# Carotene
**Carotene** is the class of orange, red, or yellow hydrocarbon pigments within the carotenoid family, distinguished from the related xanthophylls by containing no oxygen atoms. The best-known member, beta-carotene (β-carotene), has the molecular formula C₄₀H₅₆ and a molecular weight of 536.9 g/mol, and is the most abundant form of carotenoid and the most important provitamin A carotenoid, meaning the human body can convert it into vitamin A (retinol)[1](https://pubchem.ncbi.nlm.nih.gov/compound/beta-carotene). The name derives from the carrot (*Daucus carota*), a plant that accumulates high levels of these pigments in its root[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3350171/).
| Key fact | Detail |
|---|---|
| Chemical class | Hydrocarbon carotenoids (no oxygen); xanthophylls are their oxygenated derivatives[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3350171/) |
| β-carotene formula | C₄₀H₅₆, molecular weight 536.9 g/mol; two β-ionone rings joined by a conjugated polyene chain[1](https://pubchem.ncbi.nlm.nih.gov/compound/beta-carotene) • [3](https://onlinelibrary.wiley.com/doi/10.1155/bmri/4147524) |
| Provitamin A activity | β-carotene can theoretically be cleaved to yield two molecules of retinol; α-carotene, with one β-ionone ring, yields one[3](https://onlinelibrary.wiley.com/doi/10.1155/bmri/4147524) |
| Conversion factors | 1 mcg retinol activity equivalent (RAE) = 12 mcg dietary β-carotene, 24 mcg dietary α-carotene or β-cryptoxanthin, or 2 mcg supplemental β-carotene[4](https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/) |
| Absorption | The body absorbs 10%–30% of β-carotene from foods in most cases; cooking and heat treatment increase its bioavailability[4](https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/) |
| Role in plants | Accessory light-harvesting pigments absorbing 450–570 nm and photoprotective agents in photosynthetic membranes[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3350171/) |
| Safety note | Smokers should avoid β-carotene food supplements; trials in smokers found increased lung cancer risk[4](https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/) • [5](https://www.efsa.europa.eu/sites/default/files/2023-11/ul_summary_tables-version-8.pdf) |
## Structure and main forms
Carotenes are highly conjugated C40 hydrocarbon chains, sometimes cyclized at one or both ends of the molecule[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3350171/) • [6](https://www.ams.usda.gov/sites/default/files/media/Beta%20Carotene%20Synthetic%20TR.pdf). The long chain of conjugated double bonds acts as a chromophore (the part of a molecule that absorbs light and produces color) and gives the pigments their characteristic yellow-to-red colors[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3350171/). β-carotene consists of two identical substituted ring structures (β-ionone rings) at each end of the chain[6](https://www.ams.usda.gov/sites/default/files/media/Beta%20Carotene%20Synthetic%20TR.pdf).
The two principal dietary carotenes differ in provitamin A activity. β-carotene, with its two β-ionone rings, can theoretically be cleaved to yield two molecules of biologically active retinol. α-carotene has only one β-ionone ring; central cleavage yields one retinol molecule plus one α-retinol molecule with negligible vitamin A activity, so α-carotene has less vitamin A activity than β-carotene[3](https://onlinelibrary.wiley.com/doi/10.1155/bmri/4147524). Lycopene, the red pigment of tomato, is an acyclic carotene with no provitamin A activity, and serves as the precursor from which other carotenoids are formed by cyclization and functionalization[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3350171/) • [7](https://www.mdpi.com/1661-3821/5/2/13/pdf?version=1749182606).
## Biosynthesis in plants
Carotenoids are synthesized by all photosynthetic organisms and some non-photosynthetic bacteria and fungi; animals generally cannot make them and must obtain them in the diet[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3350171/). Plant carotenoids are tetraterpenes built from the 40-carbon isoprenoid phytoene, itself formed by condensation of two molecules of geranylgeranyl diphosphate (GGPP) in a reaction catalyzed by phytoene synthase, considered the main bottleneck of the pathway[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3350171/).
