Agriculture Reference
In-Depth Information
greater capacity for peptide transport immediately after birth (Boudry and Le Huërou-
Luron, personal communication). In the brush border membrane, the main systems for
di- and tri-peptides transport are the broadly specific systems B
o,+
for neutral amino
acids, B
o,+
(or y+) for cationic amino acids and X
2-
for anionic amino acids. In piglets,
a sharp decrease in uptake of various amino acids occurs in the first 24 h after birth. It
then returns to birth values at day 7 to further decline until weaning (Buddington
et al.
,
1996, Zhang
et al.
, 1997).
Fat digestion occurs in the small intestine and absorption of fatty acids is nearly
complete at the distal ileum. The whole process of fat digestion is dependent on the
exocrine pancreas that secretes three enzymes, namely, lipase, carboxyl ester hydrolase
and phospholipase A, and one cofactor, colipase, which acts in concert to digest dietary
fat. Suckling pigs digest the nutrients in sows' milk very efficiently, and an apparent
fat digestibility of 96% has been reported although the expression of pancreatic lipase,
which is the most important lipolysis enzyme is low in newborns (Jensen
et al.
, 1997).
Intestinal lipid absorption is a multistep process, traditionally divided into three steps:
apical absorption into the enterocyte, intracellular processing and subsequent release
into the lymphatic and portal circulation. After absorption, free fatty acids and sn-2
monoacylglycerols are bound to the intestinal fatty acid binding protein FABP (I-FABP)
and the liver FABP (L-FABP) to prevent their transport back to the intestinal lumen, and
to facilitate their transport to the endoplasmic reticulum where they are used for
de novo
synthesis of triglycerides and phospholipids. Little is known about the developmental
regulation of transporters, but evidence suggests that the intracellular lipid processing is
operational at birth. During the suckling period, the intestine is able to adapt to the high
lipid ingestion by increasing uptake of lipids.
15.2.4
Epithelial barrier permeability
The intestine is the first barrier for nutrients and luminal components and has a central
role in determining postnatal defence. Intestinal permeability decreases postnatally and
ingestion of colostrum accelerates this phenomena. The passage of macromolecules
(albumin, bovine serum albumin) occurs later when the animals are deprived of colostrum
(Westrom
et al.
, 1984). Cessation of
in vivo
passage of macromolecules greater than 1000
Da occurs at 3 weeks of age whereas the passage of smaller molecules persists at a rate
10 times lower than that seen immediately after birth (Westrom
et al.
, 1989). However,
in vitro
investigation of intestinal permeability using Ussing chambers demonstrates
important site-dependent developing patterns. Paracellular permeability in the jejunum
is high during the first 2 weeks of postnatal life and then decreases gradually until the end
of lactation, whereas in the ileum it increases during this period (De Quelen
et al.
, 2011,
Le Huërou-Luron
et al.
, 2010). This early passage of food and bacterial antigens through
the epithelial barrier contributes to the education of the intestinal immune system and
promotes the acquisition of tolerance (Menard
et al.
, 2010). Indeed, the presence of
proteins in the lumen of the digestive tract plays a critical role in the maturation of the
intestinal immune system (Menezes
et al.
, 2003).
