Β-Carotene

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β-Carotene
Beta-Carotin.svg
Beta-carotene-from-xtal-3D-bs-17.png
Beta-carotene-from-xtal-3D-sf.png
B-Carotene powder.jpg
Names
IUPAC name
β,β-Carotene
Systematic IUPAC name
1,1′-[(1E,3E,5E,7E,9E,11E,13E,15E,17E)-3,7,12,16-Tetramethyloctadeca-1,3,5,7,9,11,13,15,17-nonaene-1,18-diyl]bis(2,6,6-trimethylcyclohex-1-ene)
Other names
Betacarotene (INN), β-Carotene, [3] Food Orange 5, Provitamin A
Identifiers
3D model (JSmol)
3DMet
1917416
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.027.851 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 230-636-6
E number E160a (colours)
KEGG
PubChem CID
UNII
  • InChI=1S/C40H56/c1-31(19-13-21-33(3)25-27-37-35(5)23-15-29-39(37,7)8) 17-11-12-18-32(2)20-14-22-34(4)26-28-38-36(6)24-16-30-40(38,9) 10/h11-14,17-22,25-28H,15-16,23-24,29-30H2,1-10H3 X mark.svgN
    Key: OENHQHLEOONYIE-UHFFFAOYSA-N X mark.svgN
  • CC2(C)CCCC(\C)=C2\C=C\C(\C)=C\C=C\C(\C)=C\C=C\C=C(/C)\C=C\C=C(/C)\C=C\C1=C(/C)CCCC1(C)C
Properties
C40H56
Molar mass 536.888 g·mol−1
AppearanceDark orange crystals
Density 1.00 g/cm3 [4]
Melting point 183 °C (361 °F; 456 K) [4]
decomposes [5]
Boiling point 654.7 °C (1,210.5 °F; 927.9 K)
at 760 mmHg (101324 Pa)
Insoluble
Solubility Soluble in CS2, benzene, CHCl3, ethanol
Insoluble in glycerin
Solubility in dichloromethane 4.51 g/kg (20 °C) [6] = 5.98 g/L (given BCM density of 1.3266 g/cm3 at 20°C)
Solubility in hexane 0.1 g/L
log P 14.764
Vapor pressure 2.71·10−16 mmHg
1.565
Pharmacology
A11CA02 ( WHO ) D02BB01 ( WHO )
Hazards
GHS labelling:
GHS-pictogram-exclam.svg
Warning
H315, H319, H412
P264, P273, P280, P302+P352, P305+P351+P338, P321, P332+P313, P337+P313, P362, P501
NFPA 704 (fire diamond)
NFPA 704.svgHealth 0: Exposure under fire conditions would offer no hazard beyond that of ordinary combustible material. E.g. sodium chlorideFlammability 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g. canola oilInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
0
1
0
Flash point 103 °C (217 °F; 376 K) [5]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
X mark.svgN  verify  (what is  Yes check.svgYX mark.svgN ?)

β-Carotene (beta-carotene) is an organic, strongly colored red-orange pigment abundant in fungi, [7] plants, and fruits. It is a member of the carotenes, which are terpenoids (isoprenoids), synthesized biochemically from eight isoprene units and thus having 40 carbons.

Dietary β-carotene is a provitamin A compound, converting in the body to retinol (vitamin A). [8] In foods, it has rich content in carrots, pumpkin, spinach, and sweet potato. [8] It is used as a dietary supplement and may be prescribed to treat erythropoietic protoporphyria, an inherited condition of sunlight sensitivity. [9]

β-carotene is the most common carotenoid in plants. [8] When used as a food coloring, it has the E number E160a. [10] : 119  The structure was deduced in 1930. [11]

Isolation of β-carotene from fruits abundant in carotenoids is commonly done using column chromatography. It is industrially extracted from richer sources such as the algae Dunaliella salina . [12] 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. [13] Being highly conjugated, it is deeply colored, and as a hydrocarbon lacking functional groups, it is lipophilic.

