There are over 600 carotenoids in nature. Carotenoids generally
contain a conjugated polyene structure which is efficient at absorbing
light, and are the major yellow and red pigments in many fruits and
vegetables. Beta-carotene (C40 H56) and alpha-carotene are responsible for
the orange color of carrots, and lycopene for the red color of tomatoes;
astaxanthin imparts a red or pink color to lobsters and salmon. The term
"carotene" refers to carotenoids which contain only carbon and hydrogen
(e.g. beta-carotene, alpha-carotene, lycopene), while the term
"xanthophylls" refers to compounds which contain hydroxyl groups (lutein,
zeaxanthin, beta-cryptoxanthin) or keto groups (canthaxanthin) or both (astaxanthin).
Major storage sites: Because the carotenoids are
fat-soluble, they are found in fatty tissues in the body and are
transported in blood by lipoproteins. The predominant carotenoids found in
human tissues are beta-carotene, alpha-carotene, lycopene, lutein,
zeaxanthin, and beta-cryptoxanthin; their relative abundance depends on
dietary intake.
Functions: In animals some carotenoids (particularly
beta-carotene) serve as dietary precursors to vitamin A, and many of them
may function as fat-soluble antioxidants. Because they are present in much
lower concentrations than is alpha-tocopherol, some questions have been
raised about their physiological importance as antioxidants. However,
increased consumption of foods rich in carotenoids is associated with
decreased risk of some degenerative diseases, and there is some evidence
also for their role in improving immune function. In plants they serve as
antioxidants to protect the highly reactive photosystems and also act as
accessory photopigments.
Deficiency: One study has found that carotenoid
deficiency is associated with skin changes (including acne and
dermatitis). These changes were detrimental but not life-threatening. This
effect should be confirmed by additional studies before making dietary
recommendations.
Diet recommendations: No formal diet recommendation
for carotenoids has yet been established but some experts suggest intakes
of 5 to 6 mg daily (about twice the average daily American intake).
Individual dietary carotenoid consumption is quite variable.
Food sources: Carotenoids are biosynthesized only in
plants and some bacteria, thus foods of plant origin are the primary
dietary source for humans. Intestinal absorption can be poor, and depends
on the presence of dietary fat. Mild cooking (steaming) increases
bioavailability of carotenoids while overcooking can destroy some forms.
The extent of conversion of provitamin A carotenoids to vitamin A seems to
be variable but is less than 50%.
Toxicity: The carotenoids are remarkably devoid of
toxicity, and serve as good nontoxic sources of vitamin A. Massive
overconsumption of carotenoids can result in yellowing of the skin,
especially of the hand and ears (xanthosis cutis), but has no adverse
health effects. The color disappears within a week or so after reducing
intake of carotene-rich foods.
Clinical uses: Canthaxanthin and beta-carotene have
been used pharmacologically to treat erythropoietic porphyria, a disease
characterized by extreme sensitivity of the skin to sunlight.
Recent research: Most recent interest has focused on
antioxidant, anticancer, and immune-enhancing properties of carotenoids.
Research is also continuing on food carotenoids as sources of dietary
provitamin A.
