What are carotenoids

Carotenoids are dark-colored dyes (pigments) found in plant foods that can turn into a form of vitamin A.  For example, the orange color of carrots and the red color of tomatoes are due to their carotenoid components ¹⁾. There are more than 500 known carotenoids. The carotenoids that have been most studied in this regard are beta-carotene, lycopene, lutein and zeaxanthin. In part, the beneficial effects of carotenoids are thought to be due to their role as antioxidants. By far the most important provitamin A carotenoid is beta-carotene; other provitamin A carotenoids are alpha-carotene and beta-cryptoxanthin. The body converts these plant pigments into vitamin A ²⁾. Both provitamin A carotenoids and preformed vitamin A (retinol and its esterified form, retinyl ester) must be metabolized intracellularly to retinal and retinoic acid, the active forms of vitamin A, to support the vitamin’s important biological functions ³⁾. Other carotenoids found in food, such as lycopene, lutein, and zeaxanthin, are not converted into vitamin A. Most of the body’s vitamin A is stored in the liver in the form of retinyl esters.

For dietary provitamin A carotenoids—β-carotene, α-carotene, and β-cryptoxanthin—RAEs (retinol activity equivalents) have been set at 12, 24, and 24 μg, respectively ⁴⁾.

Carotenoids can be divided into two groups according to their polarity: xanthophylls (polar carotenoids such as astaxanthin, β-cryptoxanthin, lutein, and zeaxanthin) and carotene (nonpolar carotenoids such as α-carotene, β-carotene, and lycopene) ⁵⁾. The distinctive structural feature of carotenoids is the long, alternating double and single bond system, which is associated with light absorption and oxidation ⁶⁾.

The major sources of carotenoids in the human diet are fruits and vegetables, which have various colors, such as green, red, orange, and yellow ⁷⁾. Humans consume approximately 40 carotenoids from common fruits and vegetables ⁸⁾. Dark green vegetables, such as broccoli, coriander, kale, and spinach, contain a large number of chloroplasts, in which most carotenoids exist; therefore, they possess high concentrations of carotenoids ⁹⁾. As chloroplasts generally contain the most consistent carotenoid composition ¹⁰⁾, the distribution of carotenoids is similar among different plant species in this group ¹¹⁾. On the other hand, in red-, orange-, or yellow-colored fruits and vegetables, carotenoids are mainly accumulated in chromoplasts, which are usually converted from chloroplasts during ripening ¹²⁾. As chromoplasts in different plant species contain various carotenoids, the carotenoid distribution in this group is diverse ¹³⁾. Some seafood and animal foods also contain carotenoids. Animals cannot synthesize carotenoids; instead, they ingest carotenoids through foods and accumulate these molecules in their bodies. As a result, some animal foods contain carotenoids. For example, high concentrations of lutein and zeaxanthin accumulate in egg yolks ¹⁴⁾. Milk and dairy products, salmonid fish, and crustaceans also provide various carotenoids ¹⁵⁾. The main carotenoid in bovine milk is β-carotene ¹⁶⁾, whereas the major carotenoids in salmonid fish and crustaceans are astaxanthin and canthaxanthin ¹⁷⁾. In addition, some edible brown seaweeds contain fucoxanthin as a major carotenoid ¹⁸⁾.

Carotenoids are differentially distributed in various organs of the human body. Interestingly, xanthophylls account for 66–77% of the total carotenoids in the frontal and occipital lobes of the human brain ¹⁹⁾, whereas less than 40% of the total carotenoids in most tissues and plasma are reported to be xanthophylls ²⁰⁾. It was reported that the human brain contains sixteen carotenoids, with the major carotenoids being anhydrolutein, α-carotene, α-cryptoxanthin, cis- and trans-β-carotene, β-cryptoxanthin, lutein, cis- and trans-lycopene, and zeaxanthin ²¹⁾. Given carotenoids property of protecting tissues from oxidative stress and their localization in the brain, the role of carotenoids in preventing or treating oxidative stress-associated diseases, including neurodegenerative diseases, is of interest.

As carotenoids have various physiological activities, such as antioxidant activity, the amount of carotenoid in the human body is important for health. Therefore, the intake of carotenoids through the diet is associated with the prevention and treatment of various diseases, including age-related macular degeneration ²²⁾, cancer ²³⁾, cardiovascular diseases ²⁴⁾, and neurodegenerative diseases ²⁵⁾.

