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Carotenoid

Carotenoids are a class of more than 1,100 naturally occurring tetraterpenoid pigments, synthesized by , , photosynthetic , fungi, and , featuring a polyisoprenoid backbone with 40 carbon atoms and a long chain of conjugated double bonds that imparts yellow, orange, red, or purple hues to various organisms and tissues. They are classified into two main subgroups: carotenes, which are purely hydrocarbon structures such as and , and xanthophylls, which contain oxygen functionalities like and . In , carotenoids play essential roles as accessory light-harvesting pigments in and as photoprotectants that quench excess energy and prevent oxidative damage from generated under high light conditions. Humans obtain carotenoids exclusively through diet, as most animals, including humans, lack the biosynthetic enzymes, with major sources including colorful fruits and vegetables such as carrots (rich in ), tomatoes (), spinach and ( and ), and citrus fruits (). Upon ingestion, carotenoids are absorbed in the with bioavailability enhanced by dietary fats, and certain provitamin A types like , , and are cleaved by β-carotene 15,15'-monooxygenase into , serving as precursors to essential for vision, immune function, and epithelial integrity. Beyond their role in vitamin A provision, carotenoids exhibit potent properties by scavenging free radicals and , potentially reducing and linked to chronic conditions. Epidemiological evidence suggests dietary intake of carotenoids is associated with decreased risk of age-related , certain cancers (e.g., and ), , and cognitive decline, though high-dose supplements have shown mixed results and potential risks in smokers for β-carotene. Additionally, specific carotenoids like and accumulate in the , filtering and protecting against to support eye health.

Definition and Classification

Chemical Definition

Carotenoids are a class of tetraterpenoids, consisting of C40 compounds derived from the head-to-tail linkage of eight isoprenoid (C5) units, featuring a central composed of conjugated double bonds that are primarily responsible for their vibrant yellow, orange, and red coloration. These pigments exhibit a general molecular formula of C40H56 in their simplest form, with variations arising from structural modifications. The backbone structure of carotenoids is typically a long, linear polyene chain that may terminate in cyclic rings, such as β-ionone rings, at one or both ends, allowing for a range of configurations from acyclic to fully bicyclic. Carotenoids are broadly divided into two subclasses based on their chemical composition: carotenes, which are non-polar hydrocarbons lacking oxygen atoms, and , which are oxygenated derivatives bearing functional groups like hydroxyl (-OH), keto (=O), or epoxy bridges that enhance their polarity and solubility in aqueous environments. Representative examples include , a carotene with two β-ionone rings connected by a polyene chain, and , a xanthophyll featuring hydroxyl groups on its ionone rings. The discovery of carotenoids dates back to the early , when the orange pigment was first isolated in 1831 from carrot roots () by German chemist Heinrich Wackenroder, who named it "carotin" after its source. In 1837, Swedish chemist identified and named xanthophylls as the yellow pigments present in autumn leaves, distinguishing them from the carotenes in plant material. These early isolations laid the foundation for recognizing carotenoids as isoprenoid-derived compounds biosynthesized from isopentenyl pyrophosphate precursors.

Types of Carotenoids

Carotenoids are primarily classified into two broad categories based on their : carotenes, which are pigments lacking oxygen atoms, and xanthophylls, which are oxygenated derivatives of carotenes. Carotenes can be further subdivided by their ring structures, including acyclic forms such as , found in tomatoes and responsible for their red color; monocyclic variants like γ-carotene; and bicyclic types exemplified by , a common orange pigment in carrots. Xanthophylls incorporate functional groups such as hydroxyls or keto groups, as seen in , which features hydroxyl groups and accumulates in the of the , or , characterized by keto groups and imparting a pink hue to and . Apocarotenoids represent a distinct subcategory derived from the oxidative cleavage of carotenoids, resulting in shorter-chain molecules with specific biological roles. For instance, , a key component of in vision, is produced through the central cleavage of . These compounds often serve as signaling molecules or precursors in metabolic pathways across organisms. As of 2020, over 1,200 distinct carotenoids have been identified, reflecting their vast diversity across taxa. This includes unique types in , such as spheroidene, a hydroxylated carotenoid in photosynthetic like Rhodopseudomonas spheroides that aids in light harvesting, contrasted with plant and algal variants like , an allenic prevalent in and diatoms for energy transfer in . From a functional perspective, carotenoids are also grouped by their ability to serve as precursors to , with provitamin A types including , , and , which can be converted to in the body, while non-provitamin A carotenoids like lack this capacity due to structural differences.

