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Carotenoids (/kəˈrɒtɪnɔɪd/), also called tetraterpenoids, are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, and fungi.
Carotenoids give the characteristic color to pumpkins, carrots, parsnips, corn, tomatoes, canaries, flamingos, salmon, lobster, shrimp, and daffodils.
Carotenoids can be produced from fats and other basic organic metabolic building blocks by all these organisms. The only land dwelling arthropods known to produce carotenoids are aphids, and spider mites, which acquired the ability and genes from fungi.
Carotenoids is also produced by endosymbiotic bacteria in whiteflies.
Carotenoids from the diet are stored in the fatty tissues of animals, and exclusively carnivorous animals obtain the compounds from animal fat.
In the human diet, absorption of carotenoids is improved when consumed with fat in a meal.
Cooking carotenoid-containing vegetables in oil and shredding the vegetable both increase carotenoid bioavailability.
There are over 1,100 known carotenoids which can be further categorized into two classes, xanthophylls (which contain oxygen) and carotenes (which are purely hydrocarbons and contain no oxygen).
All are derivatives of tetraterpenes, meaning that they are produced from 8 isoprene molecules and contain 40 carbon atoms.
In general, carotenoids absorb wavelengths ranging from 400 to 550 nanometers (violet to green light).
This causes the compounds to be deeply colored yellow, orange, or red.
Carotenoids are the dominant pigment in autumn leaf coloration of about 15-30% of tree species, but many plant colors, especially reds and purples, are due to polyphenols.
Carotenoids serve two key roles in plants and algae: they absorb light energy for use in photosynthesis, and they provide photoprotection via non-photochemical quenching.
Carotenoids that contain unsubstituted beta-ionone rings (including β-carotene, α-carotene, β-cryptoxanthin, and γ-carotene) have vitamin A activity (meaning that they can be converted to retinol).
In the eye, lutein, meso-zeaxanthin, and zeaxanthin are present as macular pigments whose importance in visual function, as of 2016, remains under clinical research.
Biosynthesis
he basic building blocks of carotenoids are isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP).
These two isoprene isomers are used to create various compounds depending on the biological pathway used to synthesize the isomers.
Plants are known to use two different pathways for IPP production: the cytosolic mevalonic acid pathway (MVA) and the plastidic methylerythritol 4-phosphate (MEP).
In animals, the production of cholesterol starts by creating IPP and DMAPP using the MVA.
For carotenoid production plants use MEP to generate IPP and DMAPP.
The MEP pathway results in a 5:1 mixture of IPP:DMAPP.
IPP and DMAPP undergo several reactions, resulting in the major carotenoid precursor, geranylgeranyl diphosphate (GGPP).
GGPP can be converted into carotenes or xanthophylls by undergoing a number of different steps within the carotenoid biosynthetic pathway.
MEP pathway
Glyceraldehyde 3-phosphate and pyruvate, intermediates of photosynthesis, are converted to deoxy-D-xylulose 5-phosphate (DXP) using the catalyst DXP synthase (DXS).
DXP reductoisomerase reduces and rearranges the molecules within DXP[11] in the presence of NADPH, forming MEP.
Next, MEP is converted to 4-(cytidine 5’-diphospho)-2-C-methyl-D-erythritol (CDP-ME) in the presence of CTP via the enzyme MEP cytidylyltransferase.
CDP-ME is then converted, in the presence of ATP, to 2-phospho-4-(cytidine 5’-diphospho)-2-C-methyl-D-erythritol (CDP-ME2P).
The conversion to CDP-ME2P is catalyzed by the enzyme CDP-ME kinase.
Next, CDP-ME2P is converted to 2-C-methyl-D-erythritol 2,4-cyclodiphosphate (MECDP).
This reaction occurs when MECDP synthase catalyzes the reaction and CMP is eliminated from the CDP-ME2P molecule.
MECDP is then converted to (e)-4-hydroxy-3-methylbut-2-en-1-yl diphosphate (HMBDP) via HMBDP synthase in the presence of flavodoxin and NADPH.
HMBDP is reduced to IPP in the presence of ferredoxin and NADPH by the enzyme HMBDP reductase.
The last two steps involving HMBPD synthase and reductase can only occur in completely anaerobic environments.
IPP is then able to isomerize to DMAPP via IPP isomerase.
Two GGPP molecules condense via phytoene synthase (PSY), forming the 15-cis isomer of phytoene.
