Carotenoids: How Plant Pigments Work in the Body

Carotenoids are a large family of fat-soluble pigments, numbering over 1,100 known structures, that produce the yellow, orange, and red colors in fruits, vegetables, flowers, and many animals. They are synthesized primarily by plants, algae, fungi, and certain bacteria, and they serve double duty as both light-harvesting accessories in photosynthesis and shields against the damage that excess light can cause. Humans cannot make carotenoids from scratch and must get them from food, where they act as antioxidants, give rise to vitamin A, and accumulate in tissues from the retina to the skin.

Carotenes and Xanthophylls

Carotenoids split into two broad chemical families. Carotenes are pure hydrocarbons, chains of carbon and hydrogen with no oxygen atoms. Beta-carotene and lycopene are the best-known examples. Xanthophylls carry oxygen in the form of hydroxyl groups, keto groups, or epoxides attached to the same basic backbone. Lutein, zeaxanthin, and astaxanthin all belong to this oxygen-containing group. The chemical difference matters in practical ways: carotenes dissolve readily in fats and nonpolar solvents but resist mixing with water, while xanthophylls, thanks to their oxygen atoms, dissolve in a wider range of solvents including alcohols.

1PubMed Central. Chemistry, Occurrence, Properties, Applications, and Encapsulation of Carotenoids—A Review – Section: 2. Chemistry of Carotenoids

When carotenoids are cleaved, either by enzymes or by nonenzymatic oxidation, the fragments are called apocarotenoids. These breakdown products are far from waste. In plants, apocarotenoids serve as pigments, flavor compounds, hormones (the plant hormone abscisic acid starts life as a carotenoid), and signaling molecules.

2Journal of Chemistry. Research Advances on Biosynthesis, Regulation, and Biological Activities of Apocarotenoid Aroma in Horticultural Plants The saffron spice, for instance, gets both its intense color and its distinctive aroma from apocarotenoids produced when the crocus flower’s carotenoids are enzymatically cut. Many of the volatiles that give tomatoes, peaches, and tea their characteristic scents are also apocarotenoid fragments.

How Plants Build and Use Carotenoids

In plants, the biosynthetic pathway begins when phytoene synthase condenses two molecules of a precursor called geranylgeranyl diphosphate into phytoene, a colorless 40-carbon compound. This is the rate-limiting step, the bottleneck that controls how much carotenoid a plant ultimately produces.

3PubMed Central. Phytoene Synthase: The Key Rate-Limiting Enzyme of Carotenoid Biosynthesis in Plants Downstream enzymes then introduce double bonds and add oxygen groups, branching the pathway toward the hundreds of different carotenoid structures found across the plant kingdom.

Once made, carotenoids earn their keep inside plant cells as photoprotectors. During photosynthesis, chlorophyll absorbs light and converts it to chemical energy. But when light is too intense, the excess energy can generate reactive oxygen species that damage cell membranes and proteins. Carotenoids intercept this danger in two ways: they absorb excess energy directly from excited chlorophyll molecules, and they quench singlet oxygen, a particularly destructive reactive species. Carotenoids neutralize singlet oxygen at rates roughly 30 to 100 times faster than vitamin E does.

4PubMed Central. Singlet molecular oxygen-quenching activity of carotenoids: relevance to protection of the skin from photoaging

A specialized protection mechanism called the xanthophyll cycle plays a central role. Under high light, plants convert the xanthophyll violaxanthin into zeaxanthin through an intermediate called antheraxanthin. Zeaxanthin is the most powerful quencher of excess light energy in this system. Modeling of the xanthophyll cycle in land plants found that zeaxanthin’s quenching effectiveness is roughly 18 times that of violaxanthin.

5Nature Communications. Dissecting the contributions to non-photochemical quenching in a land plant under fluctuating light This cycle is not unique to land plants. Diatoms and other algae run a parallel xanthophyll cycle using different pigments, and research has shown that even the land-plant cycle pigments can drive photoprotection in algae that normally rely on their own set of xanthophylls.