Structure and Properties

Molecular Structure

Carotenoids possess a fundamental tetraterpenoid skeleton composed of a 40-carbon polyisoprenoid chain featuring 9–11 conjugated double bonds that confer near bilateral around a central double bond. This extended polyene backbone, typically represented as a linear sequence of alternating single (–CH₂–CH=) and double (–CH=CH–) bonds spanning the C5–C5' positions, enables delocalization of π-electrons and is the source of their distinctive chromophoric properties. In many carotenoids, particularly carotenes like , this chain is capped at both termini by β-ionone rings—six-membered structures with a double bond between C5 and C6, a dimethyl group at C1, and a methyl at C5, fused to the chain via C6–C7 and C1–C6 bonds. Structural variations among carotenoids arise from modifications to this core framework, including acyclic configurations, alternative cyclization patterns, and side chain alterations. Acyclic carotenoids, such as phytoene, lack end rings and exhibit a central conjugated triene system of three double bonds, resulting in a colorless, hydrocarbon formula of C₄₀H₆₄. Cyclic forms introduce one or two rings; for instance, lutein is a xanthophyll with an asymmetric structure featuring a β-ionone ring at one end (cyclohexene with the aforementioned substitutions) and an ε-ionone ring at the other (a cyclohexene ring with the endocyclic double bond between C4' and C5' and shifted methyl groups). Side chain modifications further diversify the class, such as the incorporation of acetylenic (triple) bonds in the polyene chain, exemplified by diatoxanthin, an algal carotenoid where a –C≡C– linkage replaces a double bond near one terminus, altering the conjugation pattern. Oxygen-containing functional groups in xanthophylls, like hydroxyl (–OH) moieties in lutein at C3 and C3' or keto (=O) groups in capsanthin at C6, are often attached to ring carbons, enhancing polarity while maintaining the conjugated system. Stereochemistry plays a critical role in carotenoid architecture, primarily through cis-trans (Z-E) isomerism at the conjugated bonds, with the all-trans configuration being the most stable and prevalent in nature due to minimized steric hindrance. In the all-trans form, all bonds adopt an extended planar , maximizing conjugation; however, cis isomers introduce bends in the , as in 15-cis-β-carotene, where the central 15–15' is in the Z configuration, resulting in a kinked structure observed in photosynthetic reaction centers of higher . These isomers differ in their three-dimensional profiles, with cis forms exhibiting shorter effective lengths and altered ring orientations relative to the backbone. For diagrammatic representation, the polyene is often illustrated as a zigzag line of 11 bonds flanked by single bonds and end groups, with β-ionone rings shown as fused hexagons incorporating the chain's terminal bonds, and oxygen groups marked as substituents on the ring carbons for xanthophylls.

Physical and Chemical Properties

Carotenoids display a of colors ranging from yellow to red, attributable to their selective of visible light primarily in the region. Yellow-pigmented carotenoids typically absorb between 400 and 500 nm, while those appearing red extend up to around 550 nm. For instance, , a prototypical carotenoid, shows characteristic maxima at approximately 425 nm, 451 nm, and 477 nm in organic solvents like , with a of 139,500 cm⁻¹ M⁻¹ at 451 nm. These pigments are highly lipophilic, rendering them insoluble in water (e.g., solubility < 0.001 g/L in aqueous media) but highly soluble in nonpolar organic solvents such as , , and . This hydrophobicity is quantified by high octanol-water partition coefficients, with log P values exceeding 8 for most carotenoids and reaching 14.8 for , facilitating their incorporation into membranes and extraction using organic phases. Carotenoids exhibit limited stability, undergoing rapid oxidation and cis-trans when exposed to , , or pro-oxidants like oxygen and metals. Thermal degradation accelerates with temperature; for example, the half-life of β-carotene in processed slices is 3.02 hours at 45°C but decreases to 1.43 hours at 65°C, following with an activation energy of approximately 50 kJ/mol. In solution or food matrices, exposure to 100°C can lead to significant loss within 30 minutes, emphasizing the need for protective formulations in applications. Spectroscopically, carotenoids are identified by their intense UV-Vis in the 400–550 nm range, enabling sensitive detection and quantification via intensities. provides distinctive vibrational signatures, including strong bands at ~1000 cm⁻¹ for C–C single-bond stretching and 1500–1600 cm⁻¹ for C=C double-bond stretching, which are enhanced under visible laser excitation and allow non-invasive analysis in biological samples. These compounds also efficiently quench in nearby molecules through and triplet , a property exploited in photodynamic studies.

