Carotenoids: Structural Features, Dietary Sources and Research Applications – With an Aladdin Natural Pigments Product Selection Guide

Carotenoids are a large family of fat-soluble natural pigments widely distributed in nature. They impart yellow, orange and red hues to plants, algae, and certain microorganisms and animal tissues. From a chemical perspective, most carotenoids are C₄₀ polyene compounds composed of 40 carbon atoms and characterized by a long chain of conjugated double bonds, which form the basis of both their coloration and antioxidant activity. To date, more than 700 carotenoids have been identified in nature, but only about 20–30 are commonly present in the human diet and detectable in blood.

For humans, the importance of carotenoids is mainly reflected in three aspects:

1. A subset of carotenoids serve as precursors of vitamin A.

2. They exhibit antioxidant and photoprotective activities and participate in defense against oxidative stress.

3. They may have potential health implications for vision, immune function and the modulation of chronic disease risk.

Chemical Structure and Classification

1. Fundamental Structural Features

Typical carotenoids share the following structural characteristics:

1. A C₄₀ polyene carbon chain composed of eight head-to-tail linked isoprenoid units.

(Note: the “classic” C₄₀ carotenoids built from eight isoprenoid units are the form most commonly encountered in dietary and nutritional research.)

2. A central linear polyene chain with terminal groups that may be either open-chain or cyclized.

3. A series of conjugated double bonds that determine the absorption spectrum and the color observed: the greater the number of conjugated double bonds, the more the color shifts toward orange-red or even red.

Conjugated double bonds are not only responsible for color; they also provide the structural basis for the antioxidant actions of carotenoids, such as free-radical scavenging and singlet-oxygen quenching.

2. Carotenes and Xanthophylls

According to the presence or absence of oxygen, carotenoids are generally divided into two major classes:

1. Carotenes

(1) Composed solely of carbon (C) and hydrogen (H).

(2) Common examples:

• β-Carotene: has vitamin A activity and is one of the important dietary sources of provitamin A.
• α-Carotene: also possesses a certain degree of vitamin A activity.
• Lycopene: an acyclic carotene without vitamin A activity, but with strong antioxidant capacity.

2. Xanthophylls / Xanthophyll Alcohols

(1) Oxygen-containing functional groups such as hydroxyl (–OH), carbonyl (=O) or epoxy groups are introduced onto the carotene backbone.

(2) Common examples:

• Lutein and zeaxanthin: widely present in green leafy vegetables and egg yolk, and key pigments in the macular region of the retina.
• Astaxanthin: commonly found in marine organisms and notable for its very strong antioxidant activity.
• Canthaxanthin, among others.

Among carotenoids, only a few—such as β-carotene, α-carotene and β-cryptoxanthin—possess provitamin A activity. Most others, including lycopene, lutein and astaxanthin, are not directly converted to vitamin A.

Table. Aladdin Carotenoids and Natural Pigments – Research Product Classification and Application Guide