6Plant Physiology. Both major xanthophyll cycles present in nature promote nonphotochemical quenching in a model diatom

An Unusual Evolutionary Detour in Aphids

Animals were long thought to be entirely dependent on dietary sources for their carotenoids. Then genomic sequencing of the pea aphid revealed something remarkable: the aphid genome itself contains working genes for carotenoid biosynthesis.

7PubMed. Lateral transfer of genes from fungi underlies carotenoid production in aphids Phylogenetic analysis showed these genes were acquired from fungi through horizontal gene transfer, an event where DNA jumps between distantly related organisms rather than being passed from parent to offspring. All aphids carry at least one copy of the key gene, and some species have duplicated it as many as seven times, compared with the single copy found in fungal genomes.

8Molecular Biology and Evolution. Diversification of Genes for Carotenoid Biosynthesis in Aphids following an Ancient Transfer from a Fungus

More recent work has revealed that the pathway is even more cobbled together than initially thought. Phytoene synthase, carotenoid desaturase, and a precursor-building enzyme all appear to have come from fungi, while the carotenoid cleavage oxygenase that processes the end products seems to be a native insect gene that was co-opted into the pathway.

9bioRxiv. Coordinated horizontal transfer of multiple genes assembles a carotenoid biosynthesis pathway in aphids The result is that aphids manufacture their own red and green body pigments rather than relying on plant food. This is one of the clearest examples in nature of horizontal gene transfer assembling a functional metabolic pathway in an animal.

Carotenoid Coloration and Honest Signaling in Birds

Birds are probably the most visually conspicuous consumers of carotenoids. The yellow, orange, and red feathers of species from goldfinches to flamingos depend on carotenoids obtained from the diet and deposited into growing feathers. A long-standing hypothesis in behavioral ecology holds that carotenoid-based plumage serves as an “honest signal” of health: because the same pigments are used both for immune defense and for feather color, only birds in good condition can afford to divert enough carotenoids toward ornamentation.

The evidence for this trade-off is mixed. A study of greenfinches found that males with brighter yellow breast feathers mounted stronger immune responses and showed signs of better general health.

10PubMed. Carotenoid-based plumage coloration of male greenfinches reflects health and immunocompetence A broader meta-analysis across bird species found that higher plasma carotenoid levels and redder plumage were associated with stronger cell-mediated immune responses.

11PubMed Central. What Does Carotenoid-Dependent Coloration Tell? Plasma Carotenoid Level Signals Immunocompetence and Oxidative Stress State in Birds–A Meta-Analysis But controlled experiments with American goldfinches, a species with extensive carotenoid-based color, found no trade-off: varying the amount of dietary carotenoids changed plumage and bill color but did not measurably affect immune function or disease resistance.

12Behavioral Ecology. Dietary carotenoid pigments and immune function in a songbird with extensive carotenoid-based plumage coloration The honest-signal idea is plausible but far from settled.

How You Absorb Carotenoids From Food

Because carotenoids are fat-soluble, eating them with some dietary fat dramatically improves absorption. In digestion experiments, adding even a small amount of fat to a carotenoid-rich vegetable roughly doubled to tripled the fraction that ended up in absorbable form. Oils rich in unsaturated fats, like olive or soybean oil, performed two to three times better than saturated fats like coconut oil for getting carotenoids into the absorbable mix.

13PubMed Central. Dietary fat composition, food matrix and relative polarity modulate the micellarization and intestinal uptake of carotenoids from vegetables and fruits

Interestingly, carotenes and xanthophylls respond differently to fat. Xanthophylls, being somewhat more polar, release from the food matrix more easily and form absorbable particles more readily even at lower fat levels. Carotenes need more fat to reach optimal absorption, and monounsaturated fats and long-chain triglycerides are the best vehicles for them. Saturated fats and medium-chain triglycerides, by contrast, are more helpful for xanthophyll absorption.