PSY belongs to the squalene/phytoene synthase family and is homologous to squalene synthase that takes part in steroid biosynthesis.
The subsequent conversion of phytoene into all-trans-lycopene depends on the organism.
Bacteria and fungi employ a single enzyme, the bacterial phytoene desaturase (CRTI) for the catalysis.
Plants and cyanobacteria however utilize four enzymes for this process.
The first of these enzymes is a plant-type phytoene desaturase which introduces two additional double bonds into 15-cis-phytoene by dehydrogenation and isomerizes two of its existing double bonds from trans to cis producing 9,15,9’-tri-cis-ζ-carotene.
The central double bond of this tri-cis-ζ-carotene is isomerized by the zeta-carotene isomerase Z-ISO and the resulting 9,9'-di-cis-ζ-carotene is dehydrogenated again via a ζ-carotene desaturase (ZDS).
This again introduces two double bonds, resulting in 7,9,7’,9’-tetra-cis-lycopene.
CRTISO, a carotenoid isomerase, is needed to convert the cis-lycopene into an all-trans lycopene in the presence of reduced FAD.
This all-trans lycopene is cyclized; cyclization gives rise to carotenoid diversity, which can be distinguished based on the end groups.
There can be either a beta ring or an epsilon ring, each generated by a different enzyme (lycopene beta-cyclase [beta-LCY] or lycopene epsilon-cyclase [epsilon-LCY]).
α-Carotene is produced when the all-trans lycopene first undergoes reaction with epsilon-LCY then a second reaction with beta-LCY; whereas β-carotene is produced by two reactions with beta-LCY.
α- and β-Carotene are the most common carotenoids in the plant photosystems but they can still be further converted into xanthophylls by using beta-hydrolase and epsilon-hydrolase, leading to a variety of xanthophylls.
Regulation
Carotenoids is believed that both DXS and DXR are rate-determining enzymes, allowing them to regulate carotenoid levels.
This was discovered in an experiment where DXS and DXR were genetically overexpressed, leading to increased carotenoid expression in the resulting seedlings.
Also, J-protein (J20) and heat shock protein 70 (Hsp70) chaperones are thought to be involved in post-transcriptional regulation of DXS activity, such that mutants with defective J20 activity exhibit reduced DXS enzyme activity while accumulating inactive DXS protein.
Regulation may also be caused by external toxins that affect enzymes and proteins required for synthesis.
Ketoclomazone is derived from herbicides applied to soil and binds to DXP synthase.
This inhibits DXP synthase, preventing synthesis of DXP and halting the MEP pathway.
The use of this toxin leads to lower levels of carotenoids in plants grown in the contaminated soil.
Fosmidomycin, an antibiotic, is a competitive inhibitor of DXP reductoisomerase due to its similar structure to the enzyme.
Application of said antibiotic prevents reduction of DXP, again halting the MEP pathway.
Structure and function
The structure of carotenoids allows for biological abilities, including photosynthesis, photoprotection, plant coloration, and cell signaling.
The general structure of the carotenoid is a polyene chain consisting of 9-11 double bonds and possibly terminating in rings.
This structure of conjugated double bonds leads to a high reducing potential, or the ability to transfer electrons throughout the molecule.
Carotenoids can transfer excitation energy in one of two ways:
1) singlet-singlet energy transfer from carotenoid to chlorophyll, and
2) triplet-triplet energy transfer from chlorophyll to carotenoid.
The singlet-singlet energy transfer is a lower energy state transfer and is used during photosynthesis.
The length of the polyene tail enables light absorbance in the photosynthetic range; once it absorbs energy it becomes excited, then transfers the excited electrons to the chlorophyll for photosynthesis.
The triplet-triplet transfer is a higher energy state and is essential in photoprotection.
Light produces damaging species during photosynthesis, with the most damaging being reactive oxygen species (ROS).
As these high energy ROS are produced in the chlorophyll the energy is transferred to the carotenoid’s polyene tail and undergoes a series of reactions in which electrons are moved between the carotenoid bonds in order find the most balanced state (lowest energy state) for the carotenoid.
The length of carotenoids also has a role in plant coloration, as the length of the polyene tail determines which wavelengths of light the plant will absorb.
Wavelengths that are not absorbed are reflected and are what we see as the color of a plant.
Therefore, differing species will contain carotenoids with differing tail lengths allowing them to absorb and reflect different colors.