Category	CAS No.	Catalog No.	Name	Specification / Purity	Product Positioning / Typical Applications
Annatto pigments / Natural orange-yellow carotenoids	6983-79-5	B1361400	Bixin	≥98%	Bixin, an apocarotenoid (cleavage-type carotenoid) derived from lycopene, and a typical natural orange-yellow carotenoid pigment. Suitable for food/feed colorant models, studies on the stability of natural pigments, solvent/process comparisons, etc.; can also serve as a reference standard for structure–property studies of carotenoids.
Provitamin A carotenoids (Carotene)	7488-99-5	C337473	α-Carotene	≥95%	A hydrocarbon carotenoid with provitamin A activity. Used to compare α- and β-carotene conversion rates; as an HPLC standard for plant carotenoid profiles (e.g. carrot, palm oil); and in studies of nutritional metabolism and antioxidant mechanisms.
Provitamin A carotenoids (Carotene)	7235-40-7	C425668	β-Carotene	2 mM in DMSO	Ready-to-use DMSO solution, suitable for cell experiments (provitamin A, antioxidant activity, receptor signaling pathways, etc.) and for use as a positive control in pharmacological screening or high-throughput assays; avoids instability associated with self-dissolution.
Provitamin A carotenoids (Carotene)	7235-40-7	C110502	β-Carotene	Analytical standard	Recommended as a reference standard for HPLC/UPLC quantification and for determining β-carotene content in food and biological samples; also suitable for method validation and teaching demonstrations.
Provitamin A carotenoids (Carotene)	7235-40-7	C110501	β-Carotene	Moligand™, ≥96% (HPLC)	High-purity, research-grade β-carotene suitable for in vitro and cell-based experiments, receptor and signaling pathway studies, and nutrition and metabolism models; may also be used as a standard.
Marine keto-carotenoids / Potent antioxidants	472-61-7	A390859	All-trans-Astaxanthin	≥95%	All-trans astaxanthin, suitable for studies requiring defined configuration, including antioxidant assays, mitochondrial function, aquaculture pigmentation and biomembrane protection; can be used as a structural “reference compound” within the astaxanthin series.
Vitamin A metabolites / Visual pigment precursors	116-31-4	A420746	All-trans-Retinal	Moligand™, 10 mM in DMSO	Ready-to-use all-trans-retinal solution for cell experiments on the visual cycle, opsin binding, and RA/RAR/RXR signaling; minimizes light- and oxygen-induced degradation during self-preparation.
Vitamin A metabolites / Visual pigment precursors	116-31-4	A122355	All-trans-Retinal	Moligand™, ≥98%	High-purity all-trans-retinal solid, suitable for studies on vitamin A metabolic pathways, retinal models, cis–trans isomerization, and as a standard for accurate quantification.
Macular xanthophylls / Lutein-type carotenoids	127-40-2	X109574	Lutein	Analytical standard, ≥90%	Dedicated reference standard for HPLC/UPLC analysis and quantitative determination of lutein in foods and serum; suitable for establishing analytical methods in ophthalmic nutrition and blue-light protection studies.
Macular xanthophylls / Lutein-type carotenoids	127-40-2	X109575	Lutein	≥75%	General-purpose laboratory reagent suitable for antioxidant assays, cell protection models and structure–activity relationship studies on carotenoids, where extremely high purity is not required.
Macular xanthophylls / Lutein-type carotenoids	127-40-2	X421127	Lutein	10 mM in DMSO	Ready-to-use lutein DMSO solution, convenient for cell-based experiments on retinal/macular protection, anti-blue-light and antioxidant effects; reduces problems of incomplete dissolution and degradation.
Algal xanthophyll / Fucoxanthin	3351-86-8	F303465	Fucoxanthin	Moligand™, ≥98%	High-purity fucoxanthin solid, suitable for studies on obesity and lipid metabolism, cancer, antioxidant mechanisms and bioactive components of seaweeds; also used as a standard for quantitative analysis of seaweed extracts.