Types of carotenoids

The two main groups of carotenoids are carotenes and xanthophylls. Some familiar carotenes are β-carotene and lycopene—both these carotenoids are strict hydrocarbon carotenoids, and do not possess any substituent (or even oxygen) in their structures. Xanthophylls or oxycarotenoids, which belong to the second group, are oxygen-containing molecules. Lutein and zeaxanthin are two xanthophylls with –OH groups in their structures, whereas canthaxanthin and echinenone contain =O groups. Astaxanthin has both –OH and =O groups in its structure. Furthermore, some carotenoids such as violaxanthin and diadinoxanthin contain epoxy groups, and others such as dinoxanthin and fucoxanthin have acetyl groups in their structures. The two carotenoids with acetyl groups also contain the C=C=C (allene) group in their structures, which is unique to natural products ²⁶⁾. In addition, some carotenoids such as allo-, diato-, diadino-, hetero-, croco-, pyro-, and monadoxanthin contain C≡C (acetylene) groups in their structures.

Carotenoids foods

α-Carotene

• Banana, butternut, carrot, pumpkin

β-Carotene

• Apricots, banana, broccoli, cantaloupe, carrot, dairy products, honeydew, kale, mango, nectarine, peach, pumpkin, spinach, sweet potato, tomato

Crocetin

• Gardenia fruit, saffron stigma

Crocin

• Gardenia fruit, saffron stigma

β-Cryptoxanthin

• Apple, broccoli, celery, chili, crustaceans, grape, green beans, papaya, pea, peach, peppers, salmonid fish, squashes, tangerine

Lutein

• Apple, basil, broccoli, celery, crustaceans, cucumber, dairy products, grapes, green pepper, kale, kiwi, maize, parsley, pea, pumpkin, salmonid fish, spinach, squash

Lycopene

• Grapefruit, guava, tomato, watermelon

Zeaxanthin

• Basil, crustaceans, cucumber, dairy products, honeydew, kale, maize, mango, orange, parsley, salmonid fish, spinach

Astaxanthin

• Crustaceans, algae, salmonid fish

Fucoxanthin

• Brown seaweeds

Carotenoids function

Significant evidence has shown that carotenoids can reduce oxidative damage by scavenging reactive oxygen species (ROS) and exert anti-inflammatory effects in vivo ²⁷⁾.

One such carotenoid is beta-carotene.

• Beta-carotene is an antioxidant. Antioxidants protect cells from damage caused by substances called free radicals. Free radicals are believed to contribute to certain chronic diseases and play a role in the aging processes.
• Beta-carotene may have added benefits due its ability to be converted to vitamin A.
• Food sources of carotenoids such as beta-carotene may reduce the risk for cancer.
• But beta-carotene supplements do not seem to reduce cancer risk.

Additionally, lutein and zeaxanthin may be protective in eye disease because they absorb damaging blue light that enters the eye and they decreased the risk of functional vision loss among patients with category 3 or 4 age-related macular degeneration (AMD) ²⁸⁾.

Antioxidant activity

Carotenoids have been demonstrated to be one of the most potent natural singlet oxygen scavengers, with a fast quenching rate ²⁹⁾. They can effectively neutralize reactive oxygen species (ROS) and other free radicals to provide protection against oxidation in both photosynthetic and nonphotosynthetic organisms ³⁰⁾. However, each carotenoid shows different antioxidant activities, owing to the presence of functional groups with increasing polarities as well as the number of conjugated double bonds ³¹⁾. The antioxidant property of carotenoids has inspired many epidemiological and clinical studies that have investigated if these pigment molecules are able to prevent various ROS-mediated disorders such as cancer, inflammation, retinal degeneration, and neurodegeneration. In the case of cancer, many studies have shown that carotenoid consumption is correlated with a reduced risk of several types of cancer; however, other studies have shown that the cancer-preventive effects of carotenoids are negligible or even that they are carcinogenic ³²⁾.

Lutein is a xanthophyll and the most abundant carotenoid in the human retina and brain ³³⁾. The Age-Related Eye Disease Study (AREDS) showed that a formulation consisting of vitamins C, E, β-carotene, and zinc is beneficial for the prevention of age-related macular degeneration (AMD) ³⁴⁾. In a second study, although primary analysis from the AREDS2 did not reveal a benefit of daily supplementation with lutein/zeaxanthin on AMD progression ³⁵⁾, secondary exploratory analyses suggested that lutein/zeaxanthin were helpful in reducing this risk ³⁶⁾. In addition, given that increased oxidative stress and inflammation are observed in age-related macular degeneration ³⁷⁾, lutein supplementation may improve visual function through antioxidant activity.

