560
Chapter 15
in jaundice, which normally clears spontaneously. Excessive
accumulation of bilirubin during the neonatal period, as
occurs, for example, with hemolytic disease of the newborn
(Chapter 18), carries a risk of bilirubin-induced neurological
damage during a critical phase in the development of the ner-
vous system.
Although surgery may be necessary to remove an infl
amed
gallbladder (
cholecystectomy
) or stones from an obstructed duct,
newer techniques use drugs to dissolve gallstones. Patients who
have had a cholecystectomy still make bile and transport it to
the small intestine via the bile duct. Therefore, fat digestion and
absorption can be maintained, but bile secretion and fat intake
in the diet are no longer coupled. Thus, large, fatty meals are
diffi cult to digest because of the absence of a large pool of bile
normally released from the gallbladder in response to CCK. A
diet low in fat content is usually advisable.
Lactose Intolerance
Lactose is the major carbohydrate in milk. It cannot be absorbed
directly but must fi rst be digested into its components—glucose
and galactose—which are readily absorbed by active transport.
Lactose is digested by the enzyme
lactase,
which is embedded
in the luminal plasma membranes of intestinal epithelial cells.
Lactase is usually present at birth and allows the nursing infant
to digest the lactose in breast milk. Most Asians and many
people with ancestors from certain areas of Africa undergo a
decline in lactase production at about two years of age. This
leads to
lactose intolerance
—the inability to completely digest
lactose such that its concentration increases in the small intes-
tine. Most people of Northern European descent maintain the
ability to digest lactose as adults.
Because the absorption of water requires prior absorp-
tion of solute to provide an osmotic gradient, the unabsorbed
lactose in persons with lactose intolerance prevents some of
the water from being absorbed. This lactose-containing fl
uid
is passed on to the large intestine, where bacteria digest the
lactose. They then metabolize the released monosaccharides,
producing large quantities of gas (which distends the colon,
producing pain) and short-chain fatty acids, which cause fl
uid
movement into the lumen of the large intestine, producing
diarrhea. The response to ingestion of milk or dairy products
by adults whose lactase levels have diminished varies from mild
discomfort to severely dehydrating diarrhea, according to the
volume of milk and dairy products ingested and the amount
of lactase present in the intestine. The person can avoid these
symptoms by either drinking milk in which the lactose has
been predigested or taking pills containing lactase along with
the milk.
Infl ammatory Bowel Disease
The general term infl ammatory bowel disease (IBD) com-
prises two related diseases—Crohn’s disease and ulcerative
colitis, both of which have already been mentioned earlier in
this chapter. Both diseases involve chronic infl
ammation of
the bowel. Crohn’s disease can occur anywhere along the GI
tract from the mouth to the anus, although it is most com-
mon near the end of the ileum. Colitis is confi ned to the
colon. The incidence of IBD in the United States is 7 to 11
per 100,000 people and is most common in Caucasians, par-
ticularly Ashkenazi Jews. The most common ages of onset for
IBD are in the late teens to early twenties and then again in
people older than 60.
Although the precise cause or causes of IBD are not cer-
tain, it seems that it occurs as a combination of environmental
and genetic factors. There appears to be a genetic predisposi-
tion for an abnormal response of the bowel mucosa to infec-
tion and the presence of normal luminal bacteria. Therefore,
IBD appears to result from inappropriate immune and tissue
repair responses to essentially normal microorganisms in the
intestinal lumen.
Ulcerative colitis is caused by disruption of the normal
mucosa with the presence of bleeding, edema, and ulcerations
(losses of tissue due to infl ammation). In its most extreme, the
bowel wall can get so thin and the loss of tissue so great that
holes (perforations) that go all the way through the bowel wall
can occur. Active Crohn’s disease shows infl
ammation and
thickening of the bowel wall such that the lumen can become
narrowed to the point where it may even become blocked, or
obstructed, which can be very painful.
The main symptoms of ulcerative colitis are diarrhea,
rectal bleeding, and abdominal cramps. The part of the small
intestine at the end of the ileum is the most common site of
Crohn’s disease, so the fi rst symptoms felt by patients with
this disease are often pain in the lower right abdomen and
diarrhea. Because it is often accompanied by fever due to the
exaggerated immune response, the initial symptoms can be
mistaken for acute appendicitis (Chapter 19). Because of its
obstructive nature, the abdominal pain in Crohn’s disease is
often relieved temporarily by defecation.
The current initial treatment of IBD is the use of 5-
aminosalicylate drugs, such as
sulfasalazine,
which appear to
have both antibacterial and anti-infl ammatory effects. In more
severe cases, the use of glucocorticoids as anti-infl
ammatory
agents can be very useful, although their overuse has signifi -
cant risks such as loss of bone mass. It is often helpful to make
adjustments in the diet to allow the infl amed bowel time to
heal. Finally, new drug therapy using immunosuppressive
medicines such as
tacrolimus
and
cyclosporine
show promise.
When IBD becomes severe, it is sometimes necessary to per-
form surgery to remove the diseased bowel.
Constipation and Diarrhea
Many people have a mistaken belief that, unless they have a
bowel movement every day, the absorption of “toxic” sub-
stances from fecal material in the large intestine will some-
how poison them. Attempts to identify such toxic agents in
the blood following prolonged periods of fecal retention have
been unsuccessful, and there appears to be no physiological
necessity for having bowel movements at frequent intervals.
Whatever maintains a person in a comfortable state is physi-
ologically adequate, whether this means a bowel movement
after every meal, once a day, or only once a week.
On the other hand, some symptoms—headache, loss of
appetite, nausea, and abdominal distension—may arise when
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