14LWT. Effects of dietary lipids on bioaccessibility and bioavailability of natural carotenoids15Molecular Nutrition & Food Research. Intestinal absorption of dietary carotenoids

Cooking and processing also matter. Heat breaks down cell walls and releases carotenoids from the food matrix, which generally increases the amount your body can absorb. That is why the lycopene in cooked tomatoes is more available than in raw ones. But processing can also degrade carotenoids if conditions are harsh: prolonged exposure to heat, light, and oxygen can destroy some of the pigment and convert the rest into less biologically active forms.

The Vitamin A Connection

Of the hundreds of carotenoids in the diet, only a handful can be converted to vitamin A. Beta-carotene is the most important of these provitamin A carotenoids, and the enzyme responsible for the conversion is called BCO1. This enzyme cleaves the beta-carotene molecule at its central double bond, producing retinal, the aldehyde form of vitamin A.

16PubMed Central. Enzymology of vertebrate carotenoid oxygenases For decades, researchers debated whether BCO1 inserts one or two oxygen atoms during the cleavage. Isotope-labeling experiments settled the question: BCO1 uses both atoms from molecular oxygen, making it a dioxygenase.

17PubMed Central. The Human Enzyme That Converts Dietary Provitamin A Carotenoids to Vitamin A Is a Dioxygenase

This conversion does not only happen in the gut. Fat tissue stores meaningful quantities of beta-carotene, and mouse research has shown that activating BCO1 specifically in fat cells depletes those beta-carotene stores and produces retinoids locally. The resulting vitamin A metabolites in fat tissue appear to have anti-obesity effects in mice, suggesting that beta-carotene’s biological roles extend well beyond simply being a vitamin A precursor.

18PubMed Central. The conversion of β-carotene to vitamin A in adipocytes drives the anti-obesogenic effects of β-carotene in mice

Carotenoids and Eye Health

The human retina selectively concentrates three xanthophylls: lutein, zeaxanthin, and meso-zeaxanthin. These are the only carotenoids found in the macula, the central region of the retina responsible for sharp, detailed vision.

19Advances in Nutrition. The Effect of Lutein/Zeaxanthin Intake on Human Macular Pigment Optical Density: A Systematic Review and Meta-Analysis They form a layer of macular pigment that functions as an internal blue-light filter and as an antioxidant shield, protecting the delicate photoreceptors from light-induced oxidative damage. Clinical trials have found that age-related and dietary loss of these macular carotenoids worsens phototoxic damage, while supplementation has a protective effect against damage to both the lens and the retina.

20PubMed Central. The Photobiology of Lutein and Zeaxanthin in the Eye

Lutein and zeaxanthin are found not just in the macula but also in the lens, iris, and ciliary body, suggesting a broader protective role across the eye.

21Nutrition Reviews. Potential roles of dietary zeaxanthin and lutein in macular health and function Good dietary sources include dark leafy greens like spinach and kale, egg yolks, and orange peppers. Because these are xanthophylls, they absorb well even with modest amounts of dietary fat, though a drizzle of oil on your salad still helps.

The Beta-Carotene Supplement Cautionary Tale

If carotenoids are antioxidants and come from healthy foods, it seemed logical that taking high-dose beta-carotene supplements would prevent cancer. Two large trials in the 1990s tested this idea in people at high risk for lung cancer, mainly heavy smokers and asbestos workers. The results were alarming. The CARET trial, which combined beta-carotene with vitamin A supplements, found a 28 percent increase in lung cancer incidence and a 17 percent increase in overall mortality in the supplement group compared with placebo.

22JNCI: Journal of the National Cancer Institute. The Beta-Carotene and Retinol Efficacy Trial: Incidence of Lung Cancer and Cardiovascular Disease Mortality During 6-Year Follow-up After Stopping β-Carotene and Retinol Supplements The active-treatment group had a relative risk of lung cancer of about 1.28.