In addition to their antioxidant activities, carotenoids can protect cells from the oxidative stress induced by some stressors via activation of endogenous antioxidant enzymatic activities and a reduction in DNA damage. Crocetin, a pharmacologically active metabolite of Crocus sativus L., exerts cardioprotective effects by increasing superoxide dismutase (SOD) and glutathione peroxidase activities in cardiac hypertrophy induced by norepinephrine in rats ³⁸⁾. Crocin, another component of Crocus sativus L., has also been shown to increase superoxide dismutase (SOD) activity to prevent the death of PC-12 cells during serum/glucose deprivation ³⁹⁾. Recent studies have demonstrated that marine carotenoids such as astaxanthin and fucoxanthin also display antioxidant properties by activating the antioxidant network, including superoxide dismutase (SOD) and catalase ⁴⁰⁾. In addition, β-cryptoxanthin protects human cells from H2O2-induced damage by stimulating the repair of damage caused by DNA oxidation as well as by its antioxidant activity ⁴¹⁾. Lycopene and β-carotene also provide protection against DNA damage at low concentrations ⁴²⁾. However, opposite effects have been shown at higher concentrations in cells with oxidative damage ⁴³⁾.

Antineuroinflammation

Neuroinflammation is a local response of the nervous system during neurodegeneration, trauma, and autoimmune disorders. A variety of cell types, including astrocytes, microglia, vascular cells, neutrophils, and macrophages, are involved in neuroinflammation ⁴⁴⁾. Growing evidence suggests that neuroinflammation is one of the pathological features of many neurodegenerative disorders and autoimmune diseases, such as multiple sclerosis ⁴⁵⁾. In the last decade, some carotenoids have been shown to have antineuroinflammatory effects in vivo. Among the polar xanthophylls, the ability of lutein to suppress inflammation has been demonstrated in murine retinal cells ⁴⁶⁾ and in a clinical trial studying retinal health in preterm infants ⁴⁷⁾. In addition, it has been shown that lutein reduces lipid peroxidation and proinflammatory cytokine release by suppressing the activation of the nuclear factor-κB (NF-κB) pathway in the presence of a variety of oxidative stressors ⁴⁸⁾. It has also been demonstrated that crocin and crocetin are able to suppress the production of proinflammatory cytokines and nitric oxide by lipopolysaccharide, interferon γ, and β-amyloid (Aβ) stimulation in microglial cells ⁴⁹⁾. Astaxanthin, a member of the xanthophyll family that confers the pink color in flamingos, has an anti-inflammatory effect and antioxidant activity similar to other carotenoids ⁵⁰⁾. Furthermore, astaxanthin has also been found to reduce hippocampal and retinal inflammation in streptozotocin-induced diabetic rats, alleviating cognitive deficits, retinal oxidative stress, and depression ⁵¹⁾. Fucoxanthin, another member of the marine xanthophylls, exerts anti-inflammatory effects against various stimuli through Akt, NF-κB, and mitogen-activated protein kinase pathways ⁵²⁾.

Lycopene, one of the carotenes present in large amounts in tomatoes, has been demonstrated to reduce neuroinflammatory phenotypes, depression-like behaviors, and inflammation-induced cognitive function defects in murine models ⁵³⁾. As a whole, cellular and animal models have revealed that carotenoids are potent anti-inflammatory agents in the nervous system and act through the suppression of inflammation pathways.

Modulation of autophagy

Autophagy, a catabolic process necessary for the cleanup of damaged organelles, protein complexes, and even single proteins, as well as for the recycling of nutritional building blocks, has been implicated in numerous disorders and conditions such as aging, cancer, and neurodegeneration. A growing amount of evidence strongly suggests that autophagy removes misfolded or aggregated proteins, the main features of most neurodegenerative diseases, for example, tau fibrils in Alzheimer’s disease and Lewy bodies in Parkinson’s disease ⁵⁴⁾. Recent studies have shown that some carotenoids are able to modulate autophagy in cellular and animal models. It has been recently demonstrated that lutein attenuates cobalt chloride-induced autophagy via the mTOR pathway in rat Müller cells ⁵⁵⁾, whereas it induces autophagy through the upregulation of Beclin-1 in retinal pigment epithelial cells ⁵⁶⁾. Crocin has also been shown to have a paradoxical effect on autophagy. The induction of autophagy by crocin occurs during hypoxia, and the inhibition of autophagy by crocin occurs during reperfusion ⁵⁷⁾. Lycopene has also been shown to be involved in autophagy ⁵⁸⁾. Astaxanthin has been found to attenuate autophagy in hepatic cells ⁵⁹⁾. In a model of mice traumatic brain injury, fucoxanthin has the ability to protect neuronal cells from death through the activation of autophagy and the nuclear factor erythroid 2-related factor pathway ⁶⁰⁾. The modulation of autophagy by carotenoids remains a controversial topic, and the precise molecular mechanism of this modulation remains unclear.

