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β-Carotene is an
organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ Chemistry * Organic matter, matter that has come from a once-living organism, is capable of decay or is the product ...
, strongly coloured red-orange
pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compou ...
abundant in fungi, plants, and fruits. It is a member of the
carotene The term carotene (also carotin, from the Latin ''carota'', "carrot") is used for many related unsaturated hydrocarbon substances having the formula C40Hx, which are synthesized by plants but in general cannot be made by animals (with the ex ...
s, which are
terpenoid The terpenoids, also known as isoprenoids, are a class of naturally occurring organic chemicals derived from the 5-carbon compound isoprene and its derivatives called terpenes, diterpenes, etc. While sometimes used interchangeably with "terpenes" ...
s (isoprenoids), synthesized biochemically from eight
isoprene Isoprene, or 2-methyl-1,3-butadiene, is a common volatile organic compound with the formula CH2=C(CH3)−CH=CH2. In its pure form it is a colorless volatile liquid. Isoprene is an unsaturated hydrocarbon. It is produced by many plants and animals ...
units and thus having 40
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...
s. Among the carotenes, β-carotene is distinguished by having beta-rings at both ends of the
molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and b ...
. β-Carotene is biosynthesized from
geranylgeranyl pyrophosphate Geranylgeranyl pyrophosphate is an intermediate in the biosynthesis of diterpenes and di terpenoids. It is also the precursor to carotenoids, gibberellins, tocopherols, and chlorophylls. It is also a precursor to geranylgeranylated proteins, wh ...
. In some Mucoralean fungi, β-Carotene is a precursor to the synthesis of trisporic acid. β-Carotene is the most common form of carotene in plants. When used as a
food coloring Food coloring, or color additive, is any dye, pigment, or substance that imparts color when it is added to food or drink. They come in many forms consisting of liquids, powders, gels, and pastes. Food coloring is used in both commercial food ...
, it has the
E number E numbers ("E" stands for "Europe") are codes for substances used as food additives, including those found naturally in many foods such as vitamin C, for use within the European Union (EU) and European Free Trade Association (EFTA). Commonly ...
E160a. The structure was deduced by Karrer et al. in 1930. In nature, β-carotene is a precursor (inactive form) to
vitamin A Vitamin A is a fat-soluble vitamin and an essential nutrient for humans. It is a group of organic compounds that includes retinol, retinal (also known as retinaldehyde), retinoic acid, and several provitamin A carotenoids (most notably ...
via the action of beta-carotene 15,15'-monooxygenase. Isolation of β-carotene from fruits abundant in
carotenoid Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, ...
s is commonly done using column
chromatography In chemical analysis, chromatography is a laboratory technique for the separation of a mixture into its components. The mixture is dissolved in a fluid solvent (gas or liquid) called the ''mobile phase'', which carries it through a system ( ...
. It can also be extracted from the beta-carotene rich algae, ''
Dunaliella salina ''Dunaliella salina'' is a type of halophile green unicellular micro-algae especially found in hypersaline environments, such as salt lakes and salt evaporation ponds. Known for its antioxidant activity because of its ability to create large ...
''. The separation of β-carotene from the mixture of other carotenoids is based on the polarity of a compound. β-Carotene is a non-polar compound, so it is separated with a non-polar solvent such as
hexane Hexane () is an organic compound, a straight-chain alkane with six carbon atoms and has the molecular formula C6H14. It is a colorless liquid, odorless when pure, and with boiling points approximately . It is widely used as a cheap, relative ...
. Being highly conjugated, it is deeply colored, and as a
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...
lacking functional groups, it is very
lipophilic Lipophilicity (from Greek λίπος "fat" and φίλος "friendly"), refers to the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene. Such non-polar solvents are themselves lipo ...
.


Provitamin A activity

Plant carotenoids are the primary dietary source of provitamin A worldwide, with β-carotene as the best-known
provitamin A provitamin is a substance that may be converted within the body to a vitamin. The term previtamin is a synonym. The term "provitamin" is used when it is desirable to label a substance with little or no vitamin activity, but which can be converted ...
A carotenoid. Others include α-carotene and β-cryptoxanthin. Carotenoid absorption is restricted to the
duodenum The duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear, and the terms anterior intestine or proximal intestine m ...
of the small intestine and dependent on class B scavenger receptor (SR-B1) membrane protein, which is also responsible for the absorption of
vitamin E Vitamin E is a group of eight fat soluble compounds that include four tocopherols and four tocotrienols. Vitamin E deficiency, which is rare and usually due to an underlying problem with digesting dietary fat rather than from a diet low in vi ...
(α-tocopherol). One molecule of β-carotene can be cleaved by the intestinal enzyme ''β,β-carotene 15,15'-monooxygenase'' into two molecules of vitamin A. Absorption efficiency is estimated to be between 9 and 22%. The absorption and conversion of carotenoids may depend on the form of β-carotene (e.g., cooked vs. raw vegetables, or in a supplement), the intake of fats and oils at the same time, and the current stores of vitamin A and β-carotene in the body. Researchers list these factors that determine the provitamin A activity of carotenoids: * Species of carotene * Molecular linkage * Amount in the meal * Matrix properties * Effectors * Nutrient status * Genetics * Host specificity * Interactions between factors


