Beta-carotene
Overview
 
β-Carotene is a strongly-coloured red-orange pigment
Pigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...

 abundant in plants and fruits. It is an organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

 and chemically is classified as a hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....

 and specifically as a terpenoid
Terpenoid
The terpenoids , sometimes called isoprenoids, are a large and diverse class of naturally occurring organic chemicals similar to terpenes, derived from five-carbon isoprene units assembled and modified in thousands of ways. Most are multicyclic structures that differ from one another not only in...

 (isoprenoid), reflecting its derivation from isoprene
Isoprene
Isoprene , or 2-methyl-1,3-butadiene, is a common organic compound with the formula CH2=CCH=CH2. Under standard conditions it is a colorless liquid...

 units. β-Carotene is biosynthesized from geranylgeranyl pyrophosphate
Geranylgeranyl pyrophosphate
Geranylgeranyl pyrophosphate is an intermediate in the HMG-CoA reductase pathway used by organisms in the biosynthesis of terpenes and terpenoids. In plants it is also the precursor to carotenoids, gibberellins, tocopherols, and chlorophylls....

.
It is a member of the carotene
Carotene
The term carotene is used for several related unsaturated hydrocarbon substances having the formula C40Hx, which are synthesized by plants but cannot be made by animals. Carotene is an orange photosynthetic pigment important for photosynthesis. Carotenes are all coloured to the human eye...

s, which are tetraterpenes, synthesized biochemically from eight isoprene units and thus having 40 carbons.
Encyclopedia
β-Carotene is a strongly-coloured red-orange pigment
Pigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...

 abundant in plants and fruits. It is an organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

 and chemically is classified as a hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....

 and specifically as a terpenoid
Terpenoid
The terpenoids , sometimes called isoprenoids, are a large and diverse class of naturally occurring organic chemicals similar to terpenes, derived from five-carbon isoprene units assembled and modified in thousands of ways. Most are multicyclic structures that differ from one another not only in...

 (isoprenoid), reflecting its derivation from isoprene
Isoprene
Isoprene , or 2-methyl-1,3-butadiene, is a common organic compound with the formula CH2=CCH=CH2. Under standard conditions it is a colorless liquid...

 units. β-Carotene is biosynthesized from geranylgeranyl pyrophosphate
Geranylgeranyl pyrophosphate
Geranylgeranyl pyrophosphate is an intermediate in the HMG-CoA reductase pathway used by organisms in the biosynthesis of terpenes and terpenoids. In plants it is also the precursor to carotenoids, gibberellins, tocopherols, and chlorophylls....

.
It is a member of the carotene
Carotene
The term carotene is used for several related unsaturated hydrocarbon substances having the formula C40Hx, which are synthesized by plants but cannot be made by animals. Carotene is an orange photosynthetic pigment important for photosynthesis. Carotenes are all coloured to the human eye...

s, which are tetraterpenes, synthesized biochemically from eight isoprene units and thus having 40 carbons. Among this general class of carotenes, β-Carotene is distinquished by having beta-rings at both ends of the molecule.

Carotene is the substance in carrots that colours them orange and is the most common form of carotene in plants. When used as a food colouring, it has the E number
E number
E numbers are number codes for food additives that have been assessed for use within the European Union . They are commonly found on food labels throughout the European Union. Safety assessment and approval are the responsibility of the European Food Safety Authority...

 E160a.p119

The structure was deduced by Karrer et al. in 1930. In nature, β-carotene is a precursor (inactive form) to vitamin A
Vitamin A
Vitamin A is a vitamin that is needed by the retina of the eye in the form of a specific metabolite, the light-absorbing molecule retinal, that is necessary for both low-light and color vision...

 via the action of beta-carotene 15,15'-monooxygenase
Beta-carotene 15,15'-monooxygenase
In enzymology, a beta-carotene 15,15'-monooxygenase is an enzyme that catalyzes the chemical reactionThus, the two substrates of this enzyme are beta-carotene and O2, whereas its product is retinal....

.

Isolation of β-carotene from fruits abundant in carotenoids is commonly done using column chromatography
Chromatography
Chromatography is the collective term for a set of laboratory techniques for the separation of mixtures....

. 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
Hexane is a hydrocarbon with the chemical formula C6H14; that is, an alkane with six carbon atoms.The term may refer to any of four other structural isomers with that formula, or to a mixture of them. In the IUPAC nomenclature, however, hexane is the unbranched isomer ; the other four structures...

. Being highly conjugated
Conjugated system
In chemistry, a conjugated system is a system of connected p-orbitals with delocalized electrons in compounds with alternating single and multiple bonds, which in general may lower the overall energy of the molecule and increase stability. Lone pairs, radicals or carbenium ions may be part of the...