Keto-carotenoids / Feed and pigment research	514-78-3	A299058	Canthaxanthin (E161g)	Moligand™, ≥95% (HPLC)	Canthaxanthin, a typical keto-carotenoid; suitable for research on egg yolk and fish flesh pigmentation, feed additives, antioxidant and photoprotective mechanisms, and structure–activity studies of carotenoids.
Keto-carotenoids / Feed and pigment research	514-78-3	A135943	Canthaxanthin (all-trans) (E161g)	Analytical standard, Moligand™	Canthaxanthin analytical standard predominantly in the all-trans form, used for chromatographic quantification, studies on isomer separation, and as a reference in detecting canthaxanthin in feed and food.
Macular xanthophylls / Zeaxanthin	144-68-3	Z302892	Zeaxanthin	≥96%	Zeaxanthin, one of the major high-purity macular pigments; used in ophthalmic nutrition and retinal protection research, carotenoid profile analysis and studies on synergistic mechanisms with lutein.
Non-provitamin A hydrocarbon carotenoids (Carotene)	502-65-8	L465063	Lycopene	Moligand™, ≥98% (HPLC)	High-purity lycopene suitable for antioxidant studies, cardiovascular protection, basic cancer-related research, and HPLC quantification in tomato products and nutritional supplements.
Non-provitamin A hydrocarbon carotenoids (Carotene)	502-65-8	L1452550	Lycopene	≥95%	General-purpose lycopene reagent for free radical scavenging assays, studies on food color and color stability, and teaching demonstrations.
Marine keto-carotenoids / Potent antioxidants	472-61-7	A424114	Astaxanthin	10 mM in DMSO	Ready-to-use astaxanthin solution, suitable for cell and tissue antioxidant models, mitochondrial protection and aquaculture-related cell experiments.
Marine keto-carotenoids / Potent antioxidants	472-61-7	A114383	Astaxanthin	≥98% (HPLC), mixture of cis and trans	High-purity cis–trans mixture reflecting configuration distributions in real foods/feeds; suitable for studies on metabolism, isomer stability and structure–activity relationships.
Marine keto-carotenoids / Potent antioxidants	472-61-7	A141428	Astaxanthin	≥95% (HPLC), mixture of cis and trans isomers	General research-grade astaxanthin for antioxidant, inflammation, neurological and cardiovascular cell/animal models.
Algal xanthophyll / Fucoxanthin	3351-86-8	F1499906	Fucoxanthin	Moligand™, 10 mM in DMSO	DMSO solution of the same molecule as the above “Fucoxanthin,” suitable for cell and enzymatic assays and signaling pathway screening.
Vitamin A / Retinol	68-26-8	V111674	Retinol	Moligand™, ≥95%	All-trans-retinol, the classic form of vitamin A; used in studies of vitamin A metabolism, nuclear receptor (RAR/RXR) signaling, and skin and retinal models.
Vitamin A / Retinol	68-26-8	R425433	Retinol	Moligand™, 10 mM in DMSO	Ready-to-use retinol solution, convenient for cell treatment and dose–response experiments; reduces degradation and batch-to-batch variability associated with self-dissolution.
Vitamin A / Retinol	68-26-8	R755730	Retinol	BioReagent, ≥97.5% (HPLC), ~3100 U/mg	High-activity BioReagent-grade retinol suitable for applications requiring precise quantification and activity conversion, such as nutrition fortification, enzymology and receptor studies.
Capsicum keto-carotenoids / Natural red pigments	465-42-9	C303766	Capsicum Red (oil)	Biochemical reagent	Oil-based capsicum pigment preparation mainly containing capsanthin; suitable as an experimental model for extraction of natural capsicum pigments, food coloring and color stability studies, and supercritical extraction/process development.
Capsicum keto-carotenoids / Natural red pigments	470-38-2	C347188	Capsorubin	≥90%	Capsorubin, a typical capsicum keto-carotenoid; used for color control in capsicum products, inhibition of lipid peroxidation, UV-induced damage models, and as an HPLC reference standard.