Symmetric and asymmetric cleavage

In the molecular chain between the two cyclohexyl rings, β-carotene cleaves either symmetrically or asymmetrically. Symmetric cleavage with the enzyme β,β-carotene-15,15'-dioxygenase requires an antioxidant such as
α-tocopherol α-Tocopherol is a type of vitamin E. It has E number "E307". Vitamin E exists in eight different forms, four tocopherols and four tocotrienols. All feature a chromane ring, with a hydroxyl group that can donate a hydrogen atom to reduce free r ...
. This symmetric cleavage gives two equivalent
retinal Retinal (also known as retinaldehyde) is a polyene chromophore. Retinal, bound to proteins called opsins, is the chemical basis of visual phototransduction, the light-detection stage of visual perception (vision). Some microorganisms use reti ...
molecules and each retinal molecule further reacts to give
retinol Retinol, also called vitamin A1, is a fat-soluble vitamin in the vitamin A family found in food and used as a dietary supplement. As a supplement it is used to treat and prevent vitamin A deficiency, especially that which results in xeroph ...
(vitamin A) and retinoic acid. β-Carotene is also cleaved into two asymmetric products; the product is β-
apocarotenal Apocarotenal, or ''trans''-β-apo-8'-carotenal, is a carotenoid found in spinach and citrus fruits. Like other carotenoids, apocarotenal plays a role as a precursor of vitamin A, even though it has 50% less pro-vitamin A activity than β-carotene ...
(8',10',12'). Asymmetric cleavage reduces the level of retinoic acid significantly.


Conversion factors

Since 2001, the US Institute of Medicine uses retinol activity equivalents (RAE) for their Dietary Reference Intakes, defined as follows:


Retinol activity equivalents (RAEs)

1 µg RE = 1 µg retinol 1 µg RAE = 2 µg all-''trans''-β-carotene from supplements 1 µg RAE = 12 µg of all-''trans''-β-carotene from food 1 µg RAE = 24 µg α-carotene or β-cryptoxanthin from food RAE takes into account carotenoids' variable absorption and conversion to vitamin A by humans better than and replaces the older retinol equivalent (RE) (1 µg RE = 1 µg retinol, 6 µg β-carotene, or 12 µg α-carotene or β-cryptoxanthin). RE was developed 1967 by the United Nations/
World Health Organization The World Health Organization (WHO) is a specialized agency of the United Nations responsible for international public health. The WHO Constitution states its main objective as "the attainment by all peoples of the highest possible level o ...
Food and Agriculture Organization (FAO/WHO). Another older unit of vitamin A activity is the international unit (IU). Like retinol equivalent, the international unit does not take into account carotenoids' variable absorption and conversion to vitamin A by humans, as well as the more modern retinol activity equivalent. Unfortunately, food and supplement labels still generally use IU, but IU can be converted to the more useful retinol activity equivalent as follows:


International Units

* 1 µg RAE = 3.33 IU retinol * 1 IU retinol = 0.3 μg RAE * 1 IU β-carotene from supplements = 0.3 μg RAE * 1 IU β-carotene from food = 0.05 μg RAE * 1 IU α-carotene or β-cryptoxanthin from food = 0.025 μg RAE1