Anti-angiogenic activity

Angiogenesis is the process of formation of new blood vessels from pre-existing capillaries and involves a sequence of events that are fundamental to many physiological and pathological processes ⁶¹⁾. It occurs throughout life, during both healthy and diseased conditions, and is tightly regulated under normal physiological conditions such as during embryogenesis, ovary cycling, and wound healing. Chronic, unregulated angiogenesis can lead to several anomalous angiogenic conditions such as inflammatory diseases, rheumatoid arthritis, and tumor metastasis ⁶²⁾. Tumor growth and metastasis are processes that are highly dependent on the formation of new blood vessels. Therefore, preventing angiogenesis under pathological conditions (such as cancer and other angiogenesis related diseases) is a promising approach for controlling or eradicating such diseases.

Studies involving in vivo and in vitro experiments on male C57BL/6 mice and B16F-10 cells have been used to evaluate the anti-angiogenic effects of β-carotene by Guruvayoorappan and Kuttan ⁶³⁾. Their study found that treatment with β-carotene significantly reduces the number of tumor-directed capillaries (associated with altered serum cytokine levels) formed, and suppresses the proliferation, migration, and tube formation of endothelial cells. In addition, β-carotene treatment also inhibited the activation and nuclear translocation of p65, p50, and c-Rel sub-units of nuclear factor-κB (NF-κB), as well as other transcription factors such as c-fos, activated transcription factor-2, and cyclic adenosine monophosphate response element-binding protein in B16F-10 melanoma cells ⁶⁴⁾. This study clearly showed that the anti-angiogenic effect of β-carotene occurs by affecting serum cytokine levels, and that β-carotene could inhibit the activation and nuclear translocation of transcription factors.

In another study, Sugawara et al. ⁶⁵⁾ reported that fucoxanthin (at concentrations of >10 µM) can significantly inhibit tube formation and proliferation in human umbilical vein endothelial cells (HUVECs). Fucoxanthin significantly suppressed the differentiation of endothelial progenitor cells into endothelial cells during the formation of new blood vessel. Fucoxanthin and its metabolite fucoxanthinol also suppressed the growth of microvessels during in vitro and ex vivo experiments using rat aortic rings ⁶⁶⁾. In addition, a study in 2013 used HUVECs to understand the molecular mechanisms responsible for the anti-angiogenic activity of fucoxanthin ⁶⁷⁾. The results of this study showed that fucoxanthin significantly reduced the genetic expression, and hence, mRNA levels of fibroblast growth factor 2 (FGF-2), its receptor (FGFR-1), as well as their trans-activation factor, EGR-1. However, the study found no significant changes in the mRNA levels of the vascular endothelial growth factor receptor-2 (VEGFR-2). Furthermore, fucoxanthin was found to down-regulate the phosphorylation of FGF-2-mediated intracellular signaling proteins such as extracellular signal-reduced kinase and protein kinase B (ERK1/2 and Akt). Matrigel invasion assays showed that fucoxanthin not only inhibited the migration of endothelial cells, but also inhibited their differentiation into tube-like structures by suppressing the phosphorylation of the FGF-2-mediated intracellular signaling proteins. However, there was no evidence to indicate that carotenoids activate the angiopoietins 1 and 2 (Ang1 and Ang2) pathways.