Structure-Property Relationships

The length of the conjugated polyene chain in carotenoids fundamentally influences their , particularly the of maximum (λ_max), which determines their color. As the number of conjugated double bonds (n) increases, the effective conjugation length extends, leading to a bathochromic shift toward longer wavelengths due to a decrease in the HOMO-LUMO energy gap. This relationship can be approximated by the \lambda_{\max} \approx 30.5 \times n + 100 \, \text{nm}, where n represents the number of conjugated double bonds; for example, with n = 11 exhibits absorption around 450 nm, appearing orange, while shorter-chain carotenoids like phytoene (n = 3) absorb in the UV region below 300 nm, rendering them colorless. Cyclization at the ends of the polyene chain, such as the formation of β-ionone rings in , enhances molecular rigidity compared to acyclic carotenoids like , reducing conformational flexibility and stabilizing the against twisting. This structural rigidity contributes to sharper bands and greater under . Moreover, the presence of at least one unsubstituted β-ring is essential for provitamin A activity, enabling enzymatic cleavage to , whereas acyclic or differently cyclized forms lack this capability. Oxygenation introduces polar hydroxyl, , or groups, transforming nonpolar carotenes into more hydrophilic xanthophylls, which alters and intermolecular interactions. This increased facilitates better integration into polar environments like aqueous interfaces but reduces partitioning into bilayers compared to carotenes. In terms of reactivity, oxygenation can extend effective conjugation— as seen in , where the conjugated system spans 11 double bonds due to enone functionalities—enhancing radical scavenging efficiency by stabilizing radical intermediates more effectively than in non-oxygenated analogs. Geometric isomerism along the polyene chain significantly affects stability and bioavailability, with all-trans isomers generally being more thermodynamically stable than their cis counterparts due to minimized steric repulsion between substituents. The free energy difference (ΔG) between trans and cis forms typically ranges from 2 to 5 kcal/mol, with cis isomers exhibiting higher strain and propensity for reversion to trans under heat or light. For provitamin A carotenoids like β-carotene, the all-trans form demonstrates superior bioavailability in absorption studies compared to common cis isomers such as 9-cis or 13-cis.

Biosynthesis

Precursors and Pathways

Carotenoids are synthesized from the universal C5 isoprenoid precursors and its isomer . These building blocks are condensed in a head-to-tail manner to form longer isoprenoid chains, ultimately leading to the C40 carotenoid backbone. In organisms capable of carotenoid production, IPP and DMAPP are generated through two distinct biosynthetic routes: the 2-C-methyl-D-erythritol 4-phosphate () pathway and the mevalonate (MVA) pathway. The pathway predominates in the plastids of and in the of many , providing and DMAPP for carotenoid synthesis. In , this pathway operates within membrane-bound compartments, ensuring localized production of precursors for downstream carotenoid assembly. In contrast, the MVA pathway functions in the of animals and fungi, but these organisms are generally limited to producing apocarotenoids rather than full carotenoids; animals cannot synthesize carotenoids and rely on dietary sources for modification into derivatives like retinoids. Certain fungi, however, utilize the MVA pathway to produce intact carotenoids. also possess a cytosolic MVA pathway, which can supply precursors via limited exchange with plastids, though the MEP route is primary for carotenoids. The assembly begins with successive condensations of IPP and DMAPP to yield geranylgeranyl pyrophosphate (GGPP), a C20 intermediate. Two molecules of GGPP are then dimerized to form phytoene, the first committed C40 carotenoid precursor, in a reaction catalyzed within the plastidial compartment in plants. This plastid-specific localization in plants facilitates efficient coupling with downstream processes, with transport mechanisms allowing minor contributions from cytosolic MVA-derived precursors when needed.

