Ganong's Review of Medical Physiology, 23rd Edition

454
SECTION V
Gastrointestinal Physiology

transport of glucose and galactose; it is transported instead by

facilitated diffusion from the intestinal lumen into the entero-

cytes by GLUT 5 and out of the enterocytes into the intersti-

tium by GLUT 2. Some fructose is converted to glucose in the

mucosal cells.

Insulin has little effect on intestinal transport of sugars. In

this respect, intestinal absorption resembles glucose reabsorp-

tion in the proximal convoluted tubules of the kidneys (see

Chapter 38); neither process requires phosphorylation, and

both are essentially normal in diabetes but are depressed by

the drug phlorizin. The maximal rate of glucose absorption

from the intestine is about 120 g/h.

PROTEINS & NUCLEIC ACIDS

PROTEIN DIGESTION

Protein digestion begins in the stomach, where pepsins cleave

some of the peptide linkages. Like many of the other enzymes

concerned with protein digestion, pepsins are secreted in the

form of inactive precursors

(proenzymes)

and activated in the

CLINICAL BOX 27–1

Lactose Intolerance

In most mammals and in many races of humans, intestinal

lactase activity is high at birth, then declines to low levels

during childhood and adulthood. The low lactase levels are

associated with intolerance to milk

(lactose intolerance).

Most Europeans and their American descendants retain

sufficient intestinal lactase activity in adulthood; the inci-

dence of lactase deficiency in northern and western Euro-

peans is only about 15%. However, the incidence in blacks,

American Indians, Asians, and Mediterranean populations

is 70–100%. When such individuals ingest dairy products,

they are unable to digest lactose sufficiently, and so symp-

toms such as bloating, pain, gas, and diarrhea are produced

by the unabsorbed osmoles that are subsequently digested

by colonic bacteria. Milk intolerance can be ameliorated by

administration of commercial lactase preparations, but this

is expensive. Yogurt is better tolerated than milk in intoler-

ant individuals because it contains its own bacterial lactase.

FIGURE 27–2
Brush border digestion and assimilation of the disaccharides sucrose (panel 1) and lactose (panel 2).
SGLT-1, sodium-
glucose cotransporter-1.

1

Sucrase

Isomaltase

Na+

Sucrose

Cytosol

Glucose

Fructose

2

Na+

Cytosol

Glucose

Galactose

Lactose

Lactase

SGLT-1

SGLT-1 GLUT5

Brush border

membrane