Cardioprotective activity

A study in 2005 by Hussein et al. ⁶⁸⁾ reported the anti-hypertensive effects of astaxanthin in spontaneously hypertensive rats (SHRs). In their study, Hussein et al. ⁶⁹⁾ found that oral administration of astaxanthin (at a concentration of 50 mg/kg) for 14 days led to a significant decrease in blood pressure in the SHRs. In addition, long-term administration of astaxanthin (for 5 weeks) also considerably reduced blood pressure and postponed the occurrence of heart strokes in these rats. On the 4th day of treatment, 60% of the rats in the placebo group showed signs of heart stroke, whereas none of the rats in the astaxanthin-treated group showed any signs of heart stroke. In later studies, the same authors also reported the mechanism of how astaxanthin works to prevent heart strokes ⁷⁰⁾. The authors found that SHRs treated with astaxanthin showed significantly higher levels of vasorelaxation in response to nitric oxide (NO), which enhanced thoracic aorta contractions, as compared to rats not treated with astaxanthin. These results suggest that the anti-hypertensive effect of astaxanthin is mediated by NO-related mechanisms. In addition, another study carried out on SHRs to explore the beneficial effects of astaxanthin on blood rheology found that the transit times of whole blood in astaxanthin-treated SHRs were significantly lower than those of placebo-treated SHRs. Histopathological measures, such as levels of vascular elastin in the aorta and arterial wall thickness were also improved in SHRs treated with astaxanthin ⁷¹⁾. A study by Preuss et al. ⁷²⁾ in Zucker fatty rats found that administration of 25 mg/kg of astaxanthin for one month significantly lowered systolic blood pressure. In addition, the astaxanthin treatment also decreased the activity of the renin-angiotensin system, which indicates that the lowering in blood pressure was dependent on changes in the renin-angiotensin, as well as the NO systems. Furthermore, in heat stress experiments, all rats fed with astaxanthin survived, whereas, >60% of the rats in the placebo group died.

In another study, female BALB/c mice treated with 800 mg/kg astaxanthin for eight weeks exhibited higher heart mitochondrial membrane potentials and contractility indices than mice in a placebo group ⁷³⁾. An ex vivo study of 24 adult humans showed that astaxanthin has the potential to prevent atherosclerosis by delaying the prolonged oxidation of low-density lipoprotein (LDL)-cholesterol. In this study, volunteers consumed astaxanthin at doses of 1.8, 3.6, 14.4, or 21.6 mg/day for 14 days and the LDL lag times were longer (5.0%, 26.2%, 42.3%, and 30.7%, respectively) compared with the initial day ⁷⁴⁾. Experiments by Miyawaki et al. ⁷⁵⁾ to determine the health benefits of astaxanthin extracted from Haematococcus pluvialis on human blood rheology were carried out on 20 adult men. After 10 days of astaxanthin (6 mg/day) administration, the whole blood transit time of the experimental group decreased from 52.8 ± 4.9 s to 47.6 ± 4.2 s, which is considerably lower than that of the control group (54.2 ± 6.7 s) ⁷⁶⁾. In addition, another study carried out on humans in an age group of 25–60 years [79] showed that 12 weeks of astaxanthin administration significantly decreased serum triglyceride levels, while significantly increasing high density lipoprotein (HDL)-cholesterol levels. However, LDL-cholesterol levels remained unchanged. Furthermore, astaxanthin intake increased serum adiponectin levels, which are positively correlated with changes in HDL-cholesterol levels independent of age and body mass index (BMI) ⁷⁷⁾. Fucoxanthin and its derivative fucoxanthinol show cardioprotective activity; administration of these carotenoids in an in vivo study reduced triglyceride levels in blood (high triglyceride levels in blood are related to the development of atherosclerotic vascular disease) ⁷⁸⁾. When rats were fed with 2 mg/kg of fucoxanthin or fucoxanthinol, they showed a significant reduction in triglyceride absorption in their jugular veins on being fed with non-pre-digested 10% soybean oil.

Anti-cancer activity

Numerous in vitro and in vivo studies have demonstrated the anti-cancer activities of carotenoids. The results of these studies indicate that carotenoids may prevent different types of cancers in humans, including bladder, breast, hepatic, intestinal, leukemic, lung, oral, and prostate cancer. The anti-cancer activity of carotenoids involves a variety of mechanisms, including induction of cell apoptosis and suppression of cell proliferation. In particular, one in vivo study showed that β-carotene, astaxanthin, canthaxanthin, and zeaxanthin help in reducing the sizes and numbers of liver neoplasias ⁷⁹⁾. Another study also reports that dietary intake of carotenoids can reduce the risk of developing colon cancer ⁸⁰⁾.

