224
chapter 12
Gastrointestinal Digestion and Absorption
occurs in a pancreatic islet-cell tumor (pancreatic cholera)
that produces large quantities of VIP (Table 12-1). VIP
activates adenylate cyclase by a different mechanism from
that of cholera toxin.
12.5 Thermic Effect of Food
Energy balance depends on energy intake, energy expen-
diture, and existing energy stores (Chapter 5). Energy is
expended in digestion, absorption, transport, metabolism,
and storage of food. The level varies with type of food
ingested and its metabolic rate. Energy costs for the pro-
cessing of lipids, carbohydrates, and proteins are 4%, 5%,
and 30% of their energy content, respectively. Lipogenesis
from carbohydrate also has a high metabolic cost. Part
of this energy appears as heat energy and is variously re-
ferred to as the
thermic effect of food, diet-induced thermo-
genesis,
or
specific dynamic action
of food. The magnitude
of this thermic effect depends upon the food, nutritional
state, and antecedent diet. The thermic effect of food
accounts for about
1 0
% of the daily energy expendi-
ture and exhibits interindividual variation. Activation of
the sympathetic nervous system and secretion of thyroid
hormone (Chapter 33) contribute significantly to diet-
induced thermogenesis. Some forms of obesity may result
from decreased thermic effect of food.
In laboratory rodents, diet-induced thermogenesis by
sympathetic nervous stimulation in brown adipose tissue
plays a significant role in energy expenditures. In most
mammals, brown adipose tissue is present to a much
smaller extent and is located in the interscapular, sub-
scapular, and axillary regions, the nape of the neck, along
the length of the great vessels in the thorax and abdo-
men, and in small patches between the ribs. Hibernating
animals and those adapted to living in a cold environment
have an abundant supply of brown adipose tissue. Brown
adipose tissue cells are metabolically more active than
adipocytes and contain a larger supply of mitochondria
and usually several small lipid droplets.
Norepinephrine secreted at sympathetic nerve endings
binds to /J-adrenergic receptors on brown adipose tissue
cells and initiates cAMP-dependent triacylglycerol lipase
activity (Chapter 22). Accelerated lipolysis of triacylglyc-
erol and oxidation in mitochondria of fatty acids ensue.
These mitochondria have a specific regulator mechanism
for proton conductance. Fatty acids provide substrates
and promote proton conductance, so that a proton gra-
dient generated by electron transport and required to drive
ATP synthesis is not established (Chapter 14). Uncoupling
of mitochondria leads to increased heat production. In
laboratory-inbred obese mice, obesity is associated with
defective thermogenesis in brown adipose tissue and de-
creased energy expenditure. No direct evidence links obe-
sity in humans with defective thermogenesis in brown
adipose tissue.
Supplemental Readings and References
Q. Aziz and D. G. Thompson: Brain-gut axis in health and disease.
G a s-
tro en tero lo g y
114,559 (1998).
M. J. Blaser:
H e lic o b a cte r p y lo ri
and gastric diseases.
B ritish M ed ica l J o u r-
n a l
316, 1507 (1998).
D. Branski and R. Troncone: Celiac disease: A reappraisal.
J o u rn a l o f P edi-
a trics
133, 181 (1998).
Gastrointestinal hormones in medicine.
E n d o crin o lo g y a n d M eta b o lism
C lin ics o f N o rth A m e rica
22(4), 709 (1993). This reference includes many
articles on gastrointestinal hormones.
First Multi-Disciplinary International Symposium on Supraesophageal
Complications of Reflux Disease. R. Shaker, Guest Editor.
A m erica n J o u r-
n a l o f M ed ic in e
103(5A), 1997 (The whole issue deals with gastrointes-
tinal esophageal reflux disease).
L. B. Lovat; Age-related changes in gut physiology and nutritional status.
G u t
38, 306(1996).
L. D. McBean and G. D. Miller: Allaying fears and fallacies about lac-
tose intolerance.
Jo u rn a l o f A m e rica n D ietetic A sso cia tio n
98, 671
(1998).
P. S. Mead and P. M. Griffin: Escherichia coli 0157:H7.
L a n c e t
352, 1207
(1998).
A. D. O’Brien, V. L. Tesh, A. Donohue-Rolf, et al.: Shiga toxin: Biochem-
istry, genetics, mode of action, and role in pathogenesis.
C u rren t Topics
in M icro b io lo g y a n d Im m u n o lo g y
180, 65 (1992).
R. M. Peek and M. J. Blaser: Pathophysiology of
H e lico b a cter p ylo ri-
induced gastritis and peptic ulcer disease.
A m erica n J o u rn a l o f M ed icin e
102,200(1997).
J. P. Raufman: Cholera.
A m erica n J o u rn a l o f M ed icin e
104, 386 (1997)
S. M. Schweibert, D. J. Benos, and C. M. Fuller: Cystic Fibrosis: A multi-
ple exocrinopathy caused by dysfunctions in a multifunctional transport
protein.
A m e rica n J o u rn a l o f M e d icin e
104, 576 (1998).
R. C. Stern: The diagnosis of cystic fibrosis.
N ew E n g la n d Jo u rn a l o f
M ed icin e
336,487 (1997).