Provitamin A activity

Plant carotenoids are the primary dietary source of provitamin A worldwide, with β-carotene as the best-known provitamin A carotenoid. [8] Others include α-carotene and β-cryptoxanthin. [8] Carotenoid absorption is restricted to the duodenum of the small intestine. One molecule of β-carotene can be cleaved by the intestinal enzyme β,β-carotene 15,15'-monooxygenase into two molecules of vitamin A. [8] [14] [15]

Absorption, metabolism and excretion

As part of the digestive process, food-sourced carotenoids must be separated from plant cells and incorporated into lipid-containing micelles to be bioaccessible to intestinal enterocytes. [8] If already extracted (or synthetic) and then presented in an oil-filled dietary supplement capsule, there is greater bioavailability compared to that from foods. [16]

At the enterocyte cell wall, β-carotene is taken up by the membrane transporter protein scavenger receptor class B, type 1 (SCARB1). Absorbed β-carotene is then either incorporated as such into chylomicrons or first converted to retinal and then retinol, bound to retinol binding protein 2, before being incorporated into chylomicrons. [8] The conversion process consists of one molecule of β-carotene cleaved by the enzyme beta-carotene 15,15'-dioxygenase, which is encoded by the BCO1 gene, into two molecules of retinal. [8] When plasma retinol is in the normal range the gene expression for SCARB1 and BCO1 are suppressed, creating a feedback loop that suppresses β-carotene absorption and conversion. [16]

The majority of chylomicrons are taken up by the liver, then secreted into the blood repackaged into low density lipoproteins (LDLs). [8] From these circulating lipoproteins and the chylomicrons that bypassed the liver, β-carotene is taken into cells via receptor SCARB1. Human tissues differ in expression of SCARB1, and hence β-carotene content. Examples expressed as ng/g, wet weight: liver=479, lung=226, prostate=163 and skin=26. [16]

Once taken up by peripheral tissue cells, the major usage of absorbed β-carotene is as a precursor to retinal via symmetric cleavage by the enzyme beta-carotene 15,15'-dioxygenase, which is encoded by the BCO1 gene. [8] A lesser amount is metabolized by the mitochondrial enzyme beta-carotene 9',10'-dioxygenase, which is encoded by the BCO2 gene. The products of this asymmetric cleavage are two beta-ionone molecules and rosafluene. BCO2 appears to be involved in preventing excessive accumulation of carotenoids; a BCO2 defect in chickens results in yellow skin color due to accumulation in subcutaneous fat. [17] [18]

Conversion factors

For counting dietary vitamin A intake, β-carotene may be converted either using the newer retinol activity equivalents (RAE) or the older international unit (IU). [8]

Retinol activity equivalents (RAEs)

Since 2001, the US Institute of Medicine uses retinol activity equivalents (RAE) for their Dietary Reference Intakes, defined as follows: [8] [19]

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). [19] RE was developed 1967 by the United Nations/World Health Organization Food and Agriculture Organization (FAO/WHO). [20]

International Units

Another older unit of vitamin A activity is the international unit (IU). [8] Like retinol equivalent, the international unit does not take into account carotenoid variable absorption and conversion to vitamin A by humans, as well as the more modern retinol activity equivalent. Food and supplement labels still generally use IU, but IU can be converted to the more useful retinol activity equivalent as follows: [19]

Dietary sources

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. [21] Beta-carotene is found in many foods and is sold as a dietary supplement. [8] β-Carotene contributes to the orange color of many different fruits and vegetables. Vietnamese gac (Momordica cochinchinensis Spreng.) and crude palm oil are particularly rich sources, as are yellow and orange fruits, such as cantaloupe, mangoes, pumpkin, and papayas, and orange root vegetables such as carrots and sweet potatoes. [8]

The color of β-carotene is masked by chlorophyll in green leaf vegetables such as spinach, kale, sweet potato leaves, and sweet gourd leaves. [8] [22]

The U.S. Department of Agriculture lists foods high in β-carotene content: [23]

FoodBeta-carotene

Milligrams per 100 g

Sweet potato, skinned, boiled9.4
Carrot juice9.3
Carrots, raw or boiled9.2
Kale, boiled8.8
Pumpkin, canned6.9
Spinach, canned5.9

No dietary requirement

Government and non-government organizations have not set a dietary requirement for β-carotene. [16]

Side effects

Excess β-carotene is predominantly stored in the fat tissues of the body. [8] 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. [8] [9] [16] [24]

Carotenosis

Carotenoderma, also referred to as carotenemia, is a benign and reversible medical condition where an excess of dietary carotenoids results in orange discoloration of the outermost skin layer. [8] It is associated with a high blood β-carotene value. This can occur after a month or two of consumption of beta-carotene rich foods, such as carrots, carrot juice, tangerine juice, mangos, or in Africa, red palm oil. β-carotene dietary supplements can have the same effect. The discoloration extends to palms and soles of feet, but not to the white of the eye, which helps distinguish the condition from jaundice. Carotenodermia is reversible upon cessation of excessive intake. [25] Consumption of greater than 30 mg/day for a prolonged period has been confirmed as leading to carotenemia. [16] [26]