Dietary sources

Beta-carotene is found in many foods and is sold as a
dietary supplement A dietary supplement is a manufactured product intended to supplement one's diet by taking a pill, capsule, tablet, powder, or liquid. A supplement can provide nutrients either extracted from food sources or that are synthetic in order ...
. β-Carotene contributes to the orange color of many different fruits and vegetables.
Vietnam Vietnam or Viet Nam ( vi, Việt Nam, ), officially the Socialist Republic of Vietnam,., group="n" is a country in Southeast Asia, at the eastern edge of mainland Southeast Asia, with an area of and population of 96 million, making ...
ese
gac GAC or Gac may refer to: Companies and organisations * GAC Group, a Chinese automotive company based in Guangzhou, Guangdong * GAC Ireland, an Irish bus manufacturer established with Bombardier (1980–1986) * Games Administration Committee, ...
(''Momordica cochinchinensis'' Spreng.) and crude
palm oil Palm oil is an edible vegetable oil derived from the mesocarp (reddish pulp) of the fruit of the oil palms. The oil is used in food manufacturing, in beauty products, and as biofuel. Palm oil accounted for about 33% of global oils produced fr ...
are particularly rich sources, as are yellow and orange fruits, such as
cantaloupe The cantaloupe, rockmelon (Australia and New Zealand, although cantaloupe is used in some states of Australia), sweet melon, or spanspek (Southern Africa) is a melon that is a variety of the muskmelon species (''Cucumis melo'') from the fami ...
,
mangoes A mango is an edible stone fruit produced by the tropical tree ''Mangifera indica''. It is believed to have originated in the region between northwestern Myanmar, Bangladesh, and northeastern India. ''M. indica'' has been cultivated in South ...
,
pumpkin A pumpkin is a vernacular term for mature winter squash of species and varieties in the genus ''Cucurbita'' that has culinary and cultural significance but no agreed upon botanical or scientific meaning. The term ''pumpkin'' is sometimes use ...
, and papayas, and orange root vegetables such as
carrots The carrot (''Daucus carota'' subsp. ''sativus'') is a root vegetable, typically orange in color, though purple, black, red, white, and yellow cultivars exist, all of which are domesticated forms of the wild carrot, ''Daucus carota'', nati ...
and
sweet potato The sweet potato or sweetpotato ('' Ipomoea batatas'') is a dicotyledonous plant that belongs to the bindweed or morning glory family, Convolvulaceae. Its large, starchy, sweet-tasting tuberous roots are used as a root vegetable. The young ...
es. The color of β-carotene is masked by
chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll allow plants to ...
in green
leaf vegetable Leaf vegetables, also called leafy greens, pot herbs, vegetable greens, or simply greens, are plant leaves eaten as a vegetable, sometimes accompanied by tender petioles and shoots. Leaf vegetables eaten raw in a salad can be called salad gre ...
s such as
spinach Spinach (''Spinacia oleracea'') is a leafy green flowering plant native to central and western Asia. It is of the order Caryophyllales, family Amaranthaceae, subfamily Chenopodioideae. Its leaves are a common edible vegetable consumed either f ...
,
kale Kale (), or leaf cabbage, belongs to a group of cabbage (''Brassica oleracea'') cultivars grown for their edible leaves, although some are used as ornamentals. Kale plants have green or purple leaves, and the central leaves do not form a hea ...
, sweet potato leaves, and sweet
gourd Gourds include the fruits of some flowering plant species in the family Cucurbitaceae, particularly ''Cucurbita'' and '' Lagenaria''. The term refers to a number of species and subspecies, many with hard shells, and some without. One of the ear ...
leaves. Vietnamese gac and crude palm oil have the highest content of β-carotene of any known plant sources, 10 times higher than carrots, for example. However, gac is quite rare and unknown outside its native region of Southeast Asia, and crude palm oil is typically processed to remove the carotenoids before sale to improve the color and clarity. The average daily intake of β-carotene is in the range 2–7 mg, as estimated from a pooled analysis of 500,000 women living in the US, Canada, and some European countries. The U.S. Department of Agriculture lists these 10 foods to have the highest β-carotene content per serving. :


Side effects

Excess β-carotene is predominantly stored in the fat tissues of the body. The most common side effect of excessive β-carotene consumption is carotenodermia, a physically harmless condition that presents as a conspicuous orange
skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other animal coverings, such as the arthropod exoskeleton, have different ...
tint arising from deposition of the carotenoid in the outermost layer of the
epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water rel ...
. Adults' fat stores are often yellow from accumulated carotenoids, including β-carotene, while infants' fat stores are white. Carotenodermia is quickly reversible upon cessation of excessive intakes.