Many studies indicate that β-carotene shows great potential as an anti-tumor agent. In a study in China, administration of a combination of β-carotene, vitamin E, and selenium to humans was observed to decrease the incidence of mortality due to cancer ⁸¹⁾. Many other studies have also reported an inverse relationship between ingesting carotenoids and cancer prevalence ⁸²⁾. Lycopene is one of the best studied carotenoids with respect to its potential health benefits ⁸³⁾; this is because it exhibits much higher anti-cancer potential than most other carotenoids ⁸⁴⁾. Several in vivo and in vitro studies using tumor cell lines indicate that lycopene can significantly reduce tumor cell growth ⁸⁵⁾. Nishino et al. ⁸⁶⁾ have reported that the carotenoids α-carotene, lutein, zeaxanthin, lycopene, β-cryptoxanthin, fucoxanthin, astaxanthin, capsanthin, crocetin, and phytoene exhibit greater anti-carcinogenic activity than β-carotene.

The anti-proliferative and cancer-preventive activities of fucoxanthin and fucoxanthinol are dependent on different molecules and pathways involved in the processes of cell cycle arrest, apoptosis, and metastasis ⁸⁷⁾. Furthermore, studies using human umbilical vein endothelial cells (HUVECs) have shown that fucoxanthin also has anti-angiogenic activity, which is helpful in preventing cancer. Fucoxanthin can potentially inhibit the proliferation of cancer cells by increasing intercellular communication through gap junctions in human cancer cells, which increases intracellular signaling mechanisms that promote cell cycle arrest and apoptosis. Therefore, fucoxanthin and its metabolites show great potential as chemotherapy agents if administered in the initial stages of cancer ⁸⁸⁾. In addition, fucoxanthin also lowers the viabilities of human leukemia (HL-60) cells. Fucoxanthin also shows anti-cancer activity against Caco-2, DLD-1, and HT-29, which are human colorectal adenocarcinoma cell lines. Although fucoxanthin treatment has been shown to reduce cell viability, the strength of the effect varies across cell types. After 72 h of fucoxanthin treatment (at a concentration of 15.2 mM), the viabilities of Caco-2, DLD-1, and HT-29 cells decreased to 14.8%, 29.4%, and 50.8%, respectively [89]. These remarkable reductions in cell viability levels were caused by a significant increase in cell apoptosis and DNA fragmentation ⁸⁹⁾. Kim et al. ⁹⁰⁾ reported that astaxanthin, β-carotene, and fucoxanthin show potent anti-cancer activities when tested on HL-60 cancer cells at a concentration of 7.6 mM. At this concentration, fucoxanthin caused high levels of DNA fragmentation, whereas the other two carotenoids (astaxanthin and β-carotene) did not show any significant effects on DNA fragmentation. Kim et al. ⁹¹⁾ stated that the mechanism of fucoxanthin-induced apoptosis in HL-60 cells involves the generation of ROS, which leads to cytotoxicity and apoptosis involving the cleavage of caspases-3 and -9 and poly-ADP-ribose polymerase (PARP), coupled with reductions in levels of Bcl-x. Kotake-Nara et al. ⁹²⁾ investigated the effects of fucoxanthin (at concentrations of 5 and 10 mM) on the viabilities of six types of cancer cells. Incubation with fucoxanthin for 72 h showed that five of the cancer cell lines suffered significant reductions in cell viability. In addition, comparisons of the effects of fucoxanthin and lycopene on cancer cells indicate that at the same concentrations, fucoxanthin shows higher anti-cancer effects than lycopene. Fucoxanthin is a potential chemopreventive agent for urinary bladder cancers, as it inhibits the growth and causes apoptosis in EJ-1 cells (a urinary bladder cancer cell line). Treatment with fucoxanthin significantly reduced EJ-1 cell proliferation in a dose- and time-dependent manner. Treatment with 20 mM fucoxanthin for 72 h caused a high percentage of cells to undergo apoptosis (93%), which was evident by morphological changes, DNA fragmentation, increased percentages of hypodiploid cells, and caspase-3 activity ⁹³⁾.

Anti-diabetic activity

Recent work on carotenoids suggests that these molecules may be more effective in treating and controlling diabetes than antioxidants. Studies have shown that levels of dietary carotenoids and concentrations of β-carotene in blood are inversely associated with fasting blood glucose levels and insulin resistance, respectively ⁹⁴⁾. Numerous studies have reported that carotenoids reduce the risk of type 2 diabetes mellitus development in men and women ⁹⁵⁾. It has also been observed that carotenoid intake is inversely related to HbA1c levels ⁹⁶⁾. In addition, recent findings have confirmed that carotenoids such as lycopene, lutein, and zeaxanthin can protect against diabetic retinopathy ⁹⁷⁾.

