Ganong's Review of Medical Physiology, 23rd Edition

466 SECTION V Gastrointestinal Physiology

The diseases caused by deficiency of each of the vitamins
are listed in Table 27–4. It is worth remembering, however,
particularly in view of the advertising campaigns for vitamin
pills and supplements, that very large doses of the fat-soluble
vitamins are definitely toxic. Hypervitaminosis A is charac-
terized by anorexia, headache, hepatosplenomegaly, irritabil-
ity, scaly dermatitis, patchy loss of hair, bone pain, and
hyperostosis. Acute vitamin A intoxication was first described
by Arctic explorers, who developed headache, diarrhea, and
dizziness after eating polar bear liver. The liver of this animal
is particularly rich in vitamin A. Hypervitaminosis D is asso-
ciated with weight loss, calcification of many soft tissues, and
eventual renal failure. Hypervitaminosis K is characterized
by gastrointestinal disturbances and anemia. Large doses of
water-soluble vitamins have been thought to be less likely to
cause problems because they can be rapidly cleared from the
body. However, it has been demonstrated that ingestion of
megadoses of pyridoxine (vitamin B 6 ) can produce peripheral
neuropathy.

CHAPTER SUMMARY

■ A typical mixed meal consists of carbohydrates, proteins, and

lipids (the latter largely in the form of triglycerides). Each must

be digested to allow its uptake into the body. Specific transport-

ers carry the products of digestion into the body.

■ In the process of carbohydrate assimilation, the epithelium can

only transport monomers, whereas for proteins, short peptides

can be absorbed in addition to amino acids.

■ The protein assimilation machinery, which rests heavily on the

proteases in pancreatic juice, is arranged such that these en-

zymes are not activated until they reach their substrates in the

small intestinal lumen. This is accomplished by the restricted

localization of an activating enzyme, enterokinase.

■ Lipids face special challenges to assimilation given their hydro-

phobicity. Bile acids solubilize the products of lipolysis in

micelles and accelerate their ability to diffuse to the epithelial

surface. The assimilation of triglycerides is enhanced by this

mechanism, whereas that of cholesterol and fat-soluble vitamins

absolutely requires it.

■ The catabolism of nutrients provides energy to the body in a

controlled fashion, via stepwise oxidations and other reactions.

■ A balanced diet is important for health, and certain substances

obtained from the diet are essential to life. The caloric value of

dietary intake must be approximately equal to energy expendi-

ture for homeostasis.

MULTIPLE-CHOICE QUESTIONS

For all questions, select the single best answer unless otherwise directed.

1. Maximum absorption of short-chain fatty acids produced by
   bacteria occurs in the
   A) stomach.
   B) duodenum.
   C) jejunum.
   D) ileum.
   E) colon.


2. Calcium absorption is increased by
   A) hypercalcemia.
   B) oxalates in the diet.
   C) iron overload.
   D) 1,25-dihydroxycholecalciferol.
   E) increased Na+ absorption.

D group Increase intestinal ab-

sorption of calcium and

phosphate (see Chapter

21: Hormonal Control of

Calcium & Phosphate

Metabolism & the Physi-

ology of Bone)

Rickets Fish liver Family of sterols (see Chapter 21: Hormonal Con-

trol of Calcium & Phosphate Metabolism & the

Physiology of Bone)

E group Antioxidants; cofactors in

electron transport in cy-

tochrome chain?

Ataxia and other symp-

toms and signs of

spinocerebellar dys-

function

Milk, eggs, meat,

leafy vegetables

K group Catalyze γ carboxylation

of glutamic acid residues

on various proteins con-

cerned with blood clot-

ting

Hemorrhagic phenom-

ena

Leafy green vege-

tables

aCholine is synthesized in the body in small amounts, but it has recently been added to the list of essential nutrients.

TABLE 27–4 Vitamins essential or probably essential to human nutrition.a (Continued)

Vitamin Action Deficiency Symptoms Sources Chemistry

CH 3

CH 3

H 2

H 3 C

O

α-Tocopherol;β- and γ-tocopherol also active.

HO

(C H 2 ) 3 (C H 2 ) 3 (C H 2 ) 3

CH 3

CH 3

CH CH CH

CH 3 CH 3

CH 3

H 2

O

O

CH 3

Vitamin K 3 ; a large number

of similar compounds have

biological activity.