Key Steps and Enzymes

The carotenoid biosynthesis pathway in plants proceeds through a series of enzymatic transformations in plastids, beginning with the formation of phytoene and culminating in diverse end products via desaturation, cyclization, oxygenation, and cleavage reactions. The first committed step is catalyzed by phytoene synthase (PSY), which condenses two molecules of geranylgeranyl diphosphate (GGPP), derived from the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway, into the colorless C40 hydrocarbon phytoene. This reaction establishes the symmetrical carbon backbone essential for all subsequent carotenoids. Desaturation follows, introducing conjugated double bonds to phytoene to yield the red-pigmented , which features 11 conjugated double bonds. Phytoene desaturase (PDS) performs the initial dehydrogenations, converting 15-cis-phytoene to 9,15,9'-tri-cis-ζ-carotene through the addition of four double bonds in a stereo-specific manner. Zeta-carotene desaturase (ZDS) then acts on ζ-carotene, introducing additional double bonds to complete the conversion to all-trans-, often in coordination with isomerases to adjust cis-trans configurations. These desaturases are iron-dependent enzymes embedded in membranes, and their activity is crucial for shifting the pathway from colorless to chromophore-containing intermediates. At the branch point, lycopene undergoes cyclization to form cyclic carotenoids. Lycopene β-cyclase (LCY-β) catalyzes the folding of both ends of the linear lycopene molecule into β-ionone rings, producing β-carotene with its characteristic symmetrical structure. In contrast, lycopene ε-cyclase (LCY-ε) introduces an ε-ionone ring at one terminus, yielding δ-carotene, which is further cyclized by LCY-β at the other end to form α-carotene. These cyclase enzymes, belonging to the terpenoid synthase family, determine the pathway's divergence toward provitamin A-active carotenoids like β-carotene. Further diversification occurs through oxygenation, adding functional groups to the β-rings of . Beta-carotene hydroxylase (BCH), a non-heme iron-dependent , sequentially hydroxylates the 3 and 3' positions of to produce , a key . In certain organisms like and , beta-carotene ketolase (BKT) introduces keto groups at the 4 and 4' positions, converting to echinenone and then canthaxanthin; subsequent by BCH yields , a potent . These modifications enhance polarity and bioactivity of the carotenoids. Finally, carotenoid cleavage dioxygenases (CCDs), a family of non-heme iron enzymes, oxidatively cleave internal double bonds of carotenoids to generate apocarotenoids. For instance, 9-cis-epoxycarotenoid dioxygenase (NCED), a specialized CCD, cleaves 9-cis-violaxanthin or 9-cis-neoxanthin at the 9,10 position to produce xanthoxin, the direct precursor to the plant hormone abscisic acid. Other CCDs, such as CCD1 and CCD4, contribute to volatile apocarotenoids involved in aroma and stress responses, underscoring the pathway's role in signaling.

Regulation of Biosynthesis

The regulation of carotenoid biosynthesis in occurs primarily at the and post-translational levels, ensuring precise control over flux through the pathway. often centers on the phytoene () , the rate-limiting in carotenoid production. upregulates PSY expression through phytochrome-interacting factors (PIFs), such as PIF1, which bind to the PSY promoter and activate transcription in response to photomorphogenic signals. This mechanism is crucial during seedling deetiolation, where rapid carotenoid accumulation protects emerging photosynthetic tissues. Additionally, feedback inhibition by downstream products like (ABA) derivatives modulates PSY transcription, preventing overaccumulation and maintaining . Post-translational regulation further fine-tunes PSY activity by influencing protein stability. The ORANGE (OR) protein family acts as a chaperone, directly interacting with to prevent its degradation by proteases such as Clp, thereby enhancing carotenoid levels without altering transcript abundance. In , OR proteins stabilize PSY in a dose-dependent manner, demonstrating their role as major posttranscriptional regulators across diverse plant species.30336-2) This chaperone activity is conserved, as seen in sweet potato where IbOR post-translationally controls IbPSY stability to boost tuber carotenoid content. Environmental factors integrate with these molecular controls to adapt carotenoid production to external cues. strongly induces via photoreceptors like phytochromes and cryptochromes, which signal through PIFs and other transcription factors to elevate expression. fluctuations also influence regulation; moderate promotes carotenoid accumulation in fruits by stabilizing enzymes, while extremes can repress synthesis through stress-responsive pathways. Hormones such as , derived from carotenoid cleavage, exert feedback repression on upstream genes like during or , balancing with signaling needs. Organism- and tissue-specific regulation highlights the pathway's adaptability. In tomato fruits, PSY1 expression surges during ripening, driven by ethylene and light signals, leading to massive lycopene accumulation, whereas PSY levels remain low in roots to prioritize other metabolic needs. This tissue-specific control underscores why carotenoids are abundant in chromoplast-rich fruits but minimal in etiolated or subterranean organs. Genetic engineering exploits these regulatory insights; for instance, inserting the daffodil PSY gene into rice endosperm in 2000 activated carotenoid biosynthesis in a normally colorless tissue, producing β-carotene-enriched "Golden Rice" to address vitamin A deficiency. Such interventions demonstrate how manipulating PSY regulation can enhance carotenoid output in staple crops.