Carotenoids also participate in different types of cell signaling.
They are able to signal the production of absicisic acid, which regulates plant growth, seed dormancy, embryo maturation and germination, cell division and elongation, floral growth, and stress responses.
Properties
Carotenoids belong to the category of tetraterpenoids (i.e., they contain 40 carbon atoms, being built from four terpene units each containing 10 carbon atoms).
Structurally, carotenoids take the form of a polyene hydrocarbon chain which is sometimes terminated by rings, and may or may not have additional oxygen atoms attached.
Carotenoids with molecules containing oxygen, such as lutein and zeaxanthin, are known as xanthophylls.
The unoxygenated (oxygen free) carotenoids such as α-carotene, β-carotene, and lycopene, are known as carotenes.
Carotenes typically contain only carbon and hydrogen (i.e., are hydrocarbons), and are in the subclass of unsaturated hydrocarbons.
Their color, ranging from pale yellow through bright orange to deep red, is directly linked to their structure.
Xanthophylls are often yellow, hence their class name.
The double carbon-carbon bonds interact with each other in a process called conjugation, which allows electrons in the molecule to move freely across these areas of the molecule.
As the number of conjugated double bonds increases, electrons associated with conjugated systems have more room to move, and require less energy to change states.
This causes the range of energies of light absorbed by the molecule to decrease.
As more wavelengths of light are absorbed from the longer end of the visible spectrum, the compounds acquire an increasingly red appearance.
Carotenoids are usually lipophilic due to the presence of long unsaturated aliphatic chains as in some fatty acids.
The physiological absorption of these fat-soluble vitamins in humans and other organisms depends directly on the presence of fats and bile salts.
Foods
Beta-carotene, found in pumpkins, sweet potato, carrots and winter squash, is responsible for their orange-yellow colors.
Dried carrots have the highest amount of carotene of any food per 100-gram serving, measured in retinol activity equivalents (provitamin A equivalents).
Vietnamese gac fruit contains the highest known concentration of the carotenoid lycopene.
Although green, kale, spinach, collard greens, and turnip greens contain substantial amounts of beta-carotene.
The diet of flamingos is rich in carotenoids, imparting the orange-colored feathers of these birds.
Morphology
Carotenoids are located primarily outside the cell nucleus in different cytoplasm organelles, lipid droplets, cytosomes and granules.
They have been visualised and quantified by raman spectroscopy in an algal cell.
With the development of monoclonal antibodies to trans-lycopene it was possible to localise this carotenoid in different animal and human cells.
Oxygenation
Carotenoids play an important role in biological oxygenation. In plant cells they are involved in the control of trans-membrane transport of molecular oxygen released in photosynthesis.
In animals carotenoids play an important role to support oxygen in its transport, storage and metabolism.
Transport
Carotenoids are hydrophobic and are typically present in plasma lipoproteins and cellular lipid structures.
Since molecular oxygen is also a hydrophobic molecule, lipids provide a more favorable environment for O2 solubility than in aqueous mediums.
By protecting lipids from free-radical damage, which generate charged lipid peroxides and other oxidised derivatives, carotenoids support crystalline architecture and hydrophobicity of lipoproteins and cellular lipid structures, hence oxygen solubility and its diffusion therein.
Storage
Carotenoids was first suggested that carotenoids can be involved in the intracellular depot of oxygen in 1973 by V.N. Karnaukhov.
Later it was discovered that carotenoids can also stimulate the formation of intracellular lipid droplets, which can store additional molecular oxygen.
These properties of carotenoids help animals to adapt to environmental stresses, high altitude, intracellular infections and other hypoxicconditions.
Respiration
Carotenoids, by increasing oxygen diffusion and the oxygen carrying capacity of plasma lipoproteins, can stimulate oxygen delivery into body tissues.
This improves tissue and cellular oxygenation and stimulates the growth and respiration of mitochondria.
Synergetic modality
Oxygen is required in many intracellular reactions including hydroxylation, which is important for metabolic activation of prodrugs and prohormones, such as vitamin D3.
Carotenoids not only provide support for intracellular oxygenation but can also improve efficacy of these molecules.
Carotenoids can form physical complexes with different molecules.
With hydrophobic molecules this could be self-assembly.
With amphiphilic or hydrophilic compounds the use of lycosome or supercritical CO2 technologies, or other methods, are required.
