovered at the terminal ileum (Montagne et al., 2000). This was assumed to be
due to the higher secretions of enzymes observed after an increase in the dietary protein
intake, thus leading to a higher enzymatic degradation of mucins. In pigs fed a diet
containing isolated soyabean protein, the hexosamine ileal excretion increased when the
dietary crude protein content exceeded 55 g/kg dry matter, suggesting an increased ileal
output of mucins (Mariscal-Landin et al., 1995). In isolated vascularly perfused rat
jejunum, enzymatic casein hydrolysate (0.5–5%) from bovine milk markedly stimulated
rat intestinal mucus secretion (Claustre et al., 2002). The intraluminal perfusion of βcasomorphin,
an opioid peptide released during milk digestion, reproduced this mucin
release in rat jejunum. Further data suggest that nutrient-derived opiate materials and
opioid neuropeptides may participate in the regulation of intestinal mucus discharge
(Trompette et al., 2003). Intact protein did not have any impact on mucin secretion in
isolated rat jejunum, emphasizing the importance of protein gastric digestion (Claustre et