Functions in Nature

Role in Photosynthesis and Photoprotection

Carotenoids serve as essential accessory pigments in the light-harvesting complexes of photosystems I and II in oxygenic , absorbing light in the blue-green spectrum and transferring the excitation energy to molecules with high efficiency. This energy transfer primarily occurs through (FRET), a non-radiative process driven by dipole-dipole interactions between the carotenoid donor and chlorophyll acceptor, enabling the carotenoids to extend the spectral range of light capture beyond that of chlorophyll alone. In many photosynthetic organisms, such as , this transfer achieves efficiencies exceeding 90%, ensuring that nearly all absorbed photons contribute to photochemical reactions in the reaction centers. A key photoprotective mechanism involving carotenoids is the , which dynamically adjusts to excess light conditions by interconverting specific to dissipate surplus energy as heat, thereby preventing photodamage to the photosynthetic apparatus. In , the cycle involves the reversible de-epoxidation of violaxanthin to under high light, catalyzed by the enzyme violaxanthin de-epoxidase (VDE) in the acidic , followed by epoxidation back to violaxanthin by zeaxanthin epoxidase (ZE) in low light. accumulation enhances (NPQ), a process where excess excitation energy from is safely thermalized rather than being used for , maintaining the balance between light harvesting and protection. Carotenoids also provide direct protection against generated during by scavenging , a highly reactive byproduct of excess . Beta-carotene, a prominent carotenoid in , efficiently quenches through physical energy transfer, with a second-order rate constant of approximately 10^{10} M^{-1} s^{-1}, allowing it to neutralize this oxidant before it damages lipids, proteins, or pigments. This quenching occurs in both the antenna complexes and reaction centers, contributing to the overall resilience of photosynthetic organisms under fluctuating environments. In addition to singlet oxygen scavenging, carotenoids manage triplet states to avert further , as chlorophyll triplets can sensitize formation if unquenched. Through close interactions in the pigment-protein complexes, carotenoids accept triplet energy from via triplet-triplet , rapidly dissipating it harmlessly and preventing the accumulation of long-lived, damaging triplets. This process is particularly vital in , where charge recombination under closed reaction centers would otherwise generate triplets, and is observed across diverse photosynthetic species from to higher plants.

Pigmentation in Plants and Algae

Carotenoids play a crucial role in the pigmentation of plants, becoming particularly evident during seasonal changes such as autumn leaf coloration. In temperate deciduous trees, chlorophyll degradation in the fall unmasks underlying carotenoids, which impart yellow and orange hues to the leaves. These pigments, including carotenes like beta-carotene and xanthophylls such as lutein and zeaxanthin, are synthesized in the plastids throughout the growing season but remain hidden by dominant green chlorophyll. As shorter days and cooler temperatures trigger chlorophyll breakdown, the carotenoids provide the vibrant colors that signal the end of the photosynthetic period. In flowers and fruits, carotenoids contribute to yellow, orange, and pigmentation that serves ecological functions, primarily attracting pollinators and . For instance, , a carotenoid, accumulates in ripe fruits (Solanum lycopersicum), enhancing their visual appeal to birds and mammals that aid in through consumption and excretion. Similarly, in flowers like those of marigolds ( spp.), xanthophylls produce bright tones that guide toward , facilitating . These color signals have co-evolved with animal pollinators and frugivores, promoting in angiosperms. Algal pigmentation by carotenoids also supports ecological adaptations in aquatic environments. In brown algae (Phaeophyceae), such as (Laminariales), is the dominant carotenoid, conferring the characteristic brown color while aiding in absorption for and providing protection against (UV) radiation. This pigment transfers energy to in the photosynthetic apparatus and acts as a shield against excess and in sunlit coastal waters. In dinoflagellates, which include some green-appearing species, peridinin functions in filtering within peridinin-chlorophyll-protein complexes, optimizing energy capture while mitigating photodamage and potentially aiding in by modulating visible in habitats.