Desaturation and isomerization of the colorless phytoene produce the red carotenoid lycopene. Cyclization of the ends of the lycopene chain is the first branch point: lycopene β-cyclase introduces a β-ring at both ends of lycopene in a sequential two-step reaction to form β-carotene, while a different enzyme combination yields α-carotene with one β-ring and one ε-ring[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3350171/) • [8](https://www.aocs.org/resource/carotenoid-biosynthesis-and-regulation-in-plants/). Hydroxylation of the rings then generates xanthophylls such as lutein and zeaxanthin[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3350171/).
## Function in photosynthesis
Carotenoids are integral and essential components of the photosynthetic membranes in all plants[9](https://www.annualreviews.org/content/journals/10.1146/annurev.arplant.49.1.557). They act as accessory light-harvesting pigments by absorbing in the range of 450–570 nm, where chlorophylls do not absorb efficiently, and transferring the energy to chlorophylls. Their essential role, however, is photoprotection: quenching chlorophyll triplets and singlet oxygen and dissipating excess light energy through non-photochemical quenching[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3350171/).
**Distribution within the chloroplast.** Chloroplasts have a similar carotenoid composition in all plants, with lutein (45% of the total), β-carotene (25–30%), violaxanthin (10–15%), and neoxanthin (10–15%) as the most abundant carotenoids. Carotenes, mainly β-carotene, are enriched in the photosystem reaction centers, whereas xanthophylls are most abundant in the light-harvesting complexes[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3350171/). Carotenoids become visible in leaves only when chlorophylls are degraded, producing the autumn colors of temperate plants[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3350171/).
## Provitamin A activity and bioavailability
Within the intestine, the BCMO1 enzyme (beta-carotene monooxygenase type 1) transforms provitamin A carotenoids into vitamin A, and how efficiently this happens varies with genetics.[4](https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/) Human studies using stable-isotope methods show that the conversion efficiency of dietary β-carotene to retinol ranges from 3.6:1 to 28:1 by weight, with wide variation between studies and between individuals[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC2854912/). Biofortified staple foods such as maize, rice, and cassava have shown more efficient bioconversion, roughly 3–6:1, than generally observed for vegetables, where ratios of 10–80:1 have been reported[11](https://pubmed.ncbi.nlm.nih.gov/21462118/).
Bioavailability depends on the carotenoid species, the food matrix, the amount consumed in a meal, interactions with other food components, nutritional status, and host factors[12](https://www.annualreviews.org/content/journals/10.1146/annurev.nutr.22.010402.102834). In most cases the body absorbs 10% to 30% of β-carotene from foods, while absorption of preformed retinol can reach 75% to 100%; cooking and heat treatment increase β-carotene bioavailability, and supplement absorption ranges from 8.7% to 65%[4](https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/).
## History
The earliest studies on carotenoids date to the beginning of the 19th century. β-carotene was first isolated by Wackenroder in 1831, and many other carotenoids were discovered and named during the 1800s, although their structures were unknown. The empirical formula C₄₀H₅₆ was established by Willstätter and Mieg in 1907, and the structure was elucidated by Karrer in 1930–31, the first time the structure of any vitamin or provitamin had been established; Karrer received a Nobel Prize for this work. Steenbock suggested in 1919 that a relationship existed between β-carotene and vitamin A, introducing the concept of provitamins. The first total syntheses of β-carotene were achieved in 1950, and Roche began commercial production in 1954[13](https://www.chemistry.bristol.ac.uk/motm/carotene/beta-carotene_history.html).
## Supplements and health
β-carotene is an antioxidant that inhibits free-radical damage to DNA, and it is used in the prevention and treatment of vitamin A deficiency, as a dietary supplement, and as a food colorant; under the FDA it is considered a generally recognized as safe (GRAS) substance[1](https://pubchem.ncbi.nlm.nih.gov/compound/beta-carotene) • [14](https://www.cancer.gov/publications/dictionaries/cancer-drug/def/beta-carotene) • [7](https://www.mdpi.com/1661-3821/5/2/13/pdf?version=1749182606).