Carotenoids in these complexes provide a new modality of supporting and boosting tissue oxygenation, which could be synergistically beneficial to the therapeutic objectives of different nutraceutical or pharmaceutical molecules.
Physiological effects
Reviews of epidemiological studies seeking correlations between carotenoid consumption in food and clinical outcomes have come to various conclusions:
A 2015 review found that foods high in carotenoids appear to be protective against head and neck cancers.
Another 2015 review looking at whether caretenoids can prevent prostate cancer found that while several studies found correlations between diets rich in carotenoids appeared to have a protective effect, evidence is lacking to determine whether this is due to carotenoids per se.
A 2014 review found no correlation between consumption of foods high in carotenoids and vitamin A and the risk of getting Parkinson's disease.
Another 2014 review found no conflicting results in studies of dietary consumption of carotenoids and the risk of getting breast cancer.
Carotenoids are also important components of the dark brown pigment melanin, which is found in hair, skin, and eyes.
Melanin absorbs high-energy light and protects these organs from intracellular damage.
Several studies have observed positive effects of high-carotenoid diets on the texture, clarity, color, strength, and elasticity of skin.
A 1994 study noted that high carotenoid diets helped reduce symptoms of eyestrain (dry eye, headaches, and blurred vision) and improve night vision.
Humans and other animals are mostly incapable of synthesizing carotenoids, and must obtain them through their diet.
Carotenoids are a common and often ornamental feature in animals.
For example, the pink color of salmon, and the red coloring of cooked lobsters and scales of the yellow morph of common wall lizards are due to carotenoids.
Carotenoids has been proposed that carotenoids are used in ornamental traits (for extreme examples see puffin birds) because, given their physiological and chemical properties, they can be used as visible indicators of individual health, and hence are used by animals when selecting potential mates.
Plant colors
The most common carotenoids include lycopene and the vitamin A precursor β-carotene.
In plants, the xanthophyll lutein is the most abundant carotenoid and its role in preventing age-related eye disease is currently under investigation.
Lutein and the other carotenoid pigments found in mature leaves are often not obvious because of the masking presence of chlorophyll.
When chlorophyll is not present, as in autumn foliage, the yellows and oranges of the carotenoids are predominant.
For the same reason, carotenoid colors often predominate in ripe fruit after being unmasked by the disappearance of chlorophyll.
Carotenoids are responsible for the brilliant yellows and oranges that tint deciduous foliage (such as dying autumn leaves) of certain hardwood species as hickories, ash, maple, yellow poplar, aspen, birch, black cherry, sycamore, cottonwood, sassafras, and alder.
Carotenoids are the dominant pigment in autumn leaf coloration of about 15-30% of tree species.
However, the reds, the purples, and their blended combinations that decorate autumn foliage usually come from another group of pigments in the cells called anthocyanins.
Unlike the carotenoids, these pigments are not present in the leaf throughout the growing season, but are actively produced towards the end of summer.
Bird colors and sexual selection
Dietary carotenoids and their metabolic derivatives are responsible for bright yellow to red coloration in birds.
Studies estimate that around 2956 modern bird species display carotenoid coloration and that the ability to italicize these pigments for external coloration has evolved independently many times thought avian evolutionary history.
Carotenoid coloration exhibits high levels of sexual dimorphism, meaning that male birds tend to display more vibrant coloration than females of the same species.
These differences arise due to the selection of yellow and red coloration in males by female preference.
In many species of birds, females invest greater time and resources into raising offspring than their male partners.
Therefore, it is imperative that female birds carefully select high quality mates.
Current literature supports the theory that vibrant carotenoid coloration is correlated with male quality—either though direct effects on immune function and oxidative stress, or through a connection between carotenoid metabolizing pathways and pathways for cellular respiration.
Aroma chemicals
Products of carotenoid degradation such as ionones, damascones and damascenones are also important fragrance chemicals that are used extensively in the perfumes and fragrance industry.
Both β-damascenone and β-ionone although low in concentration in rose distillates are the key odor-contributing compounds in flowers.
In fact, the sweet floral smells present in black tea, aged tobacco, grape, and many fruits are due to the aromatic compounds resulting from carotenoid breakdown.
Disease
Some carotenoids are produced by bacteria to protect themselves from oxidative immune attack.
The aureus (golden) pigment that gives some strains of Staphylococcus aureus their name is a carotenoid called staphyloxanthin.
This carotenoid is a virulence factor with an antioxidant action that helps the microbe evade death by reactive oxygen species used by the host immune system.