No risk for hypervitaminosis A

At the enterocyte cell wall, β-carotene is taken up by the membrane transporter protein scavenger receptor class B, type 1 (SCARB1). Absorbed β-carotene is then either incorporated as such into chylomicrons or first converted to retinal and then retinol, bound to retinol binding protein 2, before being incorporated into chylomicrons. The conversion process consists of one molecule of β-carotene cleaved by the enzyme beta-carotene 15,15'-dioxygenase, which is encoded by the BCO1 gene, into two molecules of retinal. When plasma retinol is in the normal range the gene expression for SCARB1 and BCO1 are suppressed, creating a feedback loop that suppresses absorption and conversion. Because of these two mechanisms, high intake will not lead to hypervitaminosis A. [16]

Drug interactions

β-Carotene can interact with medication used for lowering cholesterol. [8] Taking them together can lower the effectiveness of these medications and is considered only a moderate interaction. [8] Bile acid sequestrants and proton-pump inhibitors can decrease absorption of β-carotene. [27] Consuming alcohol with β-carotene can decrease its ability to convert to retinol and could possibly result in hepatotoxicity. [28]

β-Carotene and lung cancer in smokers

Chronic high doses of β-carotene supplementation increases the probability of lung cancer in smokers [8] [29] while its natural vitamer, retinol, increases lung cancer in smokers and nonsmokers. 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 physiological dose of β-carotene (6 mg), in contrast to high pharmacological dose (30 mg). [8] [30]

Increases in lung cancer have been attributed to the tendency of β-carotene to oxidize, [31] yet based on the pharmacokinetics of β-carotene absorption and transport through the intestine and the lack of specific β-carotene transporters, it is unlikely that β-carotene reaches the lung of smokers in sufficient quantities. [32] Additional research is required to understand the link between the increased risk of cancer and all-cause mortality following β-carotene supplementation.

Additionally, supplemental, high-dose β-carotene may increase the risk of prostate cancer, intracerebral hemorrhage, and cardiovascular and total mortality irrespective of smoking status. [8] [9]

Industrial sources

β-carotene is industrially made either by total synthesis (see Retinol § Industrial synthesis) or by extraction from biological sources such as vegetables, microalgae (especially Dunaliella salina), and genetically-engineered microbes. The synthetic path is low-cost and high-yield. [33]

Research

Medical authorities generally recommend obtaining beta-carotene from food rather than dietary supplements. [8] A 2013 meta-analysis of randomized controlled trials concluded that high-dosage (≥9.6 mg/day) beta-carotene supplementation is associated with a 6% increase in the risk of all-cause mortality, while low-dosage (<9.6 mg/day) supplementation does not have a significant effect on mortality. [34] 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. [35] However, a 2018 meta-analysis mostly of prospective cohort studies found that both dietary and circulating beta-carotene are associated with a lower risk of all-cause mortality. The highest circulating beta-carotene category, compared to the lowest, correlated with a 37% reduction in the risk of all-cause mortality, while the highest dietary beta-carotene intake category, compared to the lowest, was linked to an 18% decrease in the risk of all-cause mortality. [36]

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. [37] β-carotene content is confirmed in human retinal pigment epithelium. [16] 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. [38] Reviews reported that for intervention trials using only β-carotene, there was no change to risk of developing AMD. [8] [38] [39]

Cancer

A meta-analysis 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. [8] [40] High levels of β-carotene may increase the risk of lung cancer in current and former smokers. [8] [41] Results are not clear for thyroid cancer. [42]

Cataract

A Cochrane review looked at supplementation of β-carotene, vitamin C, and vitamin E, independently and combined, on people to examine differences in risk of cataract, 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. [43] 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. [44]

Erythropoietic protoporphyria

High doses of β-carotene (up to 180 mg per day) may be used as a treatment for erythropoietic protoporphyria, a rare inherited disorder of sunlight sensitivity, without toxic effects. [8] [9]

Food drying

Foods rich in carotenoid dyes show discoloration upon drying. This is due to thermal degradation of carotenoids, possibly via isomerization and oxidation reactions. [45]

See also

Related Research Articles

<span class="mw-page-title-main">Carotene</span> Class of compounds

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 exception of some aphids and spider mites which acquired the synthesizing genes from fungi). Carotenes are photosynthetic pigments important for photosynthesis.