, it is deeply colored, and as a hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....

 lacking functional groups, it is very lipophilic
Lipophilic
Lipophilicity, , refers to the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene. These non-polar solvents are themselves lipophilic — the axiom that like dissolves like generally holds true...

.

Provitamin A activity

Plant carotenoids are the primary dietary source of provitamin A worldwide, with β-carotene as the most well-known provitamin A carotenoid. Others include α-carotene
Alpha-carotene
α-Carotene is a form of carotene with a β-ring at one end and an ε-ring at the other. It is the second most common form of carotene.-Human physiology:...

 and β-cryptoxanthin
Cryptoxanthin
Cryptoxanthin is a natural carotenoid pigment. It has been isolated from a variety of sources including the petals and flowers of plants in the genus Physalis, orange rind, papaya, egg yolk, butter, apples, and bovine blood serum.-Chemistry:...

. Carotenoid absorption is restricted to the duodenum of the small intestine and dependent on Class B scavenger receptor (SR-B1) membrane protein, which are also responsible for the absorption of vitamin E
Vitamin E
Vitamin E is used to refer to a group of fat-soluble compounds that include both tocopherols and tocotrienols. There are many different forms of vitamin E, of which γ-tocopherol is the most common in the North American diet. γ-Tocopherol can be found in corn oil, soybean oil, margarine and dressings...

 (alpha-tocopherol). One molecule of β-carotene can be cleaved by the intestinal enzyme beta,beta-carotene 15,15'-monooxygenase into two molecules of vitamin A.

Absorption efficiency is estimated to be between 9-22%. The absorption and conversion of carotenoids may depend on the form that the β-carotene is in (cooked vs. raw vegetables, in a supplement), intake of fats and oils at the same time, and current stores of vitamin A and β-carotene in the body. Researchers list the following factors that determine the provitamin A activity of carotenoids:
  • Species of carotenoid
  • Molecular linkage
  • Amount in the meal
  • Matrix properties
  • Effectors
  • Nutrient status
  • Genetics
  • Host specificity
  • Interactions between factors

Symmetric and asymmetric cleavage

In the molecule chain between the two cyclohexyl rings β-carotene cleaves either symmetrically or asymmetrically. Symmetric cleavage with the enzyme beta,beta-carotene-15,15'-dioxygenase requires the antioxidant alpha-tocopherol. This symmetric cleavage gives two equivalent retinal molecules and each retinal molecule further reacts to give retinol (vitamin A
Vitamin A
Vitamin A is a vitamin that is needed by the retina of the eye in the form of a specific metabolite, the light-absorbing molecule retinal, that is necessary for both low-light and color vision...

) and retinoic acid. Beta-carotene is also asymmetrically cleaved into two asymmetric products. The product of asymmetric cleavage is β-apocarotenal
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

Until recently, vitamin A activity in foods was expressed as international units (IU). This is still the measurement generally used on food and supplement labels. However, it is difficult to calculate the total vitamin A activity in the diet in terms of IU, because both the absorption and conversion of carotenoids, as compared with retinol, are variable. The unit retinol equivalent (RE) was developed by the Food and Agriculture Organization of the United Nations/World Health Organization
World Health Organization
The World Health Organization is a specialized agency of the United Nations that acts as a coordinating authority on international public health. Established on 7 April 1948, with headquarters in Geneva, Switzerland, the agency inherited the mandate and resources of its predecessor, the Health...

 (FAO/WHO) in 1967. More recently in 2001, the US Institute of Medicine proposed retinol activity equivalents (RAE) for their Dietary Reference Intakes.

International Units

1 RE = 3.33 IU vitamin A activity from retinol

1 RE = 10 IU vitamin A activity from β-carotene

(In Canada, Health Canada sets 1 RE = 6.667 IU from β-carotene.)

Retinol Equivalents (REs)

1 RE = 1 µg retinol

1 RE = 6 µg β-carotene
(In Canada, Heath Canada sets 1 RE = 2 µg β-carotene.)

1 RE = 12 µg other provitamin A carotenoids

Retinol Activity Equivalents (RAEs)

1 RAE = 1 µg retinol

1 RAE = 2 µg all-trans-β-carotene as a supplement

1 RAE = 12 µg of all-trans-β-carotene in a food matrix

1 RAE = 24 µg other provitamin A carotenes in a food matrix

Sources in the diet

β-Carotene contributes to the orange color of many different fruits and vegetables. Vietnam
Vietnam
Vietnam – sometimes spelled Viet Nam , officially the Socialist Republic of Vietnam – is the easternmost country on the Indochina Peninsula in Southeast Asia. It is bordered by China to the north, Laos to the northwest, Cambodia to the southwest, and the South China Sea –...

ese gac
Gac
Momordica cochinchinensis is a Southeast Asian fruit found throughout the region from Southern China to Northeastern Australia.-Etymology:It is commonly known as gac, from the Vietnamese gấc or quả gấc...