Carotenoids are a class of more than 750 naturally occurring pigments synthesized by plants, algae, and photosynthetic bacteria.
These richly colored molecules are the sources of the yellow, orange, and red colors of many plants.
Fruit and vegetables provide most of the 40 to 50 carotenoids found in the human diet.
α-Carotene, β-carotene, β-cryptoxanthin, lutein, zeaxanthin, and lycopene are the most common dietary carotenoid.
α-Carotene, β-carotene and β-cryptoxanthin are provitamin A carotenoids, meaning they can be converted by the body to retinol.
Lutein, zeaxanthin, and lycopene are nonprovitamin A carotenoids because they cannot be converted to retinol.
Absorption, Metabolism, and Bioavailability
For dietary carotenoids to be absorbed intestinally, they must be released from the food matrix and incorporated into mixed micelles (mixtures of bile salts and several types of lipids).
Food processing and cooking help release carotenoids embedded in their food matrix and increase intestinal absorption.
Moreover, carotenoid absorption requires the presence of fat in a meal.
As little as 3 to 5 g of fat in a meal appears sufficient to ensure carotenoid absorption, although the minimum amount of dietary fat required may be different for each carotenoid.
The type of fat (e.g., medium-chain vs. long-chain triglycerides), the presence of soluble fiber, and the type and amount of carotenoids (e.g., esterified vs. non-esterified) in the food also appear to influence the rate and extent of carotenoid absorption.
Because they do not need to be released from the plant matrix, carotenoid supplements (in oil) are more efficiently absorbed than carotenoids in food.
Although carotenoids were initially thought to be absorbed within the cells that line the intestine (enterocytes) only by passive diffusion, recent investigations identified the apical membrane transporters, Scavenger Receptor-class B type I (SR-BI) and Cluster Determinant 36 (CD36), suggesting active uptake of carotenoids as well.
Within the enterocytes, provitamin A carotenoids may be cleaved by either β-carotene 15,15’-oxygenase 1 (BCO1) or by β-carotene 9’,10’-oxygenase 2 (BCO2).
BCO1 catalyzes the cleavage of provitamin A carotenoids into retinal, which is further reduced to retinol (vitamin A) or oxidized to retinoic acid (the biologically active form of vitamin A).
β-Apocarotenal derived from the cleavage of β-carotene by BCO2 can be cleaved further by BCO1 to produce retinal.
Although provitamin A carotenoids can be converted into apocarotenals by BCO2, the activity of this enzyme is higher toward nonprovitamin A carotenoids. Conversely, BCO1 shows limited affinity toward nonprovitamin A carotenoids.
Within the enterocytes, uncleaved carotenoids and retinyl esters (derived from retinol) are incorporated into triglyceride-rich lipoproteins called chylomicrons, secreted into lymphatic vessels, and then released in the bloodstream.
Triglycerides are depleted from circulating chylomicrons through the activity of an enzyme called lipoprotein lipase, resulting in the formation of chylomicron remnants.
Chylomicron remnants are taken up by the liver, where carotenoids can be cleaved by BCO1/BCO2 or incorporated into lipoproteins and secreted back into the circulation for delivery to extrahepatic tissues.
Of note, more hydrophilic molecules in the enterocytes like retinoic acid and apocarotenals can be transported directly to the liver through the portal blood system.
The conversion of provitamin A carotenoids to retinol is influenced by the vitamin A status of the individual.
The regulatory mechanism involving the intestine-specific homeobox (ISX) transcription factor can block carotenoid uptake and vitamin A production by inhibiting the expression of SR-BI and BCO1.
ISX is under the control of retinoid acid and retinoic acid receptor (RAR)-dependent mechanisms such that, when vitamin A stores are high, ISX is activated and both provitamin A carotenoid absorption and conversion to retinol are inhibited.
Conversely, during vitamin A insufficiency, the expression of both SR-BI and BCO1 is no longer repressed by ISX, allowing for provitamin A carotenoid absorption and conversion to retinol.
Interindividual variations in blood and tissue concentrations of carotenoids have been attributed to genetic differences among individuals.
Specifically, a number of single nucleotide polymorphisms (SNPs) — corresponding to changes of one nucleotide in the sequence of genes — have been identified in genes coding for proteins involved in intestinal uptake, transport, and metabolism of carotenoids.