<span class="mw-page-title-main">Lycopene</span> Carotenoid pigment

Lycopene is an organic compound classified as a tetraterpene and a carotene. Lycopene is a bright red carotenoid hydrocarbon found in tomatoes and other red fruits and vegetables.

The term Vitamin E refers to a group of eight molecular-structure related compounds that include four tocopherols and four tocotrienols. The tocopherols function as fat-soluble antioxidants which may help protect cell membranes from reactive oxygen species. Vitamin E is classified as an essential nutrient for humans. Various government organizations recommend that adults consume between 3 and 15 mg per day, while a 2016 worldwide review reported a median dietary intake of 6.2 mg per day. Sources rich in vitamin E include seeds, nuts, seed oils, peanut butter, vitamin E-fortified foods, and dietary supplements. Symptomatic vitamin E deficiency is rare, is usually caused by an underlying problem with digesting dietary fat rather than from a diet low in vitamin E. Deficiency can cause neurological disorders.

Tocopherols are a class of organic compounds comprising various methylated phenols, many of which have vitamin E activity. Because the vitamin activity was first identified in 1936 from a dietary fertility factor in rats, it was named tocopherol, from Greek τόκοςtókos 'birth' and φέρεινphérein 'to bear or carry', that is 'to carry a pregnancy', with the ending -ol signifying its status as a chemical alcohol.

<span class="mw-page-title-main">Vitamin A</span> Essential nutrient

Vitamin A is a fat-soluble vitamin that is an essential nutrient. The term "vitamin A" encompasses a group of chemically related organic compounds that includes retinol, retinyl esters, and several provitamin (precursor) carotenoids, most notably β-carotene (beta-carotene). Vitamin A has multiple functions: growth during embryo development, maintaining the immune system, and healthy vision. For aiding vision specifically, it combines with the protein opsin to form rhodopsin, the light-absorbing molecule necessary for both low-light and color vision.

<span class="mw-page-title-main">Retinol</span> Chemical compound

Retinol, also called vitamin A1, is a fat-soluble vitamin in the vitamin A family that is found in food and used as a dietary supplement. Retinol or other forms of vitamin A are needed for vision, cellular development, maintenance of skin and mucous membranes, immune function and reproductive development. Dietary sources include fish, dairy products, and meat. As a supplement it is used to treat and prevent vitamin A deficiency, especially that which results in xerophthalmia. It is taken by mouth or by injection into a muscle. As an ingredient in skin-care products, it is used to reduce wrinkles and other effects of skin aging.

<span class="mw-page-title-main">Retinoid</span> Group of tetraterpenes

The retinoids are a class of chemical compounds that are natural derivatives of vitamin A or are chemically related to it. Synthetic retinoids are used in medicine where they regulate skin health, immunity and bone disorders.

The ionones, from greek ἴον ion "violet", are a series of closely related chemical substances that are part of a group of compounds known as rose ketones, which also includes damascones and damascenones. Ionones are aroma compounds found in a variety of essential oils, including rose oil. β-Ionone is a significant contributor to the aroma of roses, despite its relatively low concentration, and is an important fragrance chemical used in perfumery. The ionones are derived from the degradation of carotenoids.

<span class="mw-page-title-main">Lutein</span> Yellow organic pigment created by plants

Lutein is a xanthophyll and one of 600 known naturally occurring carotenoids. Lutein is synthesized only by plants, and like other xanthophylls is found in high quantities in green leafy vegetables such as spinach, kale and yellow carrots. In green plants, xanthophylls act to modulate light energy and serve as non-photochemical quenching agents to deal with triplet chlorophyll, an excited form of chlorophyll which is overproduced at very high light levels during photosynthesis. See xanthophyll cycle for this topic.

<span class="mw-page-title-main">Apocarotenal</span> Chemical compound

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. The empirical chemical formula for apocarotenal is C30H40O.

<span class="mw-page-title-main">Zeaxanthin</span> Chemical compound

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, corn, saffron, goji (wolfberries), and many other plants and microbes their characteristic color.

