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What's
in a name?
Carotenoids (kuh-rah'-tuh-noydz) are the
family of compounds sometimes called 'tetraterpenes' (te-truh-ter'-peenz).
They belong to the larger class of phytochemicals known as terpenes.
Carotenoids are named, as you would expect,
after carrots. Carotenoids include the carotenes
(alpha-, beta-, delta-, and gamma-carotenes). The suffix 'oid' can be thought of as meaning 'the
family of' or 'type of' or 'relative of,' so Carotenoids include the
carotenes and their relatives. The word 'carotene' (kare'-uh-teen)
is sometimes used on supplement bottles and in older literature to refer to
a mixture of the different forms of carotene. Compounds such as lycopene (ly'-cuh-peen), lutein (loo-tee'-in),
and astaxanthin (as-tuh-zan'-thin) are carotenoids but not carotenes.
Some of the carotenoids are further
classified as xanthophylls (zan'-thuh-filz), which means 'yellow leaves.'
These are the 'oxygenated carotenoids' which include one or more oxygen
atoms in their structures. Compounds such as lutein, astaxanthin,
violaxanthin, zeaxanthin, and capsanthin are examples of xanthophylls.
Carotenoids are colorful, fat/oil-soluble pigments found in green leaves (including
dietary greens such as collards, spinach and kale) and yellow, orange and red fruits
and vegetables including sweet potatoes, squash, carrots, red peppers,
apricots, mangoes, and tomatoes. (In some plants, however, the red
coloration may be due to anthocyanins).
Astaxanthin is
a carotenoid from marine sources such as algae, salmon and trout, and crustaceans.
Commercially, it is produced either by a type of microalgae, Haematococcus
pluvialis, or from ocean krill. It is currently being marketed as a
powerful antioxidant.
Carotenoids in Nature
In living leaves carotenoids are found inside the chloroplasts where they function as accessory pigments to
chlorophyll, helping the plant harvest more light during photosynthesis. They
also serve as antioxidants by quenching
reactive oxygen species (ROS), protecting the chloroplasts from damage
and handling excess sunlight energy that might otherwise harm the plant. In
autumn tree leaves, when green chlorophyll breaks down, the beautiful reds,
oranges and yellows of carotenoids can be seen.
Carotenoids can also accumulate in non-photosynthetic tissues of plants,
such as carrot roots, sweet potato tubers, squash fruits, and corn kernels.
In these tissues, they are found in special cellular organelles called
'chromoplasts,' where they may serve as biological signal molecules and
antioxidants.
Variations on a theme
As with most phytochemicals, the term 'carotenoid' refers to a family of
compounds rather than just a single compound. There are around 600
variations known, but we will limit our discussion to the most common
carotenoids found in the diet and in herbal medicines. Some 100 different
carotenoids are generally encountered in foods; of these, about 20
have been detected in our bodies: mainly beta-carotene, lutein, and
lycopene.
Carotenoids and human health
Animals cannot synthesize carotenoids, but some can ingest them in the diet
and metabolically transform them into new variations. For humans,
carotenoids can be antioxidants that scavenge free radicals and
quench ROS. Evidence
suggests they act to minimize cell membrane damage and to help prevent lipid
peroxidation. Carotenoids can be found inside chylomicrons (the tiny
packages that transport lipids in our blood) where they help protect
cholesterol and triglycerides from oxidation. The highest carotenoid concentrations are
found in the liver, adrenals, ovaries and testes. Some 'pro-vitamin A'
carotenoids are also important as dietary precursors of vitamin A, the
pigment molecule in the retina that responds to light and initiates the
vision process. Vitamin A is also important for immune system function.
