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Carotenoids

GlossarySuccess Chemistry Staff

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.

 

FOOD SOURCES

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

lipase.

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.

 

BIOLOGICAL ACTIVITIES

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).

121

122 Johnson and Russell

β-Cryptoxanthin

HO

HO

HO

OH

OH

H

Lycopene

Lutein

Zeaxanthin

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

damage (20).

 

CONCLUSIONS

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

of carotenoids.

 

REFERENCES

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lutein and zeaxanthin diet on the concentration and distribution

of carotenoids in lipoproteins of elderly people with

and without age-related macular degeneration. Am J Clin

Nutr 2007; 85:762–769.

2. USDA. USDA-NCC Carotenoid Database for U.S. Foods—

1998, 1998.

3. Clinton SK. Lycopene: Chemistry, biology, and implications

for human health and disease. Nutr Rev 1998; 56:35–51.

4. Yeum KJ, Russell RM. Carotenoid bioavailability and bioconversion.

Annu Rev Nutr 2002; 22:483–504.

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combination of beta-carotene and vitamin A on lung cancer

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the protective role of lutein and zeaxanthin in the eye.