<span class="mw-page-title-main">Hypervitaminosis A</span> Toxic effects of ingesting too much vitamin A

Hypervitaminosis A refers to the toxic effects of ingesting too much preformed vitamin A. Symptoms arise as a result of altered bone metabolism and altered metabolism of other fat-soluble vitamins. Hypervitaminosis A is believed to have occurred in early humans, and the problem has persisted throughout human history. Toxicity results from ingesting too much preformed vitamin A from foods, supplements, or prescription medications and can be prevented by ingesting no more than the recommended daily amount.

Megavitamin therapy is the use of large doses of vitamins, often many times greater than the recommended dietary allowance (RDA) in the attempt to prevent or treat diseases. Megavitamin therapy is typically used in alternative medicine by practitioners who call their approach orthomolecular medicine. Vitamins are useful in preventing and treating illnesses specifically associated with dietary vitamin shortfalls, but the conclusions of medical research are that the broad claims of disease treatment by advocates of megavitamin therapy are unsubstantiated by the available evidence. It is generally accepted that doses of any vitamin greatly in excess of nutritional requirements will result either in toxicity or in the excess simply being metabolised; thus evidence in favour of vitamin supplementation supports only doses in the normal range. Critics have described some aspects of orthomolecular medicine as food faddism or even quackery. Research on nutrient supplementation in general suggests that some nutritional supplements might be beneficial, and that others might be harmful; several specific nutritional therapies are associated with an increased likelihood of the condition they are meant to prevent.

<span class="mw-page-title-main">Carotenoid oxygenase</span>

Carotenoid oxygenases are a family of enzymes involved in the cleavage of carotenoids to produce, for example, retinol, commonly known as vitamin A. This family includes an enzyme known as RPE65 which is abundantly expressed in the retinal pigment epithelium where it catalyzed the formation of 11-cis-retinol from all-trans-retinyl esters.

<span class="mw-page-title-main">Carotenosis</span> Skin discoloration caused by carotenoids

Carotenosis is a benign and reversible medical condition where an excess of dietary carotenoids results in orange discoloration of the outermost skin layer. The discoloration is most easily observed in light-skinned people and may be mistaken for jaundice. Carotenoids are lipid-soluble compounds that include alpha- and beta-carotene, beta-cryptoxanthin, lycopene, lutein, and zeaxanthin. The primary serum carotenoids are beta-carotene, lycopene, and lutein. Serum levels of carotenoids vary between region, ethnicity, and sex in the healthy population. All are absorbed by passive diffusion from the gastrointestinal tract and are then partially metabolized in the intestinal mucosa and liver to vitamin A. From there they are transported in the plasma into the peripheral tissues. Carotenoids are eliminated via sweat, sebum, urine, and gastrointestinal secretions. Carotenoids contribute to normal-appearing human skin color, and are a significant component of physiologic ultraviolet photoprotection.

<span class="mw-page-title-main">Vitamin A deficiency</span> Disease resulting from low Vitamin A concentrations in the body

Vitamin A deficiency (VAD) or hypovitaminosis A is a lack of vitamin A in blood and tissues. It is common in poorer countries, especially among children and women of reproductive age, but is rarely seen in more developed countries. Vitamin A plays a major role in phototransduction, so this deficiency impairs vision, often presenting with nyctalopia. In more severe VAD cases, it can progress to xerophthalmia, keratomalacia, and complete blindness.

β-Cryptoxanthin is a natural carotenoid pigment. It has been isolated from a variety of sources including the fruit of plants in the genus Physalis, orange rind, winter squashes such as butternut, papaya, egg yolk, butter, apples, and bovine blood serum.

<span class="mw-page-title-main">Beta-carotene 15,15'-dioxygenase</span> Mammalian protein found in Homo sapiens

In enzymology, beta-carotene 15,15'-dioxygenase, (EC 1.13.11.63) is an enzyme with systematic name beta-carotene:oxygen 15,15'-dioxygenase (bond-cleaving). In human it is encoded by the BCO1 gene. This enzyme catalyses the following chemical reaction

Vitamins occur in a variety of related forms known as vitamers. A vitamer of a particular vitamin is one of several related compounds that performs the functions of said vitamin and prevents the symptoms of deficiency of said vitamin.

A co-carcinogen is a chemical that promotes the effects of a carcinogen in the production of cancer. Usually, the term is used to refer to chemicals that are not carcinogenic on their own, such that an equivalent amount of the chemical is insufficient to initiate carcinogenesis. A chemical can be co-carcinogenic with other chemicals or with nonchemical carcinogens, such as UV radiation.

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