Distribution in Nature and Major Dietary Sources

Carotenoids are synthesized mainly by plants, algae and certain microorganisms. Humans and common livestock/fish species essentially do not synthesize carotenoids de novo and must obtain them from the diet, after which they can accumulate in tissues. Accordingly, dietary carotenoids in humans can be broadly categorized into three source types: plant-based foods, animal-based foods, and carotenoid preparations added to foods.

The table below summarizes common source categories, representative foods and their major carotenoid components for quick reference:

Major source category	Representative foods / products	Main carotenoid components	Notes
Green leafy vegetables	Spinach, kale, rapeseed/mustard greens, bok choy, pea shoots, etc.	Lutein, β-carotene, violaxanthin, neoxanthin	Major dietary sources of lutein and part of dietary β-carotene. Chloroplasts contain relatively stable carotenoid combinations.
Yellow/orange roots and vegetables/fruits	Carrot, sweet potato, pumpkin	β-Carotene, α-carotene	Typical provitamin A–rich foods; darker orange color usually indicates higher carotenoid content.
Red or orange-red fruits	Tomato, watermelon, pink grapefruit, etc.	Lycopene	Rich in non–vitamin A-type carotenoids; mainly used in antioxidant-related research.
Yellow cereals and fruits/vegetables	Yellow corn, yellow sweet pepper, mango, papaya, apricot, persimmon, etc.	Lutein, zeaxanthin, some β-carotene	One of the key dietary sources commonly examined in ocular nutrition research.
Vegetable oils and special fruits	Red palm oil, Gac fruit, etc.	α-Carotene, β-carotene, lycopene, etc.	Red palm oil and Gac fruit are considered particularly carotenoid-rich foods.
Eggs	Egg yolk	Lutein, zeaxanthin, small amounts of β-carotene	Yolk color is influenced by carotenoids in feed (e.g. zeaxanthin, marigold extract) and is often studied in the context of ocular nutrition.
Dairy products	Milk, cream, butter, cheese	Mainly β-carotene	Concentrations are relatively low but impart a pale yellow color to milk fat. Ruminants accumulate carotenoids from forage.
Aquatic animals and fish roe	Salmon, trout, shrimp, crab, fish roe, etc.	Astaxanthin, canthaxanthin, etc.	High astaxanthin levels in muscle and shells are a major contributor to pink/red coloration; after heating, release of bound carotenoids intensifies the red color.
Food colorants and additives	Beverages, confectionery, dairy products, etc. containing β-carotene, capsanthin/capsorubin, annatto (E160b), etc.	Formula-dependent, commonly β-carotene, capsanthin/capsorubin, bixin/norbixin, etc.	Used as colorants and nutritional fortifiers. Annatto extracts (bixin, norbixin) have been evaluated by JECFA and EFSA and are considered safe within specified intake limits.

In summary, dark green and orange-yellow fruits and vegetables are the most important natural dietary sources of carotenoids. Animal-derived sources mainly reflect the composition of the feed (e.g. aquaculture species, egg yolk and milk fat), while in the food industry, β-carotene, capsicum pigments and annatto pigments are widely used as colorants and nutritional fortifying ingredients.

Physiological Functions and Potential Health Implications

Most of the current evidence on the health effects of carotenoids comes from in vitro experiments, animal studies and epidemiological observations, while results from randomized controlled trials in humans are not entirely consistent. Therefore, the effects discussed in this section should be understood as potential health benefits with a biological rationale and some supporting evidence, rather than as drug-level therapeutic effects.

1. Provitamin A Function

• Some carotenoids (such as β-carotene, α-carotene and β-cryptoxanthin) can be enzymatically cleaved in vivo to form retinal, which is then converted to vitamin A.
• In populations with low intake of animal-derived foods, these provitamin A carotenoids are among the important dietary sources of vitamin A.
• At the same time, humans can obtain vitamin A directly from animal liver, whole-fat dairy products and fortified foods. Thus, carotenoids should not be described as the only or absolutely predominant source of vitamin A.

2. Antioxidant and Photoprotective Effects

1. Due to their long-chain conjugated double-bond structures, carotenoids can scavenge free radicals and quench singlet oxygen, thereby contributing to the defense against oxidative stress at sites such as cell membranes.

2. Lutein and zeaxanthin:

• These pigments are enriched in the macular region of the retina, where they filter part of high-energy blue light and reduce photo-oxidative damage.
• Multiple studies suggest an association between higher intake or status of these carotenoids and a lower risk of age-related macular degeneration (AMD) and cataract. At present, however, most evidence is correlative or mechanistic and cannot be directly interpreted as proof that they “prevent or treat specific eye diseases.”

3. Astaxanthin:

• Widely studied as a highly potent antioxidant.
• Shows potential roles in reducing oxidative stress and inflammatory responses, and in protecting the cardiovascular and nervous systems.
• In in vitro models, its antioxidant activity is often higher than that of some traditional antioxidants.