Specifically, SNPs within genes coding for SR-BI, CD36, and BCO1 are suspected to affect the expression and/or activity of these proteins and, in turn, individual carotenoid status.
What are carotenoids?
Carotenoids are pigments in plants, algae, and photosynthetic bacteria.
These pigments produce the bright yellow, red, and orange colors in plants, vegetables, and fruits.
Carotenoids act as a type of antioxidant for humans.
There are more than 600 different types of carotenoids.
Some can be converted into vitamin A when released into the body.
A few of the most common carotenoids include:
-alpha carotene
-beta carotene
-beta cryptoxanthin
-lutein
-zeaxanthin
-lycopene
Carotenoids must be consumed through the diet.
They are best absorbed through a source of fat.
Foods rich in carotenoids include:
-yams
-kale
-spinach
-watermelon
-cantaloupe
-bell peppers
-tomatoes
-carrots
-mangoes
-oranges
How do carotenoids work?
Carotenoids are fat-soluble compounds, meaning they are best absorbed with fat.
Unlike some protein-rich foods and vegetables, cooking and chopping carotenoid-rich foods increase the strength of the nutrients when they enter the bloodstream.
Carotenoids are classified into two main groups: xanthophylls and carotenes.
Both types of carotenoids have antioxidant properties.
In addition, some carotenoids can be converted into vitamin A, an essential component for human health and growth.
These provitamin A carotenoids include alpha carotene, beta carotene, and beta cryptoxanthin.
Non-provitamin A carotenoids include lutein, zeaxanthin, and lycopene.
Xanthophylls
Xanthophylls contain oxygen and sometimes have more of a yellow pigment.
Xanthophyll carotenoids protect you from too much sunlight.
They are most associated with eye health.
Lutein and zeaxanthin fall under the xanthophyll category.
Foods that fall under the xanthophyll category include:
-kale
-spinach
-summer squash
-pumpkin
-avocado
-yellow-fleshed fruits
-corn
-egg yolks
Carotenes
Carotenes do not contain oxygen and are associated with more of an orange pigment.
Carotene carotenoids play a significant role in helping plants grow.
Beta carotene and lycopene fall under this category of carotenoids.
Foods in the carotene category include:
-carrots
-cantaloupe
-sweet potatoes
-papaya
-pumpkin
-tangerines
-tomatoes
-winter squash
Agricultural Uses
Carotenoids are a large class of pigments usually located in the thylakoid membranes of the grana in chloroplasts, in the form of carotenoprotein complexes.
The general structure of carotenoids is that of aliphatic and aliphatic-alicyclic polyenes with a few aromatic-polyenes.
They are widely distributed in plants and act as photosynthetic pigments in cells that lack chlorophyll.
They have the same basic structure as vitamin A, and are converted into vitamin A in animal livers.
More than 300 carotenoids are known and this number is on the rise.
There are several biochemical functions in which carotenoids play a role, apart from their well-known role as photosynthetic pigments.
Carotenoids act as blue light harvesting pigments, protect biological systems from photodynamic damage and are safe food colorants.
Carotenoids are plant pigments responsible for bright red, yellow and orange hues in many fruits and vegetables.
These pigments play an important role in plant health.
People who eat foods containing carotenoids get protective health benefits as well.
Carotenoids are a class of phytonutrients ("plant chemicals") and are found in the cells of a wide variety of plants, algae and bacteria.
They help plants absorb light energy for use in photosynthesis.
They also have an important antioxidant function of deactivating free radicals — single oxygen atoms that can damage cells by reacting with other molecules, according to the Linus Pauling Institute at Oregon State University.
Carotenoids also act as antioxidants in the human body.
They have strong cancer-fighting properties, according to the Physicians Committee for Responsible Medicine.
Some carotenoids are converted by the body to vitamin A, which is essential to vision and normal growth and development.
Carotenoids also have anti-inflammatory and immune system benefits and are sometimes associated with cardiovascular disease prevention.
Sources of carotenoids
Carotenoid-containing foods are often red, yellow or orange, but not always.
Louis Premkumar, a professor of pharmacology at Southern Illinois University School of Medicine and author of "Fascinating Facts about Phytonutrients in Spices and Healthy Food" (Xlibris, 2014), told Live Science that carrots, yams, sweet potatoes, papaya, watermelon, cantaloupe, mangos, spinach, kale, tomatoes, bell peppers and oranges are among the fruits and vegetables in which carotenoids can be found.
