Introduction to Human Nutrition

180 Introduction to Human Nutrition

● decreased serum cholesterol and decreased urinary
excretion of 17-ketosteroids, refl ecting the impair-
ment of steroidogenesis
● decreased acetylation of p-aminobenzoic acid, sul-
fonamides and other drugs, refl ecting reduced
availability of acetyl-CoA for these reactions
● increased susceptibility to upper respiratory tract
infections.

There is no evidence on which to estimate panto-
thenic acid requirements. Average intakes are between
3 mg/day and 7 mg/day, and since defi ciency does not
occur, such intakes are obviously more than adequate
to meet requirements.

Non-nutritional uses of pantothenic acid

Blood levels of pantothenic acid have been reported
to be low in patients with rheumatoid arthritis; some
workers have reported apparently benefi cial effects of
supplementation, but these reports remain uncon-
fi rmed and there are no established pharmacological
uses of the vitamin.
Pantothenic acid defi ciency in rats leads to a loss of
fur color and at one time pantothenic acid was known
as the “anti-grey hair factor.” There is no evidence that
the normal graying of hair with age is related to pan-
tothenic acid nutrition, or that pantothenic acid sup-
plements have any effect on hair color. Its use in
shampoo is not based on any evidence of effi cacy.
Pantothenic acid has very low toxicity; intakes of up
to 10 g/day of calcium pantothenate (compared with a
normal dietary intake of 2–7 mg/day) have been given
for up to 6 weeks with no apparent ill-effects.

8.14 Vitamin C (ascorbic acid)

Vitamin C is a vitamin for only a limited number of
vertebrate species: humans and the other primates, the

guinea pig, bats, the passeriform birds, and most fi shes.

Ascorbate is synthesized as an intermediate in the gulo-

nolactone pathway of glucose metabolism; in those

vertebrate species for which it is a vitamin, one enzyme

of the pathway, gulonolactone oxidase, is absent.

The vitamin C defi ciency disease, scurvy, has been

known for many centuries and was described in the

Ebers papyrus of 1500 bc and by Hippocrates. The

Crusaders are said to have lost more men through

scurvy than were killed in battle, while in some of the

long voyages of exploration of the fourteenth and

fi fteenth centuries up to 90% of the crew died from

scurvy. Cartier’s expedition to Quebec in 1535 was

struck by scurvy; the local native Americans taught

him to use an infusion of swamp spruce leaves to

prevent or cure the condition.

Recognition that scurvy was due to a dietary defi -

ciency came relatively early. James Lind demonstrated

in 1757 that orange juice and lemon juice were pro-

tective, and Cook kept his crew in good health during

his circumnavigation of the globe (1772–1775) by

stopping frequently to take on fresh fruit and vegeta-

bles. In 1804 the British Navy decreed a daily ration

of lemon or lime juice for all ratings, a requirement

that was extended to the merchant navy in 1865.

The structure of vitamin C is shown in Figure 8.19;

both ascorbic acid and dehydroascorbic acid have

vitamin activity. Monodehydroascorbate is a stable

radical formed by reaction of ascorbate with reactive

oxygen species, and can be reduced back to ascorbate

by monodehydroascorbate reductase. Alternatively, 2

mol of monodehydroascorbate can react together to

yield 1 mol each of ascorbate and dehydroascorbate.

Dehydroascorbate may either be reduced to ascorbate

or undergo hydration to diketogulonate and onward

metabolism.

Vitamin C is found in fruits and vegetables. Very

signifi cant losses occur as vegetables wilt, or when

CH O

OH OH

O

HO

CH 2 OH

CH O

O OH

O

HO

CH 2 OH

.

CH O

O O

O

HO

CH 2 OH

Ascorbate Monodehydroascorbate

(semidehydroascorbate)

Dehydroascorbate

Figure 8.19 Vitamin C (ascor-

bic acid, monodehydroascor-

bate and dehydroascorbate).