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SECTION V
Gastrointestinal Physiology
converted into glycerophospholipids that participate in chy-
lomicron formation. The acylation of glycerophosphate and
the formation of lipoproteins occur in the rough endoplasmic
reticulum. Carbohydrate moieties are added to the proteins in
the Golgi apparatus, and the finished chylomicrons are
extruded by exocytosis from the basal or lateral aspects of the
cell.
Absorption of long-chain fatty acids is greatest in the upper
parts of the small intestine, but appreciable amounts are also
absorbed in the ileum. On a moderate fat intake, 95% or more
of the ingested fat is absorbed. The processes involved in fat
absorption are not fully mature at birth, and infants fail to
absorb 10–15% of ingested fat. Thus, they are more susceptible
to the ill effects of disease processes that reduce fat absorption.
SHORT-CHAIN FATTY
ACIDS IN THE COLON
Increasing attention is being focused on short-chain fatty ac-
ids (SCFAs) that are produced in the colon and absorbed from
it. SCFAs are two- to five-carbon weak acids that have an av-
erage normal concentration of about 80 mmol/L in the lumen.
About 60% of this total is acetate, 25% propionate, and 15%
butyrate. They are formed by the action of colonic bacteria on
complex carbohydrates, resistant starches, and other compo-
nents of the dietary fiber, that is, the material that escapes di-
gestion in the upper gastrointestinal tract and enters the colon.
Absorbed SCFAs are metabolized and make a significant
contribution to the total caloric intake. In addition, they exert
a trophic effect on the colonic epithelial cells, combat inflam-
mation, and are absorbed in part by exchange for H
- , helping
to maintain acid–base equilibrium. SCFAs are absorbed by
specific transporters present in colonic epithelial cells. SCFAs
also promote the absorption of Na
+
, although the exact mech-
anism for coupled Na
+
–SCFA absorption is unsettled.ABSORPTION OF VITAMINS
& MINERALS
VITAMINS
Absorption of the fat-soluble vitamins A, D, E, and K is defi-
cient if fat absorption is depressed because of lack of pancreat-
ic enzymes or if bile is excluded from the intestine by
obstruction of the bile duct. Most vitamins are absorbed in the
upper small intestine, but vitamin B
12
is absorbed in the ileum.
This vitamin binds to intrinsic factor, a protein secreted by the
stomach, and the complex is absorbed across the ileal mucosa
(see Chapter 26).
Vitamin B
12
absorption and folate absorption are Na
+
-inde-
pendent, but all seven of the remaining water-soluble vita-
mins—thiamin, riboflavin, niacin, pyridoxine, pantothenate,
biotin, and ascorbic acid—are absorbed by carriers that are
Na
+
cotransporters.CALCIUM
A total of 30–80% of ingested calcium is absorbed. The absorp-
tive process and its relation to 1,25-dihydroxycholecalciferol
are discussed in Chapter 23. Through this vitamin D deriva-
tive, Ca
2+
absorption is adjusted to body needs; absorption is
increased in the presence of Ca
2+
deficiency and decreased in
the presence of Ca
2+
excess. Ca
2+
absorption is also facilitated
by protein. It is inhibited by phosphates and oxalates because
these anions form insoluble salts with Ca
2+
in the intestine.
Magnesium absorption is also facilitated by protein.IRON
In adults, the amount of iron lost from the body is relatively
small. The losses are generally unregulated, and total body
stores of iron are regulated by changes in the rate at which it is
absorbed from the intestine. Men lose about 0.6 mg/d, largely
in the stools. Women have a variable, larger loss averaging
about twice this value because of the additional iron lost dur-
ing menstruation. The average daily iron intake in the United
States and Europe is about 20 mg, but the amount absorbed is
equal only to the losses. Thus, the amount of iron absorbed is
normally about 3–6% of the amount ingested. Various dietary
factors affect the availability of iron for absorption; for exam-
ple, the phytic acid found in cereals reacts with iron to form
insoluble compounds in the intestine, as do phosphates and
oxalates.
Most of the iron in the diet is in the ferric (Fe
3+
) form,
whereas it is the ferrous (Fe
2+
) form that is absorbed. Fe
3+
reductase activity is associated with the iron transporter in theFIGURE 27–6
Intracellular handling of the products of lipid
digestion.
Absorbed fatty acids (FA) and monoglycerides (MG) are re-
esterified to form triglyceride (TG) in the smooth endoplasmic reticu-
lum. Apoproteins synthesized in the rough endoplasmic reticulum are
coated around lipid cores, and the resulting chylomicrons are secreted
from the basolateral pole of epithelial cells by exocytosis.
Rough ERGolgiSmooth ER FA/MGChylomicronsSynthesis of TG and
phospholipids
Synthesis of
apolipoproteins
Apolipoprotein
glycosylation
ExocytosisTG