**Evidence from large trials.** Clinical trials in smokers found harm rather than benefit. The Carotene and Retinol Efficacy Trial (CARET), which randomized 18,314 current and former smokers and asbestos-exposed workers to 30 mg β-carotene plus 25,000 IU [retinyl palmitate](https://www.edgepedia.org/retinyl-palmitate) daily, was ended prematurely after a mean of 4 years because the supplements increased lung cancer risk by 28%, death from lung cancer by 46%, and all-cause mortality by 17%. The Alpha-[Tocopherol](https://www.edgepedia.org/tocopherol), Beta-Carotene (ATBC) Cancer Prevention Study, in 29,133 male smokers taking 20 mg/day β-carotene for 5–8 years, found an 18% increase in lung cancer risk and an 8% higher overall death rate among those taking β-carotene[4](https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/). In the Age-Related Eye Disease Study 2 (AREDS2), more lung cancers were discovered in the group receiving 15 mg β-carotene than in the group without it (23 versus 11 cases), and 31 of the 34 affected participants were former smokers[4](https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/). On this basis, the [European Food Safety Authority](https://www.edgepedia.org/european-food-safety-authority) advises that smokers should avoid consuming food supplements containing β-carotene[5](https://www.efsa.europa.eu/sites/default/files/2023-11/ul_summary_tables-version-8.pdf).
## References
1. [Beta-Carotene, CID 5280489, PubChem](https://pubchem.ncbi.nlm.nih.gov/compound/beta-carotene)
2. [Ruiz-Sola & Rodríguez-Concepción (2012). Carotenoid Biosynthesis in Arabidopsis: A Colorful Pathway. The Arabidopsis Book.](https://pmc.ncbi.nlm.nih.gov/articles/PMC3350171/)
3. [Marine Carotenoids: A Critical Review of Bioactivities, Bioavailability, and Therapeutic Potential. BioMed Research International (2025).](https://onlinelibrary.wiley.com/doi/10.1155/bmri/4147524)
4. [Vitamin A and Carotenoids, Health Professional Fact Sheet, NIH Office of Dietary Supplements](https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/)
5. [EFSA NDA Panel summary tables of tolerable upper intake levels, version 8 (November 2023)](https://www.efsa.europa.eu/sites/default/files/2023-11/ul_summary_tables-version-8.pdf)
6. [β-Carotene (Synthetic), Technical Report, USDA Agricultural Marketing Service](https://www.ams.usda.gov/sites/default/files/media/Beta%20Carotene%20Synthetic%20TR.pdf)
7. [Comparative Assessment of Nutraceuticals for Supporting Skin Health, MDPI](https://www.mdpi.com/1661-3821/5/2/13/pdf?version=1749182606)
8. [Carotenoid Biosynthesis and Regulation in Plants, AOCS](https://www.aocs.org/resource/carotenoid-biosynthesis-and-regulation-in-plants/)
9. [Cunningham & Gantt (1998). Genes and Enzymes of Carotenoid Biosynthesis in Plants. Annual Review of Plant Biology 49:557-583.](https://www.annualreviews.org/content/journals/10.1146/annurev.arplant.49.1.557)
10. [Tang (2010). Bioconversion of dietary provitamin A carotenoids to vitamin A in humans. American Journal of Clinical Nutrition.](https://pmc.ncbi.nlm.nih.gov/articles/PMC2854912/)
11. [Provitamin A carotenoid bioavailability: what really matters? Proceedings of the Nutrition Society / PubMed](https://pubmed.ncbi.nlm.nih.gov/21462118/)
12. [Yeum & Russell (2002). Carotenoid Bioavailability and Bioconversion. Annual Review of Nutrition 22:483-504.](https://www.annualreviews.org/content/journals/10.1146/annurev.nutr.22.010402.102834)
13. [Beta-carotene history, School of Chemistry, University of Bristol](https://www.chemistry.bristol.ac.uk/motm/carotene/beta-carotene_history.html)
14. [Definition of beta carotene, NCI Drug Dictionary, National Cancer Institute](https://www.cancer.gov/publications/dictionaries/cancer-drug/def/beta-carotene)
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*Topic: Encyclopedia › Life and health › Biological foundations › Biochemistry and metabolism › Metabolites, cofactors, and biomolecules › Metabolite records › Metabolic intermediates › Plant specialized metabolism intermediates*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Sep 30, 2026; Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license