Coloration in Animals

Animals, unlike plants and some microorganisms, cannot synthesize carotenoids de novo and must acquire them through their diet to produce pigmentation. These pigments are deposited in various tissues, including feathers, scales, , and exoskeletons, where they contribute to vibrant yellows, oranges, reds, and pinks that serve ecological and behavioral functions. For instance, in birds such as , the characteristic pink coloration arises from the accumulation of beta-carotene and other carotenoids ingested from and crustaceans in their diet; without this dietary input, their would remain pale or gray. Similarly, like obtain from and , which imparts their reddish hues to and flesh, enhancing or signaling in environments. In many species, carotenoid-based coloration plays a key role in , acting as an honest signal of health and genetic quality because the pigments are costly to acquire and metabolize. Male finches, for example, display red beak and plumage coloration derived from dietary carotenoids like beta-carotene, which females prefer as it indicates better nutritional status and resistance to parasites; experimental studies from the demonstrated that females consistently chose males with brighter red traits in trials. This preference correlates with male pairing success, as redder individuals secure mates more readily, underscoring how carotenoid access influences reproductive outcomes. Invertebrates also utilize carotenoids for coloration, often through dietary uptake and biochemical modification. Crustaceans such as and lobsters derive from their algal and planktonic food sources, which is esterified and bound to proteins in their exoskeletons to produce the striking red pigmentation observed upon cooking or molting; this coloration aids in among or serves as a warning signal. In , metabolic modifications like esterification further enhance stability in , preventing degradation during growth and ensuring durable color expression that persists through environmental stresses.

Occurrence and Sources

Natural Occurrence

Carotenoids are ubiquitous pigments found across a wide range of , with over 1,100 distinct structures identified in nature, primarily synthesized by , , and photosynthetic . These compounds exhibit remarkable diversity in their distribution, contributing to the coloration and physiological adaptations in various ecosystems, from terrestrial to aquatic and extremophilic microbes. Surveys of neotropical vascular , encompassing 86 from 64 families, highlight the prevalence of carotenoids as key components of pigmentation. In , carotenoids are most abundant in photosynthetic tissues such as chloroplasts, where they serve as essential accessory pigments alongside chlorophylls, with concentrations varying by species and environmental conditions. For instance, accumulates at levels of approximately 8-10 mg per 100 g in carrots (), a where it imparts the characteristic hue. In non-photosynthetic or fruit tissues, different carotenoids predominate; tomatoes (Solanum lycopersicum), for example, contain at 0.9-7.7 mg per 100 g fresh weight, responsible for their red coloration. Broader analyses indicate that carotenoids are present in over 600 unique forms across plant species, underscoring their widespread occurrence in leaves, , fruits, and flowers. Algae represent another major reservoir of carotenoids, particularly in environments, where brown seaweeds like (Laminariales order) produce as a dominant , giving them their characteristic golden-brown appearance and aiding in light harvesting. This carotenoid is prevalent in macro and diatoms, contributing to the of aquatic ecosystems. In photosynthetic , carotenoids can comprise a significant portion of , similar to their role in higher . Among microorganisms, and display notable carotenoid diversity adapted to specific niches. Photosynthetic and synthesize compounds like myxol, a glycosylated found in such as , which supports membrane stability in oxygenic phototrophs. Non-photosynthetic , including , produce staphyloxanthin, a C30 carotenoid that imparts a golden pigmentation and enhances survival in oxidative environments. In extremophilic settings, halophilic like those in the Haloferax accumulate bacterioruberin, a C50 carotenoid that protects against high salinity and UV radiation, exemplifying microbial adaptations in hypersaline habitats. Overall, this microbial production highlights the ecological abundance of carotenoids beyond eukaryotic producers, with estimates of up to 850 known variants across prokaryotic domains.