Animals cannot manufacture carotenoids themselves; they have to get it in their diets.
Carotenoids need to be consumed with a fat in order for the body to absorb them.
According to the Linus Pauling Institute at Oregon State University, carotenoids need to leave the food they came in and become part of mixed micelles, which are combinations of bile salts and lipids.
The presence of a fat makes this process possible.
The carotenoid family
There are more than 600 types of carotenoids.
The most common ones in the Western diet, and the most studied, are alpha-carotene, beta-carotene, beta-cryptoxanthin, lutein, zeaxanthin and lycopene, according to the Linus Pauling Institute.
There are two broad classifications of carotenoids: carotenes and xanthophylls, said Premkumar.
The difference between the two groups is chemical: xanthophylls contain oxygen, while carotenes are hydrocarbons and do not contain oxygen.
Also, the two absorb different wavelengths of light during a plant’s photosynthesis process, so xanthophylls are more yellow while carotenes are orange.
Nutritionally, there is another, potentially more useful, grouping of carotenoids: provitamin A and non-provitamin A.
Provitamin A carotenoids can be turned into vitamin A (retinol) in the intestine or liver.
Vitamin A is an important component to human health.
It helps maintain eye health, healthy mucus membranes and immunity.
Alpha-carotene, beta-carotene and beta-cryptoxanthin are provitamin A carotenoids; lutein, zeaxanthin and lycopene are not.
Xanthophylls
Lutein and zeaxanthin
Lutein and zeaxanthin are associated primarily with eye health.
Studies often do not separate lutein and zeaxanthin because they are the only carotenoids found in the retina.
“Lutein and zeaxanthin are accumulated in human retina at the macula lutea, which is responsible for central vision and protects the retina from blue light, which may cause ionization and damage the retina,” explained Premkumar.
Scientists seem to know more about lutein, and supplements typically contain much more lutein than zeaxanthin.
Lutein and zeaxanthin are likely “effective in age-related macular degeneration (AMD), a leading cause of blindness,” said Premkumar.
“A six-year study from the National Eye Institute concluded that lutein reduces the risk of AMD.
It has been shown to reduce the incidence of cataract (lens opacity) and light sensitivity if consumed in adequate quantities on a daily basis.”
Premkumar noted that lutein could also be good for the heart.
“Lutein is known to prevent the formation of atherosclerosis, which is composed of plaques that restrict blood flow to the heart muscle; when occluded, it fully leads to a heart attack,” he said.
When lutein is in the blood, it can have an antioxidant effect on cholesterol, thereby preventing cholesterol from building up in the arteries and clogging them.
A study published in Circulation found that participants who added lutein supplements to their diets had less arterial wall thickening than those who did not.
Good sources of lutein and zeaxanthin include kale, spinach, turnip greens, summer squash, pumpkin, paprika, yellow-fleshed fruits and avocado, said Premkumar.
Lutein is also available through enriched eggs.
A study published in the Journal of Nutrition found that lutein from enriched eggs was absorbed better than lutein from spinach or supplements.
Beta-cryptoxanthin
Beta-cryptoxanthin is a xanthophyll carotenoid that is also provitamin A.
It can be a source of vitamin A, but it produces half as much as beta-carotene.
Premkumar listed papaya, mango and oranges as good sources of it.
Beta-cryptoxanthin is typically found in yellow foods, such as corn and bell peppers, and is present in yellow-colored dairy products, such as egg yolks and butter.
Beta-cryptoxanthin may be helpful in reducing the risk of inflammatory polyarthritis, which includes rheumatoid arthritis.
Scientists suspect this is because its antioxidant abilities can reduce chronic inflammation.
In a large-scale European study published in the American Journal of Clinical Nutrition, researchers found that participants who developed inflammatory polyarthritis had 40 percent less beta-crytpxanthin than those who did not.
Participants who consumed the most beta-cryptoxanthin were significantly less likely to develop inflammatory polyarthritis.
The researchers advised that a modest increase in beta-cryptoxanthin, such as a glass of orange juice a day, could be helpful in preventing arthritis.
Carotenes
Beta-carotene
Of the provitamin A carotenoids, beta-carotene is the most powerful when it comes to turning into vitamin A; twice as much beta-carotene becomes vitamin A than does alpha-carotene or beta-cryptoxanthin.
Beta-carotene was the first and is the most widely studied of the carotenoids.
It seems to be capable of both positive and negative effects, especially for smokers taking it as a supplement.