Excessive intakes and vitamin A toxicity

The proportion of carotenoids absorbed decreases as dietary intake increases. Within the intestinal wall (
mucosa A mucous membrane or mucosa is a membrane that lines various cavities in the body of an organism and covers the surface of internal organs. It consists of one or more layers of epithelial cells overlying a layer of loose connective tissue. It i ...
), β-carotene is partially converted into vitamin A (
retinol Retinol, also called vitamin A1, is a fat-soluble vitamin in the vitamin A family found in food and used as a dietary supplement. As a supplement it is used to treat and prevent vitamin A deficiency, especially that which results in xeroph ...
) by an
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
,
dioxygenase Dioxygenases are oxidoreductase enzymes. Aerobic life, from simple single-celled bacteria species to complex eukaryotic organisms, has evolved to depend on the oxidizing power of dioxygen in various metabolic pathways. From energetic adenosine ...
. This mechanism is regulated by the individual's vitamin A status. If the body has enough vitamin A, the conversion of β-carotene decreases. Therefore, β-carotene is considered a safe source of vitamin A and high intakes will not lead to
hypervitaminosis A Hypervitaminosis A refers to the toxic effects of ingesting too much preformed vitamin A (retinyl esters, retinol, and retinal). Symptoms arise as a result of altered bone metabolism and altered metabolism of other fat-soluble vitamins. Hypervi ...
.


Drug interactions

β-Carotene can interact with medication used for lowering
cholesterol Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell memb ...
. Taking them together can lower the effectiveness of these medications and is considered only a moderate interaction. β-Carotene should not be taken with
orlistat Orlistat, sold under the brand name Xenical among others, is a medication used to treat obesity. Its primary function is preventing the absorption of fats from the human diet by acting as a lipase inhibitor, thereby reducing caloric intake. It ...
, a weight-loss medication, as orlistat can reduce the absorption of β-carotene by as much as 30%.
Bile acid sequestrant The bile acid sequestrants are a group of resins used to bind certain components of bile in the gastrointestinal tract. They disrupt the enterohepatic circulation of bile acids by combining with bile constituents and preventing their reabsorption fr ...
s and
proton-pump inhibitor Proton-pump inhibitors (PPIs) are a class of medications that cause a profound and prolonged reduction of stomach acid production. They do so by irreversibly inhibiting the stomach's H+/K+ ATPase proton pump. They are the most potent inhibitor ...
s can also decrease absorption of β-carotene. Consuming alcohol with β-carotene can decrease its ability to convert to retinol and could possibly result in
hepatotoxicity Hepatotoxicity (from ''hepatic toxicity'') implies chemical-driven liver damage. Drug-induced liver injury is a cause of acute and chronic liver disease caused specifically by medications and the most common reason for a drug to be withdrawn fr ...
.


β-Carotene and lung cancer in smokers

Chronic high doses of β-carotene supplementation increases the probability of lung cancer in smokers. The effect is specific to supplementation dose as no
lung The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of ...
damage has been detected in those who are exposed to cigarette smoke and who ingest a physiologic dose of β-carotene (6 mg), in contrast to high pharmacologic dose (30 mg). Therefore, the
oncology Oncology is a branch of medicine that deals with the study, treatment, diagnosis and prevention of cancer. A medical professional who practices oncology is an ''oncologist''. The name's etymological origin is the Greek word ὄγκος (''ó ...
from β-carotene is based on both cigarette smoke and high daily doses of β-carotene. Increases in lung cancer may be due to the tendency of β-carotene to oxidize, and may hasten oxidation more than other food colors such as
annatto Annatto ( or ) is an orange-red condiment and food coloring derived from the seeds of the achiote tree ('' Bixa orellana''), native to tropical America. It is often used to impart a yellow or orange color to foods, but sometimes also for its f ...
. A β-carotene breakdown product suspected of causing cancer at high dose is ''trans''-β-apo-8'-carotenal (common
apocarotenal Apocarotenal, or ''trans''-β-apo-8'-carotenal, is a carotenoid found in spinach and citrus fruits. Like other carotenoids, apocarotenal plays a role as a precursor of vitamin A, even though it has 50% less pro-vitamin A activity than β-carotene ...
), which has been found in one study to be mutagenic and genotoxic in cell cultures which do not respond to β-carotene itself. Additionally, supplemental β-carotene may increase the risk of
prostate cancer Prostate cancer is cancer of the prostate. Prostate cancer is the second most common cancerous tumor worldwide and is the fifth leading cause of cancer-related mortality among men. The prostate is a gland in the male reproductive system that su ...
,
intracerebral hemorrhage Intracerebral hemorrhage (ICH), also known as cerebral bleed, intraparenchymal bleed, and hemorrhagic stroke, or haemorrhagic stroke, is a sudden bleeding into the tissues of the brain, into its ventricles, or into both. It is one kind of bleed ...
, and cardiovascular and total mortality in people who smoke
cigarette A cigarette is a narrow cylinder containing a combustible material, typically tobacco, that is rolled into thin paper for smoking. The cigarette is ignited at one end, causing it to smolder; the resulting smoke is orally inhaled via the opp ...
s or have a history of high-level exposure to
asbestos Asbestos () is a naturally occurring fibrous silicate mineral. There are six types, all of which are composed of long and thin fibrous crystals, each fibre being composed of many microscopic "fibrils" that can be released into the atmosphere b ...
.