In Human Foods

Carotenoids are abundant in various human foods, particularly fruits, , and , serving as key dietary contributors to intake. Vegetables like carrots are rich in , with raw carrots containing approximately 8.8 mg per 100 g. Fruits such as tomatoes provide significant , averaging 3 mg per 100 g in fresh form. , including , offers , with sockeye salmon supplying about 4 mg per 100 g. These examples highlight deeply pigmented foods as primary sources, where carotenoids contribute to the vibrant colors and nutritional profiles of everyday diets. Bioavailability of carotenoids is influenced by dietary and preparation factors. Co-ingestion with fats enhances , as minimal amounts of 3–5 g of dietary fat per meal can increase uptake by 3–5 times, facilitating formation in the gut. Cooking methods also play a role; for instance, heat processing tomatoes boosts lycopene release from the food matrix, improving its accessibility compared to raw consumption. These factors underscore the importance of meal composition for optimizing carotenoid utilization from foods. Food processing impacts carotenoid stability, with varying retention rates depending on the method. Heat generally preserves 70–80% of carotenoids, such as in emulsions, though prolonged exposure can lead to or . During , oxidation causes losses of 20–50%, influenced by oxygen exposure, temperature, and packaging; for example, in beverages may decline by 30–32% over 42 days at 10–20°C. These effects emphasize the need for controlled conditions to maintain carotenoid levels in processed products. In the average Western diet, daily intake of provitamin A carotenoids, primarily , , and , is estimated at 2–3 mg, derived mainly from fruits and vegetables. This level reflects typical consumption patterns, with variations based on dietary habits and regional food availability.

Health Effects

Nutritional Roles

Carotenoids play a vital role in primarily through their provitamin A activity and properties. Certain carotenoids, such as , , and , serve as precursors to (retinol), which is essential for , immune function, , and cellular communication. The enzyme 15,15'-oxygenase 1 (BCO1) catalyzes the central oxidative cleavage of at the 15-15' double bond in the intestinal mucosa, symmetrically producing two molecules of retinal (vitamin A aldehyde) from one molecule of . This retinal is then converted to retinol or retinoic acid, meeting the body's needs when dietary preformed is insufficient. The efficiency of this conversion varies by individual factors like and diet, but for dietary , the standard equivalency is approximately 12 μg of yielding 1 μg of retinol activity equivalents (RAE), reflecting challenges in plant-based sources. Beyond provitamin A functions, carotenoids exhibit potent antioxidant activity by quenching reactive oxygen species, including peroxyl radicals, thereby protecting lipids, proteins, and DNA from oxidative damage in cell membranes and lipoproteins. This radical-scavenging capacity helps mitigate inflammation and supports overall cellular health. For instance, lycopene demonstrates high antioxidant potential, underscoring its role in neutralizing free radicals more effectively than some other carotenoids. Specific carotenoids like and are crucial for eye health, accumulating in the macula lutea of the to form macular that filters and reduces . These xanthophylls also contribute to immune modulation by enhancing production and T-cell responses, indirectly through pathways or direct effects. Evidence from clinical studies suggests a daily intake of 10 mg and 2 mg for optimal eye protection, though average consumption in Western diets falls below this at 1-2 mg. To meet requirements from carotenoids, the Recommended Dietary Allowance (RDA) is 700 μg RAE per day for adult women and 900 μg RAE for men, equivalent to about 8.4-10.8 mg of β-carotene assuming full conversion efficiency.

Disease Prevention and Risks

Carotenoids, particularly , have been associated with a reduced risk of through dietary intake, with epidemiologic evidence indicating potential reductions of 10-20% in risk. Similarly, , often combined with in supplements, has demonstrated efficacy in preventing age-related (AMD); the Age-Related Eye Disease Study 2 (AREDS2) trial reported a 25% reduction in progression to advanced AMD among participants with intermediate disease. Low intake of provitamin A carotenoids like beta-carotene can contribute to , which affects an estimated 190 million preschool-aged children globally (as of 2023) and manifests in symptoms such as night blindness. This deficiency arises primarily from inadequate dietary sources in regions with limited access to carotenoid-rich foods, exacerbating risks of blindness and increased mortality. Excessive supplementation with beta-carotene poses risks, particularly for smokers; the Beta-Carotene and Retinol Efficacy Trial () found a 28% higher incidence of among participants receiving beta-carotene and compared to . Overconsumption of carotenoids from foods or supplements can also lead to carotenodermia, a benign condition characterized by yellow-orange skin discoloration due to elevated blood carotene levels. Carotenoid is influenced by dietary factors, with soluble fibers reducing by trapping carotenoids in the gut, thereby lowering uptake. Conversely, dietary fats enhance , as even small amounts (3-5 grams per meal) facilitate the incorporation of fat-soluble carotenoids into micelles for intestinal .

