section 12.3

Digestion and Absorption of Major Food Substances

213

Outside

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.

302,

111

(1980).]

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

intolerance.

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

populations.

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

diarrhea.

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-

hydrates.

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

3

is trapped as NHj ions (Chapter 9).