Research

Medical authorities generally recommend obtaining beta-carotene from food rather than dietary supplements. Research is insufficient to determine whether a minimum level of beta-carotene consumption is necessary for human health and to identify what problems might arise from insufficient beta-carotene intake.


Macular degeneration

Age-related macular degeneration (AMD) represents the leading cause of irreversible blindness in elderly people. AMD is an oxidative stress, retinal disease that affects the macula, causing progressive loss of central vision. β-carotene content is confirmed in human retinal pigment epithelium. Reviews reported mixed results for observational studies, with some reporting that diets higher in β-carotene correlated with a decreased risk of AMD whereas other studies reporting no benefits. Reviews reported that for intervention trials using only β-carotene, there was no change to risk of developing AMD.


Cancer

A meta-analysus concluded that supplementation with β-carotene does not appear to decrease the risk of cancer overall, nor specific cancers including: pancreatic, colorectal, prostate, breast, melanoma, or skin cancer generally. High levels of β-carotene may increase the risk of lung cancer in current and former smokers. This is likely because beta-carotene is unstable in cigarette smoke-exposed lungs where it forms oxidized metabolites that can induce carcinogen-bioactivating enzymes. Results are not clear for thyroid cancer. In a single, small clinical study published in 1989, natural beta-carotene appeared to reduce premalignant gastric lesions.


Cataract

A
Cochrane review Cochrane (previously known as the Cochrane Collaboration) is a British international charitable organisation formed to organise medical research findings to facilitate evidence-based choices about health interventions involving health professi ...
looked at supplementation of β-carotene, vitamin C, and vitamin E, independently and combined, on people to examine differences in risk of
cataract A cataract is a cloudy area in the lens of the eye that leads to a decrease in vision. Cataracts often develop slowly and can affect one or both eyes. Symptoms may include faded colors, blurry or double vision, halos around light, trouble ...
, cataract extraction, progression of cataract, and slowing the loss of visual acuity. These studies found no evidence of any protective effects afforded by β-carotene supplementation on preventing and slowing age-related cataract. A second meta-analysis compiled data from studies that measured diet-derived serum beta-carotene and reported a not statistically significant 10% decrease in cataract risk.


Nanotechnology

Dispersed β-carotene molecules can be encapsulated into
carbon nanotube A scanning tunneling microscopy image of a single-walled carbon nanotube Rotating single-walled zigzag carbon nanotube A carbon nanotube (CNT) is a tube made of carbon with diameters typically measured in nanometers. ''Single-wall carbon na ...
s enhancing their
optical properties The optical properties of a material define how it interacts with light. The optical properties of matter are studied in optical physics, a subfield of optics. The optical properties of matter include: * Refractive index *Dispersion * Transmittan ...
. Efficient energy transfer occurs between the encapsulated dye and nanotube — light is absorbed by the dye and without significant loss is transferred to the nanotube. Encapsulation increases chemical and thermal stability of β-carotene molecules; it also allows their isolation and individual characterization.


See also

* Sunless tanning with beta-carotene *
Vitamin A Vitamin A is a fat-soluble vitamin and an essential nutrient for humans. It is a group of organic compounds that includes retinol, retinal (also known as retinaldehyde), retinoic acid, and several provitamin A carotenoids (most notably ...
*
Retinol Retinol, also called vitamin A1, is a fat-soluble vitamin in the vitamin A family found in food and used as a dietary supplement. As a supplement it is used to treat and prevent vitamin A deficiency, especially that which results in xeroph ...
*
Lycopene Lycopene is an organic compound classified as a tetraterpene and a carotene. Lycopene (from the neo-Latin '' Lycopersicum'', the tomato species) is a bright red carotenoid hydrocarbon found in tomatoes and other red fruits and vegetables. Occu ...
*
Lutein Lutein (;"Lutein"
*
Zeaxanthin Zeaxanthin is one of the most common carotenoids in nature, and is used in the xanthophyll cycle. Synthesized in plants and some micro-organisms, it is the pigment that gives paprika (made from bell peppers), corn, saffron, goji (wolfberries) ...
*
Carotenoids Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, co ...


References


External links


USDA Webpage on β-carotene Content of Gac
– Fatty Acids and Carotenoids in Gac (Momordica Cochinchinensis Spreng) Fruit. {{DEFAULTSORT:Carotene, Beta- Carotenoids Vitamin A Hydrocarbons Chemical tests Tetraterpenes Cyclohexenes