Biology of Disease

ABSORPTION OF THE PRODUCTS OF DIGESTION

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been described in some patients. Malnutrition and a low-protein
diet are the primary contributing factors to morbidity. In rare
cases, severe central nervous system (CNS) damage may lead to
death.

Plasma concentrations of amino acids are usually normal in
Hartnup’s disease. However, it may be diagnosed by demon-
strating increased concentrations of neutral amino acids, such
as tryptophan, glutamine, valine, phenylalanine, leucine and
isoleucine in the urine, with normal levels of proline, hydroxy-
proline and arginine, which differentiates Hartnup’s disease from
other causes of gross aminoaciduria. Other tryptophan metabo-
lites, including kynurenine and serotonin, are also found in the
urine and neutral amino acids occur in the feces. Urinary indole
derivatives, for example 5-hydroxyindoleacetic acid, may be

demonstrated following an oral tryptophan load (Chapter 10).

In selected patients, diagnosis may require the transport defect

be identified in vitro using a jejunal sample obtained by biopsy.

Most patients can be treated with a high-protein diet which over-

comes the deficient transport of neutral amino acids because

dipeptides can be actively absorbed as shown in Figure 11.18

and by paracellular routes. Indeed, in the developed world,

cases presenting with extreme clinical symptoms are rare prob-

ably because of the protein rich diet available. Patients who are

symptomatic are advised to avoid excessive exposure to sunlight

and use sunscreens with a skin protection factor of at least 15.

Dietary supplements of 40 to 200 mg niacin daily relieve the

pellagra-like symptoms. Psychiatric treatment may help patients

with severe CNS damage.

Lacteal

GIT lumen

Tight junction

Free fatty acid

Monoacylglycerol

Mixed micelle

(with fat soluble vitamins)

Triacylglycerol

Chylomicron

MM

MM

Figure 11.20The absorption of the products of

triacylglycerol digestion and their reformation in

an enterocyte. Transport movements are denoted

by colored lines, chemical transformations in

black.See text for general details.

num. Dietary iron occurs in nonheme and heme forms and their absorption
by duodenal enterocytes is by different mechanisms. Dietary nonheme iron
occurs in ferrous (Fe(II)2+) and ferric (Fe(III)3+) forms. Ferrous iron is solu-
ble up to a pH of about 7 but the predominant form, ferric iron, is sparingly
soluble above pH 3 and is not available for absorption and must be reduced
before it can be transported across the intestinal epithelium. A ferrireductase