section 12.3
Digestion and Absorption of Major Food Substances
FIGURE 12-11
Model for transport of Na+ and K+ by Na+,K+-ATPase. Inside the cell, Na+ initiates the one-way ion exchange cycle by
the phosphorylation of an aspartate residue at the active site by ATP, which eventually leads to the translocation of Na+
and K+ . Conformational changes of the enzyme occur during the exchange of ions. [Reproduced, with permission, from
K. J. Sweadner and S. M. Goldin, Active transport of sodium and potassium ions.
New Engl. J. M ed.
is rare and is transmitted as an autosomal recessive trait.
Prompt diagnosis and a lactose-free diet assure the infant’s
normal growth. Since hydrolysis of lactose by lactase
is rate-limiting, any mucosal damage will cause lactose
In full-term human infants, lactase activity attains peak
values at birth and remains high throughout infancy. As
milk intake decreases, lactase levels drop and lactose in-
tolerance may develop. The extent of the decrease of lac-
tase activity distinguishes lactose-tolerant from intolerant
Ingestion of milk (or lactose) by individuals who have
lactose intolerance leads to a variety of symptoms (bloat-
ing, cramps, flatulence, and loose stools). Severity of the
symptoms depends on the amount of lactose consumed
and the enzyme activity. Symptoms disappear with elim-
ination of lactose from the diet. Eight ounces of milk
contains about 12 g of lactose. Some individuals show
symptoms with consumption of as little as 3 ounces of
milk. Some lactose-intolerant individuals can drink milk
if it is consumed along with meals and show no reac-
tion to milk added to cereal or coffee. Lactose-depleted
milk or fermented milk products with negligible amounts
of lactose are good substitutes for milk. The intestinal
problems are due primarily to osmotic effects of lac-
tose and its metabolites in the colon. The lactose not
absorbed in the small intestine increases the osmolar-
ity and causes water to be retained in the lumen. In
the colon, it is metabolized by bacterial enzymes to a
number of short-chain acids, further increasing osmolar-
ity and aggravating fluid reabsorption. The bacterial fer-
mentation also yields gaseous products (H2, C 02, and
CH4), hence the bloating, flatulence, and sometimes frothy
A lactose tolerance test is performed by oral administra-
tion of a load of lactose
( 1 0 0
g in an adult), measurement of
blood glucose at specific time intervals, and occurrence of
characteristic symptoms. The values for blood glucose are
compared with those from an oral glucose tolerance test
performed the previous day on the same individual. The
hydrogen breath test can also be used in the assessment
of lactose malabsorption. Hydrogen is a product of bacte-
rial fermentation of undigested sugar in the colon and is
not a product of human metabolism. Some of this hydro-
gen is absorbed and excreted by the lungs. Measurement
of the amount of hydrogen in the expired air by gas chro-
matographic studies provides a measure of the unabsorbed
carbohydrate in the colon. Both tests can be adapted for
the diagnosis of malabsorption disorders for other carbo-
Intolerance of Other Carbohydrates
Intolerance to
sucrose and a-limit dextrins may be due to deficiency of
sucrase-a-dextrinase or to a defect in glucose-galactose
transport. These disorders are rare autosomal recessive
traits; clinical problems can be corrected by removing
the offending sugars from the diet. Lactulose, a. synthetic
disaccharide consisting of galactose and fructose with a
/1(1 -> 4) linkage, is hydrolyzed not in the small intestine
but in the colon and is converted to products similar to
those derived from lactose fermentation. It has been used
in the treatment of patients with liver disease. Normally,
ammonia (NH3) produced in the GI tract is converted in the
liver to urea (Chapter 17), hence, in patients with severe
liver disease, blood ammonia levels increase. Absorption
of ammonia can be decreased by administration of lactu-
lose, which acidifies the colonic constituents so that NH
is trapped as NHj ions (Chapter 9).
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