Most studies on carotenoids and diabetes report the importance of carotenoids in dietary intake for the prevention and treatment of type 2 diabetes mellitus ⁹⁸⁾. A recent study by Sugiura et al. ⁹⁹⁾ shows that in middle-aged and older Japanese patients, serum levels of α-carotene and β-cryptoxanthin are associated with lower incidences of type 2 diabetes mellitus. In addition, another study that investigated the interactions between serum concentrations of carotenoids and smoking with the incidence of diabetes mellitus over a time span of 15 years ¹⁰⁰⁾ showed that the incidence of type 2 diabetes mellitus is inversely associated with serum concentrations of carotenoids in nonsmokers. A similar result was also obtained by Ylonen et al. ¹⁰¹⁾, who showed that serum concentrations of lutein, zeaxanthin, lycopene, α-carotene, and β-carotene were significantly lower in diabetic subjects. Most of these studies also report that there is an association between carotenoid intake and reductions in the risk of developing type 2 diabetes mellitus ¹⁰²⁾.

Astaxanthin, which is one of the best studied carotenoids, shows great potential in preventing and treating diabetes. Astaxanthin has higher antioxidant activity than other carotenoids such as lutein, β-carotene, and zeaxanthin ¹⁰³⁾, and can be consumed safely by humans ¹⁰⁴⁾. In db/db mice (a well-known obesity model for type 2 diabetes mellitus), treatment with astaxanthin decreases glucose tolerance, enhances serum insulin levels, and attenuates blood glucose levels. These results indicate that astaxanthin has protective antioxidant effects that can help in the preservation of pancreatic β-cell function ¹⁰⁵⁾. Bhuvaneswari et al. ¹⁰⁶⁾ have also reported similar anti-diabetic effects in high-fat, high-fructose diet HFFD mice. The effect of astaxanthin on metabolic syndrome has also been investigated in a rat experimental model. Astaxanthin was found to decrease blood glucose and triglyceride levels, as well as enhance serum levels of HDL-cholesterol and adiponectin ¹⁰⁷⁾. Interestingly, recent studies have reported that astaxanthin primarily targets the peroxisome proliferator-activated receptor (PPARγ), which plays a pivotal role in carbohydrate metabolism. These studies also report that astaxanthin not only binds to PPARγ, but the carotenoid also affects the mRNA levels of this protein ¹⁰⁸⁾. These results are consistent with another study that reports the anti-hyperglycemic effects of astaxanthin ¹⁰⁹⁾.

Another important carotenoid, β-carotene, has been investigated in detail regarding its usefulness in the treatment of diabetes. Hozumi et al. ¹¹⁰⁾ reported a significantly inverse correlation between serum concentrations of β-carotene and serum levels of HbA1c in diabetic patients. Arnlov et al. ¹¹¹⁾ also reported that impaired insulin sensitivity is linked to low serum concentrations of β-carotene. In a study conducted by the European Prospective Investigation into Cancer and Nutrition–Netherlands (EPIC), investigations on 37,846 men and women revealed an inverse association between dietary intake of β-carotene and the risk of type 2 diabetes mellitus development ¹¹²⁾, a result similar to that obtained by Coyne et al. ¹¹³⁾ in a population-based study in Queensland, Australia. Furthermore, serum levels of β-carotene are reported to be important determinants of metabolic syndrome outcome ¹¹⁴⁾. Although β-carotene is well-studied with respect to its usefulness in preventing or treating diabetes, other carotenoids such as lutein have not been well investigated. Katyal et al. ¹¹⁵⁾ found that lutein can lower streptozotocin (STZ)-induced hyperglycemia and shows significant antioxidant effects in the kidneys of diabetic rats.

Anti-obesity activity

Obesity is a condition where excessive accumulation and storage of fat in the body occurs, leading to inordinate increases in body weight ¹¹⁶⁾. Obesity leads to, and exacerbates several conditions, particularly those related to cardiovascular diseases, type 2 diabetes mellitus, obstructive sleep apnea, certain types of cancer, osteoarthritis, and depression ¹¹⁷⁾. Since the first half of this century, obesity has been one of the foremost issues of concern regarding public health. In developing countries, increased industrialization has increased the incidence of obesity in teenagers and senior citizens, causing a worrying health trend ¹¹⁸⁾. Therefore, the search for safe anti-obesity agents is now of great importance.