3. Immune Modulation and Chronic Disease Risk

1. Based on multiple review articles, carotenoids, through antioxidant effects and modulation of gene expression and cellular signaling pathways, appear to be associated with immune function, cardiovascular health, the risk of certain cancers and neurodegenerative diseases.

2. However, the existing evidence is still dominated by observational and mechanistic studies, and results from human intervention trials remain inconsistent. Therefore, carotenoids should not be regarded as medicines for the prevention or treatment of any specific disease.

4. Practical Considerations for Diet and Supplements

1. The more widely accepted approach is:

• To obtain carotenoids primarily through a varied diet rich in vegetables and fruits, rather than relying on high-dose single-compound supplements.
• To include more dark green, orange and red fruits and vegetables in daily meals in order to naturally increase carotenoid intake.

2. High-dose carotenoid supplements should be used with caution:

• Some large-scale studies have indicated that long-term use of high-dose β-carotene supplements in smokers may actually increase the risk of certain cancers.
• Therefore, blind, long-term high-dose supplementation is not recommended. If supplementation is considered necessary, it should preferably be undertaken under professional guidance.

Industrial Production and Applications

Industrial carotenoid production can be broadly divided into three categories: chemical synthesis, biosynthesis/fermentation, and natural extraction.

1. Chemical Synthesis

Since the mid-20th century, total synthesis routes for β-carotene, lycopene and other carotenoids have been successively established, enabling industrial-scale production. Synthetic carotenoids have the following features:

1. High structural purity and stable composition.

2. Suitable for standardized use in foods, feeds and pharmaceuticals.

3. In some markets and among some consumers, however, their synthetic origin raises concerns and preference differences regarding “natural vs. non-natural” ingredients.

2. Biosynthesis and Fermentation Technologies

The biosynthesis of carotenoids using microorganisms (bacteria, yeast, fungi), algae or transgenic plants has become a rapidly developing area in recent years:

1. Metabolic pathways are engineered through genetic modification to enhance the expression of key enzymes in carotenoid biosynthesis.

2. Large-scale fermentation processes are used to produce β-carotene, lutein, astaxanthin and others, which are then extracted and purified to obtain final products.

Biosynthesis combines relatively high yields with a source that is perceived as closer to “natural,” and is expected to partially replace chemical synthesis in the future.

3. Natural Extraction

Direct extraction from carotenoid-rich natural raw materials is currently a highly attractive source of “natural pigments/nutrients” in the food and health-product sectors:

1. Raw materials: tomato (lycopene), palm oil (α/β-carotene), marigold (lutein), red chili pepper (capsicum carotenoids), etc.

2. Processes:

• Organic solvent extraction (e.g. ethanol, vegetable oils): low-cost and technologically mature, but residual solvents must be controlled.
• Supercritical CO₂ extraction: uses the solvation capacity of CO₂ in its supercritical state. This approach offers solvent-free residues, mild processing conditions and better protection of heat-sensitive active components, making it suitable for high value-added products.

Carotenoids Quick Reference for Experiments: Structures, Properties and Common Compounds

1. Three Key Points to Clarify Before Starting Experiments

(1) Structural Classification

Category	Structural features	Representative compounds
Carotenes	Contain only C and H, no O; mostly yellow–orange to red in color	β-Carotene, α-carotene, lycopene
Xanthophylls	C₄₀ backbone bearing oxygenated groups such as hydroxyl, keto or epoxy groups	Lutein, zeaxanthin, astaxanthin, canthaxanthin, bixin, violaxanthin, neoxanthin, etc.

(2) Is It a Provitamin A?

Provitamin A activity	Typical compounds	Experimental notes
Yes	β-Carotene, α-carotene, β-cryptoxanthin	Enables design of experiments on “carotenoids → retinal/retinol/retinoic acid” metabolic pathways, and on absorption and conversion efficiency.
No (but with antioxidant/other functions)	Lycopene, lutein, zeaxanthin, astaxanthin, fucoxanthin, etc.	More suitable for functional studies on antioxidant activity, photoprotection and metabolic regulation, rather than vitamin A supply.