 (Momordica Cochinchinensis Spreng.) and crude palm oil
Palm oil
Palm oil, coconut oil and palm kernel oil are edible plant oils derived from the fruits of palm trees. Palm oil is extracted from the pulp of the fruit of the oil palm Elaeis guineensis; palm kernel oil is derived from the kernel of the oil palm and coconut oil is derived from the kernel of the...

 are particularly rich sources, as are yellow and orange fruits, such as cantaloupe
Cantaloupe
"Rockmelon" redirects here, for the band see Rockmelons. See also Cantaloupe .Cantaloupe refers to a variety of Cucumis melo, a species in the family Cucurbitaceae which includes nearly all melons and squashes. Cantaloupes range in size from...

, mangoes and papayas, orange root vegetables such as carrots and yams
Yam (vegetable)
Yam is the common name for some species in the genus Dioscorea . These are perennial herbaceous vines cultivated for the consumption of their starchy tubers in Africa, Asia, Latin America and Oceania...

 and in green leafy vegetables such as spinach
Spinach
Spinach is an edible flowering plant in the family of Amaranthaceae. It is native to central and southwestern Asia. It is an annual plant , which grows to a height of up to 30 cm. Spinach may survive over winter in temperate regions...

, kale
Kale
Kale is very high in beta carotene, vitamin K, vitamin C, lutein, zeaxanthin, and reasonably rich in calcium. Kale, as with broccoli and other brassicas, contains sulforaphane , a chemical with potent anti-cancer properties. Boiling decreases the level of sulforaphane; however, steaming,...

, sweet potato
Sweet potato
The sweet potato is a dicotyledonous plant that belongs to the family Convolvulaceae. Its large, starchy, sweet-tasting, tuberous roots are an important root vegetable. The young leaves and shoots are sometimes eaten as greens. Of the approximately 50 genera and more than 1,000 species of...

 leaves, and sweet gourd leaves. Vietnamese gac and crude palm oil
Palm oil
Palm oil, coconut oil and palm kernel oil are edible plant oils derived from the fruits of palm trees. Palm oil is extracted from the pulp of the fruit of the oil palm Elaeis guineensis; palm kernel oil is derived from the kernel of the oil palm and coconut oil is derived from the kernel of the...

 have by far the highest content of β-carotene of any known fruit or vegetable, 10 times higher than carrots for example. However, gac is quite rare and unknown outside its native region of SE Asia, and crude palm oil
Palm oil
Palm oil, coconut oil and palm kernel oil are edible plant oils derived from the fruits of palm trees. Palm oil is extracted from the pulp of the fruit of the oil palm Elaeis guineensis; palm kernel oil is derived from the kernel of the oil palm and coconut oil is derived from the kernel of the...

 is typically processed to remove the cartenoids 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 USA, Canada and some European countries.

The U.S. Department of Agriculture lists the following 10 foods to have the highest β-carotene content per serving.
Item Grams per serving Serving size Milligrams β-carotene per serving Milligrams β-carotene per 100 g
Carrot juice, canned 236 1 cup 22.0 9.3
Pumpkin, canned, without salt 245 1 cup 17.0 6.9
Sweet potato, cooked, baked in skin, without salt 146 1 potato 16.8 11.5
Sweet potato, cooked, boiled, without skin 156 1 potato 14.7 9.4
Spinach, frozen, chopped or leaf, cooked, boiled, drained, without salt 190 1 cup 13.8 7.2
Carrots, cooked, boiled, drained, without salt 156 1 cup 13.0 8.3
Spinach, canned, drained solids 214 1 cup 12.6 5.9
Sweet potato, canned, vacuum pack 255 1 cup 12.2 4.8
Carrots, frozen, cooked, boiled, drained, without salt 146 1 cup 12.0 8.2
Collards, frozen, chopped, cooked, boiled, drained, without salt 170 1 cup 11.6 6.8

Side effects

The most common side effect of excessive β-carotene consumption is carotenodermia, a physically harmless condition that presents as a conspicuous orange skin tint arising from deposition of the carotenoid in the outermost layer of the epidermis. Chronic, high doses of synthetic
Synthetic
Synthesis, the combination of two or more parts, whether by design or by natural processes. Furthermore, it may imply being prepared or made artificially, in contrast to naturally.-In the sense of combination:* Synthetic position, a concept in finance...

