709
CHAPTER EIGHTEEN
Nutrition and Metabolism
Fat-Soluble Vitamins
Fat-soluble vitamins dissolve in fats, so they associate with
lipids and are infl uenced by the same factors that affect lipid
absorption. For example, bile salts in the intestine promote
absorption of fat-soluble vitamins. As a group, the fat-soluble
vitamins are stored in moderate quantities in various tissues,
which is why excess intake can lead to overdose conditions.
Fat-soluble vitamins resist the effects of heat, so cooking and
food processing do not usually destroy them.
PRACTICE
29
What are vitamins?
30
How are vitamins classif
ed?
31
How do bile salts aF
ect the absorption o± ±at-soluble vitamins?
Vitamin A
exists in several forms, including retinol and
retinal (retinene). Body cells synthesize this vitamin from a
group of yellowish plant pigments, provitamins called
car-
otenes
(f
g. 18.10)
. Excess vitamin A or its precursors are
mainly stored in the liver, which regulates their concentra-
tion in the body. An adult’s liver stores enough vitamin A
to supply body requirements for a year. Infants and children
usually lack such reserves and are therefore more likely to
develop vitamin A de±
ciencies if their diets are inadequate.
from a debilitating illness might consume more carbohy-
drates, whereas a bodybuilder might eat extra protein to
hasten muscle development. An infant also needs to gain
weight rapidly, best accomplished by drinking human milk,
which has more total carbohydrate than prepared formulas.
The high fat content of human milk is important for the
rapid growth of the infant’s brain, where many neurons are
ensheathed in lipids.
PRACTICE
26
What is desirable weight?
27
Distinguish between being overweight and obese.
28
Under what conditions is weight gain desirable?
18.6
VITAMINS
Vitamins
(vi
tah-minz) are organic compounds (other
than carbohydrates, lipids, and proteins) required in small
amounts for normal metabolism, that body cells cannot syn-
thesize in adequate amounts. Vitamins are essential nutri-
ents that must come directly from foods or indirectly from
provitamins,
which are precursor substances.
Vitamins are classi±
ed on the basis of solubility, because
some are soluble in fats (or fat solvents) and others are sol-
uble in water.
Fat-soluble
vitamins are A, D, E, and K; the
water-soluble
group includes the B vitamins and vitamin C.
Table 18.6
lists, and corrects, some common misconceptions
about vitamins.
DiF
erent species have diF
erent vitamin requirements. ²or example,
ascorbic acid is a required vitamin (C) in humans, guinea pigs, and
Indian ±ruit bats, but not in other animals, which can manu±acture
their own.
H
3
C
H
3
C
C
CC
H
2
CH
2
C
C
H
2
H
2
Beta carotene
H
3
C
CH
3
CH
3
CH
3
(CH
C
CH
CH
CH
CH)
2
CH
3
CH
3
(CH
C
CH
CH)
2
H
2
C
CC
C
C
C
Retinal (retinene)
H
3
C
CH
3
CH
3
CH
3
(CH
C
CH
C
H
O
CH)
2
H
2
C
H
2
C
C
C
C
C
Retinol
H
3
C
CH
3
CH
3
CH
3
(CH
C
CH
C
OH
H
H
CH)
2
H
2
C
H
2
C
C
C
C
C
H
2
H
2
H
2
FIGURE 18.10
A molecule o± beta carotene can react to ±orm two molecules o± retinal, which, in turn, can react to ±orm retinol.
TABLE
18.6
|
Vitamin Fallacies and Facts
Fallacy
Fact
The more vitamins,
the better
Too much o± a water-soluble vitamin results in
excretion o± the vitamin through urination; too
much o± a ±at-soluble vitamin can harm health
A varied diet provides
all needed vitamins
Many people do need vitamin supplements,
particularly pregnant and breast-±eeding women
Vitamins provide
energy
Vitamins do not directly supply energy; they aid
in the release o± energy ±rom carbohydrates, ±ats,
and proteins
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