216
chapter
12
Gastrointestinal Digestion and Absorption
OH
h 2o
r A
COOH
p-Aminobenzoic acid (PABA)
Absorbed and excreted in urine
as p-aminobenzoylglycine
F I G U R E 1 2 -1 2
H ydrolysis o f a synthetic tripeptide by Chymotrypsin in the small intestine.
The hydrolysis yields PABA, w hich is absorbed and eventually excreted in
the urine. The concentration o f PABA is a m easure o f exocrine pancreatic
function.
than one transport system; within each group, amino
acids compete with each other for transport. These sys-
tems are specific for
L -
Amino acids.
D -
Amino acids are
transported by a passive diffusion process. Amino acid
transport in the renal cells may use similar systems
(Chapter 39). Entry of amino acids into cell compart-
ments elsewhere in the body may require different trans-
port systems (e.g., the /-glutamyl cycle, Chapter 17). In
the enterocyte, amino acids may be metabolized or trans-
ported to the liver. Glutamate, glutamine, aspartate, and
asparagine are metabolized in the enterocyte (Chapter 17).
Small amounts of protein (e.g., dietary, bacterial, and vi-
ral) may be absorbed intact by nonselective pinocytosis.
This absorption may be more common in neonatal life.
Absorption of food proteins (or antigenic peptides from
them) can cause allergic manifestations, whereas bacte-
rial and viral antigens stimulate immunity by produc-
tion and secretion of secretory IgA (Chapter 35). Since
thyrotropin-releasing hormone (pGlu-His-Pro-NH2) is
resistant to hydrolysis, it is effective if taken orally
(Chapter 33).
D iso rd ers o f P ro tein D ig estio n a n d A b so rp tio n
The principal causes of protein maldigestion and
malabsorption are diseases of the exocrine pancreas and
small intestine. Primary isolated deficiency of pepsinogen
or pepsin, affecting protein assimilation, has not been de-
scribed. Deficiencies of trypsinogen and enteropeptidase
are rare. Defects in neutral amino acid transport
(Hartnup
disease),
in basic amino acids and cystine (
cystinuria
),
dicarboxylic aminoaciduria, and aminoglycinuria have
been reported. The clinical severity of these disorders is
usually minimal and relates to the loss of amino acids or
relative insolubility of certain amino acids in the urine.
In cystinuria, for example, cystine can precipitate in
acidic urine to form stones. In Hartnup disease, severe
nutritional deficiencies are uncommon, since the essential
amino acids are absorbed as dipeptides or oligopeptides.
Tryptophan, a precursor of nicotinamide (a vitamin; see
Chapter 38), and NAD+ and NADP+ (Chapter 17) are lost
in this disease; skin and neuropsychiatric manifestations
characteristic of nicotinamide deficiency respond to oral
nicotinamide supplementation.
Lipids
Dietary fat provides energy in a highly concentrated
form and accounts for 40-45% of the total daily energy
intake (100 g/day in the average Western diet). Lipids
contain more than twice the energy per unit mass than
carbohydrates and proteins (Chapter 5). The efficiency
of fat absorption is very high; under normal conditions,
almost all ingested fat is absorbed, with less than 5%
appearing in the feces. The predominant dietary lipid
is triacylglycerol (previously called triglyceride), which
contains three long-chain (16-carbon or longer) fatty
acids (Chapter 18). The dietary lipids include essential
fatty acids (Chapter 18) and the lipid-soluble vitamins A,
D, E, and K (Chapters 36-38).
Digestion and absorption of lipids involves the coordi-
nated function of several organs but can be divided into
three phases: luminal, intracellular, and secretory.
In tra lu m in a l P h a se
Digestion of lipid in the mouth and stomach is minimal.
However, lipases secreted by lingual glands at the base of
the tongue are active at acid pH and initiate the hydrolysis
of triacylglycerol without a requirement for bile acids.
The fatty acids released stimulate the release of CCK and
a flow of bile and pancreatic juice. The free fatty acids
also stabilize the surface of triacylglycerol particles and
promote binding of pancreatic colipase. This phase aids in
the optimal action of pancreatic lipase and is particularly
important in disorders of pancreatic function or secretion
(e.g., in prematurity, cystic fibrosis, congenital deficiency
of pancreatic lipase).