Other Applications

Aroma Compounds

Carotenoids degrade to form a variety of volatile apocarotenoids that contribute significantly to the aromas of fruits, flowers, teas, and spices. These degradation products arise primarily through oxidative cleavage of the carotenoid polyene chain, yielding compounds with low thresholds that impart characteristic scents even in trace amounts. A prominent example is the oxidative cleavage of to β-ionone, which produces a , floral scent essential to the aroma profiles of and various flowers. In , this occurs during withering and stages, where carotenoid cleavage dioxygenase 4 (CsCCD4) catalyzes the formation of β-ionone from , enhancing the beverage's floral notes. Similarly, in flowers, PhCCD1 regulates β-ionone emission, contributing to their nocturnal fragrance. Other notable carotenoid-derived aromas include safranal, derived from the oxidative degradation of (a cleavage product of ) in stigmas, which imparts a spicy, scent responsible for the spice's distinctive . In roses, β-damascenone forms from neoxanthin via oxidative cleavage, delivering a honey-like, fruity aroma that defines the flower's bouquet despite its low concentration. These aroma compounds emerge through both enzymatic and non-enzymatic pathways. Enzymatically, carotenoid dioxygenases (CCDs) perform site-specific oxidative cleavages on carotenoid double bonds, generating apocarotenoid aldehydes or ketones. Non-enzymatically, exposure to heat, light, or during processing or storage induces random oxidations, further breaking down carotenoids into volatiles, as seen in drying or fruit ripening. In perfumery, synthetic ionones, which mimic the violet scent of natural β-ionone, are widely used to replicate floral and woody notes in fragrances. Global annual production of β-ionone exceeds 4,000 metric tons, underscoring its commercial importance in the industry.

Industrial and Commercial Uses

Carotenoids serve as natural colorants in the , with beta-carotene designated as E160a and approved by the (FDA) for use in foods, including beverages such as water-based flavored drinks and malt beverages. This approval allows its application in amounts consistent with good manufacturing practices to impart yellow-to-orange hues without the need for certification. , another carotenoid, is widely incorporated into salmon feed to enhance the pink coloration of farmed flesh, mimicking the natural pigmentation from wild diets; the global astaxanthin market, largely driven by applications, reached USD 1.69 billion in 2024 and is projected to grow to USD 1.1 billion in 2025. In supplements and , is utilized for its photoprotective properties against (UV) , helping to filter harmful and reduce oxidative damage to cells when applied topically. It is commonly formulated into creams and serums at concentrations around 0.5-1% to improve hydration, elasticity, and defense. The global + support market was valued at USD 1.12 billion in , reflecting growing demand for natural ingredients in personal care formulations. Biotechnological production of carotenoids has advanced through microbial , with Escherichia coli strains engineered to express lycopene biosynthetic pathways achieving yields of up to 3.52 g/L in fed-batch cultures. These strains incorporate genes for key enzymes like phytoene synthase and lycopene cyclase, optimizing precursor supply from the to enhance productivity. For astaxanthin, algal cultures of Haematococcus pluvialis are a primary commercial method, employing two-stage cultivation in photobioreactors to induce accumulation under stress conditions like high light and nutrient limitation, yielding up to 4-7% of dry biomass as astaxanthin. Emerging applications include nanotechnology-based encapsulation to address carotenoid instability, where polymeric nanocapsules or nanoemulsions protect against oxidation and thermal degradation during processing and storage, retaining 70-80% bioactivity post-pasteurization; as of 2025, advancements in these techniques continue to improve delivery in and supplements. Additionally, carotenoids from sources like pequi fruit or are explored as sustainable dyes, offering a non-toxic alternative to synthetic azo compounds in textiles and due to their vibrant yellow-orange pigmentation and biodegradability.