Carotenoids are a family of compounds of over 600 fat soluble
plant pigments, which provide much of the color
seen in nature.
For example, carotenoids are responsible
for the red color of tomatoes and the orange color of
carrots. Apart from their aesthetic role, carotenoids are
considered to be beneficial in the prevention of various
diseases, including certain cancers and eye disease. The
beneficial effects of carotenoids are attributed to a small
portion of the hundreds of carotenoids found in nature,
given that only about two dozen are found in human
blood and tissue, and only two in the eye. The carotenoids
that have been most studied in this regard are -carotene,
-carotene, cryptoxanthin, lycopene, lutein, and zeaxanthin.
(See also separate chapters on “-Carotene,”
“Lutein,” and “Lycopene.”) -Carotene and lycopene
belong to a class of carotenoids called carotenes and are
highly fat soluble. Cryptoxanthin, lutein, and zeaxanthin
belong to a class of carotenoids called xanthophylls.
Because xanthophylls contain at least one hydroxyl
group, they are more polar than that of carotenes. Thus, –
carotene, -carotene, and lycopene tend to predominate in
low-density lipoproteins (LDL) in the circulation, whereas
high-density lipoproteins (HDL) are major transporters of
cryptoxanthin, lutein, and zeaxanthin (1). In part, the protection
against chronic disease by carotenoids is thought
to be through their antioxidant activity. Lutein and
zeaxanthin are thought to have an additional protective
role of absorbing damaging blue light that enters the eye.
The most common carotenoids in the U.S. diet are
-carotene, -carotene, -cryptoxanthin, lycopene, lutein,
and zeaxanthin (2). Rich sources of – and -carotene
include carrots and winter squashes. Foods high in
-cryptoxanthin are orange and red fruits and vegetables
such as pumpkin, papayas, and red peppers. Tomatoes account
for approximately 80% of the dietary lycopene (3).
Dark green leafy vegetables such as spinach and kale are
rich sources of lutein and zeaxanthin. The relatively low
bioavailability of carotenoids from foods is, in part, due
to their association with proteins in the plant matrix (4).
Disruption of the plant matrix with chopping, blending,
or cooking can increase carotenoid bioavailability (5).
ABSORPTION AND METABOLISM
Carotenoids, being fat soluble, follow the same intestinal
absorption path as dietary fat. Carotenoids are released
from food matrices and solubilized in the gut. This is done
in the presence of fat and conjugated bile acids. As little
as 3 to 5 g of fat in a meal is sufficient for carotenoid absorption
(5,6). Absorption is affected by the same factors
that influence fat absorption. Thus, the absence of bile
or any generalized malfunction of the lipid absorption
system (e.g., small intestinal disease, pancreatic disease)
will interfere with the absorption of carotenoids. Chylomicrons
are responsible for the transport of carotenoid from
the intestinal mucosa to the bloodstream via the lymphatics
for delivery to tissues. Carotenoids are transported in
the plasma exclusively by lipoproteins, being carried predominately
by LDL (carotenes) and HDL (xanthophylls)
(7). The delivery of carotenoids to extrahepatic tissues is
accomplished through the interaction of lipoprotein particles
with receptors and the degradation of lipoprotein
Plant sterols reduce the absorption of cholesterol in
the gut, in part by competing with cholesterol when they
are incorporated into the mixed micelles (8). Stanol and
steryl esters reduce plasma concentrations of carotenoids
(9,10). It is thought that, during the absorption process in
the intestine, plant sterols could displace not only cholesterol
but also these lipophilic molecules and replace them
in incorporation into mixed micelles.
To date, the only known essential function of carotenoids
is as a source of vitamin A. The carotenoids in this category
are -carotene, -carotene, and -cryptoxanthin (11). The
vitaminAactivity of -carotene in food is 1/12 that of vitamin
A (retinol), and vitamin A activity of both -carotene
and -cryptoxanthin is 1/24 that of vitamin A (11).
Most carotenoids have antioxidant activity. –
Carotene and others carotenoids have antioxidant properties
as shown in in vitro and animal models. Mixtures of
carotenoids or associations with others antioxidants (e.g.,
vitamin E) can increase their activity against free radicals
(12). The use of animal models for studying carotenoids is
limited because most of the animals do not absorb or metabolize
carotenoids similarly to humans. Epidemiologic
studies have shown an inverse relationship between
the presence of various cancers and dietary or blood
carotenoid levels (13). However, three out of four intervention
trials using high-dose -carotene supplements did
not show protective effects against cancer or cardiovascular
disease. Rather, the high-risk populations (smokers
and asbestos workers) in these high-dose intervention trials
showed an increase in cancer and angina cases (14–17).
122 Johnson and Russell
Figure 1 Structures of the major dietary carotenoids.
Therefore, carotenoids may promote health when taken at
dietary levels but may have adverse effects when taken
in high dose by subjects who smoke or who have been
exposed to asbestos.
The long chain of alternating double and single
bonds is a characteristic of all carotenoids (Fig. 1). This
feature allows them to absorb light in the visible range
of the light spectrum (18). This may be of particular
importance in the macula of the retina, where lutein
and zeaxanthin are highly concentrated to the exclusion
of all other carotenoids (19). In the macula, lutein
and zeaxanthin absorb blue light to reduce the amount
of light that reaches critical visual structures, thereby
providing some protection from light-induced oxidative
Epidemiologic studies support a protective role of
carotenoids in the prevention of certain major diseases,
including certain cancer and eye disease. The hypothesis
that these antioxidant nutrients may protect against
certain diseases is plausible, given the putative role of
oxidative damage in the etiology or these diseases. However,
clinical trials have suggested that supplementation
with high dose of -carotene may have an adverse effect
on the incidence of cancer in smokers and workers
exposed to asbestos. Current recommendations include
diets high in fruits and vegetables, which are rich sources
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