 β-carotene supplements have been associated with increased rate of lung cancer
Lung cancer
Lung cancer is a disease characterized by uncontrolled cell growth in tissues of the lung. If left untreated, this growth can spread beyond the lung in a process called metastasis into nearby tissue and, eventually, into other parts of the body. Most cancers that start in lung, known as primary...

 among those who smoke. Additionally, supplemental β-carotene may increase the risk of prostate cancer, intracerebral hemorrhage, and cardiovascular and total mortality in people who smoke cigarettes or have a history of high-level exposure to asbestos.

β-Carotene is stored in the liver and many other organs ("golden ovaries"). Rat studies show that the body cannot convert such stored β-carotene into vitamin A, even if a deficit develops. Heavy consumption of synthetic β-carotene additive from a variety of foods, plus from natural sources, may result in saturating the liver's storage capacity for fat soluble vitamins, so that reserves of other fat soluble vitamins, e.g. vitamin D and vitamin A, are not created - in countries far from the Equator, the summer storage of vitamin D, to be drawn upon during the darker winter, may be particularly important, not least in preventing osteoporosis and other vitamin D-deficiency related problems. In many cases the food color annatto
Annatto
Annatto, sometimes called roucou or achiote, is a derivative of the achiote trees of tropical regions of the Americas, used to produce a yellow to orange food coloring and also as a flavoring...

 can be used instead of β-carotene, and is not deposited in the body.

β-Carotene has a high tendency to oxidize, more so than most food fats, and may thus to some extent hasten oxidation more than other food colours such as annatto.

β-Carotene and lung cancer in smokers

A high dose of synthetic
Synthetic
Synthesis, the combination of two or more parts, whether by design or by natural processes. Furthermore, it may imply being prepared or made artificially, in contrast to naturally.-In the sense of combination:* Synthetic position, a concept in finance...

 β-Carotene supplementation increases the probability of lung cancer in cigarette smokers, although the validity of this statement has been put into question.
The effect is specific to supplementation dose as no lung 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
Oncology is a branch of medicine that deals with cancer...

 from β-carotene is based on both cigarette smoke and high daily doses of β-carotene.

There have been at least two suggestions for the mechanism for the observed harmful effect of high-dose β-carotene supplementation in this group. None has so-far gained wide acceptance.

A common explanation of the high dose effect is that when retinoic acid
Retinoic acid
Retinoic acid is a metabolite of vitamin A that mediates the functions of vitamin A required for growth and development. Retinoic acid is required in chordate animals which includes all higher animals from fishes to humans...

 is liganded to RAR-beta (Retinoic Acid Receptor beta), the complex binds AP1
AP-1 (transcription factor)
In the field of molecular biology, the activator protein 1 is a transcription factor which is a heterodimeric protein composed of proteins belonging to the c-Fos, c-Jun, ATF and JDP families. It regulates gene expression in response to a variety of stimuli, including cytokines, growth factors,...

 (Activator Protein 1). AP1
AP-1 (transcription factor)
In the field of molecular biology, the activator protein 1 is a transcription factor which is a heterodimeric protein composed of proteins belonging to the c-Fos, c-Jun, ATF and JDP families. It regulates gene expression in response to a variety of stimuli, including cytokines, growth factors,...

 is a transcription factor that binds to DNA and in downstream events promote cell proliferation. Therefore, in the presence of retinoic acid
Retinoic acid
Retinoic acid is a metabolite of vitamin A that mediates the functions of vitamin A required for growth and development. Retinoic acid is required in chordate animals which includes all higher animals from fishes to humans...

, the retinoic acid:RAR-beta complex binds to AP1
AP-1 (transcription factor)
In the field of molecular biology, the activator protein 1 is a transcription factor which is a heterodimeric protein composed of proteins belonging to the c-Fos, c-Jun, ATF and JDP families. It regulates gene expression in response to a variety of stimuli, including cytokines, growth factors,...

 and inhibit AP-1 from binding to DNA. In that case, AP1
AP-1 (transcription factor)
In the field of molecular biology, the activator protein 1 is a transcription factor which is a heterodimeric protein composed of proteins belonging to the c-Fos, c-Jun, ATF and JDP families. It regulates gene expression in response to a variety of stimuli, including cytokines, growth factors,...

 is no longer expressed, and cell proliferation does not occur. Cigarette smoke increases the asymmetric cleavage of β-carotene, decreasing the level of retinoic acid
Retinoic acid
Retinoic acid is a metabolite of vitamin A that mediates the functions of vitamin A required for growth and development. Retinoic acid is required in chordate animals which includes all higher animals from fishes to humans...

 significantly. This can lead to a higher level of cell proliferation in smokers, and consequently, a higher probability of lung cancer.

Another β-carotene breakdown product suspected of causing cancer at high dose is trans-β-apo-8'-carotenal (common apocarotenal
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.

External links

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