Biology of Disease

the urine. This property is exploited in the xylose absorption test to assess

the malabsorption of carbohydrates. The patient fasts overnight, empties the

bladder and drinks a 500 cm^3 solution containing 5 g xylose. In normal indi-

viduals, the serum xylose concentration increases above 1.3 mmol dm–3 one

hour after the test. After 5 h, the concentration of xylose in the urine increases

to more than 7.0 mmol dm–3. Significantly lower concentrations of xylose

occur in the serum of patients with carbohydrate malabsorption. However,

care must be exercised since some bacteria colonizing the small intestine are

capable of metabolizing xylose and a number of renal diseases can also lead

to reduced concentrations.

Malabsorption of fat can occur in a number of pancreatic and intestinal dis-

orders. Bacteria colonizing the small intestine may break down bile acids,

reducing their effective concentration and causing malabsorption. A fecal

fat test can assess fat malabsorption. The test involves collecting feces over

a period of three days after which their fat content is assessed chemically.

Normally, up to 5 g of fat is lost in the feces each day but more is lost during

malabsorption giving rise to steatorrhea.

X]VeiZg&&/ DISORDERS OF THE GASTROINTESTINAL TRACT, PANCREAS, LIVER AND GALL BLADDER

(%- W^dad\nd[Y^hZVhZ

Figure 11.35Schematic showing the clinical features associated with malabsorption.

Depression

Night blindness

Anemia

Angular stomatitis

Glossitis

Bleeding gums

Follicular hyperkeratosis

Acrodermatitis enteropathica

Koilonychia

Paresthesia, tetany

Clubbing

Osteomalacia, rickets

Muscle-wasting

Proximal myopathy

Peripheral neuropathy

Peripheral edema

Purpura, bruising,

impaired wound healing

Steatorrhoea,

watery diarrhea

Figure 11.34Photographs of gastrointestinal

tract biopsies from (A) a healthy person and (B) a

celiac patient. Note the degeneration of the villi.