Introduction to Human Nutrition

134 Introduction to Human Nutrition

a role in vision, as the prosthetic group of the light-
sensitive proteins in the retina, and a major role in the
regulation of gene expression and tissue differentia-
tion. Defi ciency is a major public health problem in
large areas of the world, and prevention of vitamin A
defi ciency is one of the three micronutrient priorities
of the World Health Organization (WHO) (the other
two are iron and iodine).

Vitamers and international units

Two groups of compounds, shown in Figure 8.1, have
vitamin A activity: retinol, retinaldehyde, and retinoic

acid (preformed vitamin A); and a variety of caro-

tenes and related compounds (collectively known as

carotenoids) that can be cleaved oxidatively to yield

retinaldehyde, and hence retinol and retinoic acid.

Those carotenoids that can be cleaved to yield retinal-

dehyde are known as provitamin A carotenoids.

Preformed vitamin A (mainly as retinyl esters) is

found only in foods of animal origin. The richest

source by far is liver, which may contain suffi cient

vitamin A to pose a potential problem for pregnant

women, since retinol is teratogenic in excess. Caro-

tenes are found in green, yellow, and red fruits and

CH 2 OH

CH 3

H 3 C CH 3 CH^3 CH^3

H

C

CH 3

H 3 C CH 3 CH^3 CH^3

O

COO-

CH 3

H 3 C CH 3 CH^3 CH^3

CH 3

H 3 C CH 3 CH^3

COO-

H 3 C

CH 3

H 3 C CH 3 CH^3 CH^3

CH 3 CH 3

H 3 C

H 3 C CH 3

CH 3

H 3 C CH 3 CH^3 CH^3

CH 3 CH 3

H 3 C

H 3 C CH 3

Retinol

Retinaldehyde

all-trans-Retinoic acid

9-cis-Retinoic acid

α-Carotene

β-Carotene

Figure 8.1 The major vitamin A vitamers and vitamin A active carotenoids.