Two studies of showed that smokers and former asbestos workers who took beta- carotene supplements increased their risk of lung cancer, according to the Linus Pauling Institute.
Doctors currently advise smokers not to take beta-carotene supplements.
Large amounts of beta-carotene from food, however, do not seem to carry this risk; the worst they can do is temporarily turn your skin orange, according to the National Institutes of Health.
Cantaloupe, mangoes, papaya, carrots, sweet potatoes, spinach, kale and pumpkin are good sources of beta-carotene, said Premkumar.
Beta-carotene gives orange foods their color; in fact, the word carotene comes from the Latin word for carrot.
Beta-carotene may help protect against sunburn, according to a meta-analysis published in Photochemistry and Photobiology.
The researchers looked at several studies and found that participants who took beta-carotene supplements for 10 weeks had lower rates of sunburn.
For each month of additional supplementation, the protection level increased.
Beta-carotene may help lower the risk of metabolic syndrome, at least in middle-age and elderly men, a study published in the Journal of Nutrition found.
Metabolic syndrome is characterized by high blood pressure, high blood sugar, abnormal cholesterol levels and excess fat around the waist.
The men with the most beta-carotene intake had the lowest risk of metabolic syndrome, as well as reduced waist circumference.
Scientists suspect this is the result of beta-carotene’s antioxidant activities.
Early studies suggested that beta-carotene was associated with a reduced risk of lung cancer, according to a review published in the Journal of Nutrition.
More recent studies have shown that relationship to be unreliable, although other carotenoids like alpha-carotene, lycopene and beta-cryptoxanthin have shown promise.
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Alpha-carotene
Alpha-carotene produces half the vitamin A that beta-carotene does.
Alpha-carotene is found in similar foods to beta-carotene and is often studied in conjunction with that carotenoid, though it is rarer and less well-understood.
Recently, scientists have been paying more attention to alpha-carotene, and have found some potential longevity benefits, in addition to the vitamin A goodness alpha-carotene can provide.
A study published in Archives of Internal Medicine found a correlation between alpha-carotene intake and longevity.
Looking at results from the 14-year study, researchers found that high blood levels of alpha-carotene were inversely associated with cancer death, cardiovascular disease and all other illness causes.
The correlation between high levels of alpha-carotene and a lower risk of death from diabetes and lower respiratory disease were especially high.
It is worth noting that because alpha-carotene is not widely available in supplement form, these participants were getting their alpha-carotene from fruits and vegetables.
A Japanese study published in the Journal of Epidemiology found that participants with the highest blood levels of alpha-carotene were less likely to die from heart disease — even less likely than participants with high beta-carotene levels.
Together with lycopene, alpha-carotene was associated with reduced risk of lung cancer in a study of two large cohorts published in the American Journal of Clinical Nutrition.
Good sources of alpha-carotene include pumpkin, carrots, tomatoes, collards, tangerines, winter squash and peas, said Premkumar.
Lycopene
Lycopene is a bright red pigment responsible for the color of watermelons, tomatoes, guavas and grapefruit.
Other good sources include papaya, carrots, asparagus, red cabbage, red bell peppers and parsley.
The lycopene in tomatoes is absorbed much more easily if the tomatoes are cooked, according to the Linus Pauling Institute.
“Lycopene can act as a potent antioxidant,” said Premkumar.
In a test tube study published in Archives of Biochemistry and Biophysics, researchers found that, of all the carotenoids, lycopene was most effective at deactivating singlet oxygen (a harmful free radical).
This may be because lycopene has a unique molecule shape that is highly effective in deactivating free radicals.
Lycopene is also associated with reduced prostate cancer risk.
A large-scale study of nearly 50,000 men published in the Journal of the National Cancer Institute found an inverse relationship between lycopene levels and prostate cancer risk.
Men with the highest levels of lycopene were 21 percent less likely to develop prostate cancer than those with the lowest lycopene levels.
These men got their lycopene from tomatoes, which demonstrated the effectiveness of lycopene from food sources rather than supplements.
However, the effects may have come from other nutrients in tomatoes.
Lycopene may promote bone health and help prevent the development of osteoporosis, said Premkumar.
Additionally, lycopene may decrease stroke risk, at least in men.
Together with alpha-carotene, lycopene was associated with reduced risk of lung cancer in a study of two large cohorts published in the American Journal of Clinical Nutrition.