(3) Common Experimental Properties of Carotenoids

(a) Strong lipophilicity

Prefer organic solvents such as THF, acetone, ethanol, hexane and DMSO. In cell experiments, pay close attention to the final DMSO/ethanol concentration.

Note: Solvent selection should match carotenoid polarity. Hydrocarbon-type carotenoids (e.g. β-carotene) are better suited to nonpolar solvents (e.g. hexane), whereas polyhydroxy xanthophylls (e.g. lutein) dissolve better in alcohols. When using THF, be aware that it easily forms peroxides; extra caution is required in long-term storage and in experiments requiring precise quantification.

(b) Extremely sensitive to light, oxygen and heat

• Prone to oxidation and isomerization from all-trans to cis forms.
• Under light exposure, the color may fade and HPLC peak shape/area may change.

(c) Distinct spectroscopic characteristics

• Typically show pronounced absorption peaks in the 400–500 nm region.
• Well-suited for qualitative, quantitative and stability studies using UV–Vis or HPLC–PDA.

Clarifying these three points before choosing standards and designing your experimental strategy will help you avoid many unnecessary detours.

2. Overview of Common Carotenoids and Typical Research Uses

Application scenario	Representative compounds	Key features	Typical research uses (examples)
1. Provitamin A & metabolism studies	β-Carotene	Classic provitamin A; cleaved to form retinal/retinoic acid	Studies of carotenoid→vitamin A conversion in liver and intestine models (cells/mice); comparison of cleavage efficiency under different enzymes/conditions; calibration standards for β-carotene quantification in serum or food.
	α-Carotene, β-cryptoxanthin	Also possess provitamin A activity, with different natural sources	Comparing absorption and conversion rates of different provitamin A carotenoids; standard substances for carotenoid profile analysis in serum/foods.
	Retinol, retinal	Key intermediates in vitamin A metabolism downstream of carotenoids	Linking the full pathway: β-carotene → retinal → retinol / retinoic acid; visual/retinal models, RAR/RXR signaling axis research; LC–MS/HPLC methods simultaneously measuring carotenoids and vitamin A metabolites.
2. Antioxidant · Cell protection · Eye-related research	Lycopene	Non–vitamin A hydrocarbon carotenoid with strong antioxidant capacity	Positive control in DPPH, ABTS, ORAC and other free radical scavenging assays; modeling “high-tomato diets” and their effects on oxidative stress and lipid peroxidation.
	Lutein & zeaxanthin	Major macular pigments with light-filtering and antioxidant functions	Anti-blue-light and anti-ROS protection experiments in retinal/macular cell models; HPLC marker components in ophthalmic nutritional supplements and fortified foods; carotenoid profile standards for samples such as plant leaves and egg yolk.
	Astaxanthin	Marine keto-carotenoid, widely recognized as a potent antioxidant	Positive control in in vitro/cellular oxidative stress models; studies on fish flesh/shell coloration in aquaculture; mitochondrial protection, inflammation and cardio-cerebrovascular models.
3. Food pigments · Natural colorants	Capsanthin & capsorubin	Major red keto-carotenoids in chili pepper	Studies on color stability, storage and processing-induced color changes in chili products; component identification and quantification in capsicum pigment preparations; evaluation of delayed lipid oxidation and antioxidant performance in foods.
	Bixin & norbixin	Orange-yellow pigments from annatto, complementary in oil and aqueous phases	Studies on solubility behavior and stability versus pH and ionic strength in aqueous/oil systems; student experiments on “retention of natural pigments under different processing conditions”; comparison of stability and safety between synthetic and natural colorants.
4. Plant photosynthesis & stress physiology	Violaxanthin & neoxanthin	Xanthophylls involved in the V–A–Z lutein cycle and ABA biosynthesis	Studies of the V–A–Z xanthophyll cycle and non-photochemical quenching (NPQ); carotenoid profile changes under light stress, drought and salinity; mechanisms of abscisic acid (ABA) biosynthesis and signal transduction.

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