23PubMed. Effects of a combination of beta carotene and vitamin A on lung cancer and cardiovascular disease

The trial was stopped early. Follow-up after supplementation ceased showed the excess risk gradually faded but did not vanish immediately. The leading explanation is that at pharmacological doses in the oxidative environment of a smoker’s lung, beta-carotene can shift from antioxidant to pro-oxidant, actually promoting DNA damage rather than preventing it. The lesson was stark: eating carotenoid-rich foods is consistently linked to health benefits, but isolating a single carotenoid and taking it in large supplemental doses is a different proposition entirely, particularly for smokers.

Cardiovascular Associations

While supplementation trials for beta-carotene were disappointing, observational data on carotenoid levels in the blood paint a more encouraging picture for heart health. Multiple large cohort studies have found that higher circulating levels of various carotenoids are linked to lower rates of cardiovascular disease events and death.

24PubMed Central. The Roles of Carotenoid Consumption and Bioavailability in Cardiovascular Health A study of hypertensive adults found that those with the highest blood levels of alpha-carotene had about a 32 percent lower rate of cardiovascular death, while those with high beta-cryptoxanthin and lycopene levels showed reductions of about 29 and 26 percent, respectively.

25PubMed Central. Associations of Serum Carotenoids With Risk of All-Cause and Cardiovascular Mortality in Hypertensive Adults

Even noninvasive measures of carotenoid status have shown associations. Research using skin carotenoid scores, measured by shining light through the skin and detecting the pigment, found that higher skin carotenoid levels correlated with lower blood pressure and reduced odds of metabolic syndrome.

26PubMed. Skin carotenoids status as a potential surrogate marker for cardiovascular disease risk determination in middle-aged and older adults The caveat is that these are observational findings. People with high blood or skin carotenoid levels tend to eat more fruits and vegetables, exercise more, and smoke less, so the carotenoids may be a marker of a healthy lifestyle rather than the direct cause of the cardiovascular benefit. No large trial has yet demonstrated that carotenoid supplements reduce heart disease.

Skin Protection From the Inside

Carotenoids accumulate in the skin after you eat them, and the resulting pigment offers a small but measurable degree of sun protection. Clinical studies have shown that supplementation with beta-carotene or mixed carotenoids for about 12 weeks reduces the skin’s sunburn response. One study estimated the resulting internal sun-protection factor at roughly 2.4 for beta-carotene alone and about 3.0 when combined with vitamin E.

27The American Journal of Clinical Nutrition. Carotenoids and tocopherols reduce erythema in humans induced by solar UV-light A separate trial comparing pure beta-carotene with a mixed carotenoid supplement found that both offered similar protection against UV-induced redness after 12 weeks.

28The Journal of Nutrition. Supplementation with β-Carotene or a Similar Amount of Mixed Carotenoids Protects Humans from UV-Induced Erythema

More recent research has expanded beyond sunburn to show that carotenoids also reduce UVA-induced pigmentation and suppress molecular markers of photoaging, including enzymes that break down collagen.

29PubMed. Role of ingestible carotenoids in skin protection: A review of clinical evidence To be clear, a sun-protection factor in the range of 2 to 3 is nowhere close to replacing sunscreen. Think of it as a subtle background layer of defense that builds up over weeks of consistent carotenoid-rich eating, not a substitute for topical protection.

Golden Rice and Biofortification

The fact that rice endosperm, the part of the grain people eat, naturally produces no carotenoids made it a target for biofortification. Golden Rice was developed by introducing two enzymes, phytoene synthase and phytoene desaturase, into the rice grain under a promoter that activates specifically in the endosperm. These enzymes establish a beta-carotene production pathway where none previously existed, giving the grain its distinctive golden-yellow color.

30PubMed Central. Golden Rice is an effective source of vitamin A

Since the first version in 2000, researchers have continuously refined Golden Rice to boost its provitamin A content, achieve regulatory compliance, and breed the trait into locally adapted rice varieties.

31PubMed. Golden Rice-Lessons learned for inspiring future metabolic engineering strategies and synthetic biology solutions More recent protocols use synthetic gene constructs and high-throughput transformation methods to produce hundreds of transgenic plant lines per construct, accelerating the search for lines with the highest endosperm carotenoid levels.