Free carotenoids,
everywhere One excellent, and
often overlooked, source of carotenoids is the Dandelion flower (Taraxacum
officinale ). Analyses have detected beta-carotene, chrysanthemumxanthin,
cryptoxanthin, cryptoxanthin epoxide, flavoxanthin, lutein, taraxanthin, and
violaxanthin. As suggested by studies indicating mixed carotenoids can act
as cancer-preventing antioxidants, Dandelion flowers should be a beneficial
addition to the diet. |
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Common carotenoids and their sources |
|
Compound |
Pronunciation |
Sources |
| Astaxanthin |
as-tuh-zan'-thin |
Haematococcus pluvialis (a
microalga); Xanthophyllomyces dendrorhous (a yeast); Euphausia
superba (krill); crustaceans, shellfish, salmon & trout |
| Bixin |
biks'-in |
Seed coats of Annatto; widely used as a
food colorant |
| Canthaxanthin |
kan-thuh-zan'-thin |
crustaceans, salmon & trout, egg yolks;
used commercially in poultry and fish feed |
| Capsanthin |
kap-san'-thin |
red peppers, paprika |
| Capsorubin |
kap-soh-roo'-bin |
red peppers, paprika |
| Carotenes |
kare'-uh-teenz |
|
| Alpha-carotene |
al'-fuh- |
Many of the same sources as beta-carotene
(below), and apples, corn, peaches, potatoes, watermelon; palm oil |
| Beta-carotene |
bay'-tuh- |
Common in green leaves (spinach, collards,
kale; darker leaves have more beta-carotene than lighter-colored ones);
green peppers, orange vegetables including carrots, squash, sweet
potatoes; palm oil |
| Delta-carotene |
del'-tuh- |
a precursor of alpha-carotene and lutein; many
yellow-red fruits & vegetables, corn |
| Gamma-carotene |
gam'-uh- |
a precursor of beta-carotene and lutein; in apricots,
carrots, corn, sweet potatoes, tomatoes, watermelon; palm oil |
| Cryptoxanthin |
krip-toh-zan'-thin |
apples, apricots, bell peppers, corn,
lemons, oranges, persimmons, papaya, paprika |
| Lycopene |
ly'-cuh-peen |
tomato juice, sauce, paste & fresh
tomatoes; catsup; watermelon, pink grapefruit, papaya, pink guava |
| Lutein |
loo-tee'-in |
common in green leaves; carrots, corn, some
fruits, tomatoes. Commercial source: Marigold petals |
| Violaxanthin |
vy-ol-uh-zan'-thin |
common in green leaves; yellow vegetables |
| Zeaxanthin |
zee-uh-zan'-thin |
common in green leaves; corn, some fruits,
paprika, peppers (esp. orange ones) |
|
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Toxicology
Carotenoids are generally non-toxic at the levels found in
foods, herbs, and properly used supplements. A benign coloration of the skin
may result from excessive intake of beta-carotene or even of carrot juice.
Several years ago, canthaxanthin was marketed in extremely
high-concentration pills as a sunless tanning agent; the FDA banned these
when toxicity symptoms arose (including deposition of canthaxanthin crystals
in the retina). Although it has GRAS status and is used as a food colorant
(in amounts not to exceed 30 mg/pint or pound), canthaxanthin has been
demonstrated to cause retinal changes 50% of the time at a cumulative dose
level of 37 g and 100% of the time following a cumulative dose of 60 g.1,2
Some research has indicated that isolated, synthetic
beta-carotene supplements may actually be damaging to smokers and drinkers.
One study3 found an increase in cancer rates among 29,000 men who
took 20 mg/day of beta-carotene; this increase did not occur if vitamin E
was taken along with the beta-carotene. The same study indicated that diets
high in beta-carotene were protective against cancer. This evidence suggests
that the higher levels of free radicals in the bodies of smokers and
drinkers can damage isolated beta-carotene, turning it into a pro-oxidant as
opposed to an antioxidant. To prevent this, the beta-carotene should be
accompanied by other antioxidants as found in foods. Another large study4
corroborated these findings.
High levels of supplemental vitamin A have demonstrated
significant toxicity, inducing blurred vision, headaches, hair loss,
pathological skin changes, and birth defects. However, the body limits
conversion of carotenes to vitamin A, so these effects have not been
observed with similarly high carotenoid intakes. Even so, the wisdom of Nature
suggests that excessive intake of supplemental carotenoids might have subtle
or long-term deleterious effects. 1 Harnois et
al, Arch Ophthalmol, 107: 538-40, 1989.
2 Espaillat et al, Arch Ophthalmol, 117: 412-3, 1999.
3
Alpha-Tocopherol, Beta
Carotene Cancer Prevention Study Group. New England Journal of
Medicine 330, 1029-1035, 1994.
4 Cancer Research 54(Suppl.), 2038S-2043S, 1994.
2006 Dual role of
{beta}-carotene in combination with cigarette smoke aqueous extract on the
formation of mutagenic lipid peroxidation products in lung membranes:
dependence on pO2.
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&dopt=Abstract&db=PubMed&list_uids=16704989
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