32PubMed. Molecular protocol to develop β-carotene-biofortified rice events via molecular optimization Golden Rice remains one of the most prominent examples of metabolic engineering for nutritional purposes, developed specifically to address vitamin A deficiency in rice-dependent diets. Its real-world rollout has been slow and politically contentious, but the underlying science demonstrated that an entirely absent biosynthetic pathway could be engineered into a staple crop.

Astaxanthin in Aquaculture and Beyond

Wild salmon get their pink-to-red flesh color from astaxanthin, a xanthophyll they accumulate by eating krill and other crustaceans. Farmed salmon do not have access to those wild food sources, so astaxanthin is added to their feed, and it accounts for a meaningful fraction of aquaculture feed cost. Most commercial astaxanthin is still produced by chemical synthesis, though natural production from the microalga Haematococcus pluvialis and the yeast Xanthophyllomyces dendrorhous is growing.

33PubMed Central. Optimization of microbial cell factories for astaxanthin production: Biosynthesis and regulations, engineering strategies and fermentation optimization strategies

Pigmentation trials in rainbow trout found that synthetic astaxanthin actually produced more intense flesh color at the same dose than natural astaxanthin from algae or yeast.

34PubMed Central. Comparison of the Retention Rates of Synthetic and Natural Astaxanthin in Feeds and Their Effects on Pigmentation, Growth, and Health in Rainbow Trout (Oncorhynchus mykiss) In Atlantic salmon, however, fish fed natural astaxanthin from several microbial sources achieved comparable flesh color and oxidative status to those on synthetic astaxanthin, even though the synthetic-fed fish retained more total astaxanthin in their muscle. Among natural sources, the yeast Phaffia rhodozyma showed the most promising results for salmon flesh coloration.

35Aquaculture. Effect of natural carotenoids obtained from Haematococcus pluvialis, Paracoccus carotinifaciens, and Phaffia rhodozyma on flesh pigmentation and related biochemical mechanisms in Atlantic salmon (Salmo salar L.)

Beyond coloring fish flesh, astaxanthin has attracted attention as an antioxidant for human supplements. Natural astaxanthin from H. pluvialis has been described as having antioxidant activity roughly 10 to 100 times greater than that of vitamin E.

36Aquaculture Research. The Red Gold of Aquaculture: The Dual Role of Astaxanthin in Aquatic Nutrition and Human Health Scaling up natural production remains a challenge, and researchers are engineering non-native microorganisms like E. coli and brewer’s yeast to produce astaxanthin through fermentation as an alternative. One emerging approach uses food-processing wastewater as a nutrient source for microalgal growth, combining waste remediation with pigment production.

37Fermentation. Valorization of Food Processing Wastewater for Astaxanthin Production by the Mixotrophic Fermentation of Microalgae: A Review

Carotenoids as Food Colorants

Using carotenoid-rich plant extracts to color food is an ancient practice, but the modern food-color industry traces its carotenoid era to 1950, when beta-carotene was first fully synthesized in the laboratory. Since then, several synthetic carotenoids have become standard food additives. In the United States, beta-carotene, beta-apo-8′-carotenal, and canthaxanthin are all accepted color additives exempt from batch certification.

38PubMed. Carotenoids as food colorants Carotenoids appeal to food manufacturers because they are nontoxic, provide a wide color spectrum from yellow through orange to red, and remain reasonably stable under the mildly acidic and low-oxygen conditions common in many processed foods.

39Journal of Agriculture and Food Research. Natural bio-colorant and pigments: Sources and applications in food processing

Their main weakness is sensitivity to oxygen and light over time. A brightly colored juice or snack will fade if stored in clear packaging under fluorescent lights. Encapsulation technologies, packaging choices that block light and oxygen, and blending carotenoids with antioxidants like tocopherols are all used commercially to extend shelf life. As consumer demand shifts toward “clean label” ingredients, carotenoids derived from algae, marigold flowers, and paprika are increasingly replacing synthetic dyes in products ranging from dairy to confectionery.