Wang et al. ¹¹⁹⁾ reported that in obese individuals, there is an excessive accumulation of adipose tissue in organs that have large numbers of fat cells. Obesity is thought to result from adipocyte hypertrophy and the recruitment of new adipocytes from precursor cells. For this reason, the regulation of adipogenesis may be a potential strategy for the treatment of obesity. Okada et al. ¹²⁰⁾ reported that the chemical structures of carotenoids are important for suppression of adipocyte differentiation; investigations on 13 naturally occurring carotenoids have revealed that molecules with keto or epoxy groups, as well as epoxy-hydroxy carotenoids, hydroxyl-carotenoid, and keto-hydroxy carotenoid have no suppressive effects on adipocyte differentiation. The study found that only fucoxanthin and neoxanthin could significantly suppress adipocyte differentiation, suggesting that the presence of the allenic bond is an important factor for carotenoids to exhibit anti-obesity functions. From these results, it could be hypothesized that carotenoids containing an allenic group and an additional hydroxyl group may be effective in controlling adipocyte differentiation.

Maeda et al. ¹²¹⁾ used a mouse model to show that oral intake of fucoxanthin could significantly decrease the amount of abdominal white adipose tissue in obese mice. In addition, the study also found that this treatment had no such effects on normal mice kept on normal diets. This indicates that fucoxanthin specifically suppresses adiposity in obese mice. This study suggests that the anti-obesity effect of fucoxanthin is mediated by alterations in the functioning of lipid-regulating enzymes that could raise plasma adipokine levels and promote higher expression levels of uncoupling protein 1 (UCP1) and β3-adrenergic receptor (Adrb3) in abdominal fat tissues. UCP1, which is abundant in the inner membrane of the mitochondria, is specifically expressed at high levels in brown adipocytes. UCP1 can dissipate energy by uncoupling the process of oxidative phosphorylation, which then produces heat instead of ATP. It is well-known that brown adipose tissue (BAT) plays a vital role in the prevention and treatment of obesity ¹²²⁾. The role of UCP1 in BAT is known to be a significant component of the regulatory system governing whole-body energy expenditure, and the protein is thought to be important in preventing the development of obesity ¹²³⁾. Increasing UCP1 expression in BAT could be considered as a useful anti-obesity treatment option ¹²⁴⁾. However, in humans, most of the body fat is stored in white adipose tissue ¹²⁵⁾. Furthermore, white adipose tissue has now been recognized to function as an endocrine and active secretory organ as its produces biologically active mediators known as adipokines ¹²⁶⁾. Fucoxanthin is likely to emerge as an important and attractive anti-obesity agent ¹²⁷⁾. However, further studies are needed to clarify the various molecular mechanisms and intracellular signaling pathways that are involved in the anti-obesity activities of fucoxanthin. These studies indicate that natural pigments may play a vital role in the treatment and prevention of obesity, as these molecules may act as regulators of lipid metabolism in fat tissues. Natural pigments obtained from microalgae can be used in functional foods and pharmaceuticals, as these substances can be obtained at relatively low production costs, exhibit low cytotoxicity, and have gained wide acceptance as food supplements. Among the different types of carotenoids, fucoxanthin derived from marine algae may be considered a promising food supplement and weight-loss drug for the prevention and management of obesity.

Beauty-enhancing effect

Skin has naturally occurring antioxidant agents which can block the effects of ROS and suppress cell disruption and damage ¹²⁸⁾. However, when high levels of ROS are produced by ultraviolet (UV)-exposure, these defenses may not provide adequate protection. Apoptosis and necrosis are the two major modes of cell death that occur due to the accumulation of ROS in cells; excessive cell death can lead to wrinkling and dryness of skin. ROS accumulation also plays an important role in photo-aging conditions such as cutaneous inflammation, melanoma, and skin cancer ¹²⁹⁾. Natural pigments can be used as therapeutic agents to overcome these problems. As many consumers prefer naturally derived compounds in their cosmetics, there is an increasing global demand for naturally derived carotenoids rather than those synthetized chemically. Due to this demand, the price of natural pigments isolated from algae is roughly double (~700 Euros/kg) of that of synthetic products ¹³⁰⁾.

Astaxanthin is an excellent antioxidant, exhibiting higher antioxidant activity than vitamins C and E; furthermore, this molecule aids in the preservation of proteins and essential lipids in human lymphocytes as it boosts superoxide dismutase and catalase enzyme activities ¹³¹⁾. Tominaga et al. ¹³²⁾ reported that both topical and oral use of astaxanthin can suppress skin hyper-pigmentation, inhibit synthesis of melanin, and improve the condition of all skin layers. Fucoxanthin has been reported to suppress tyrosinase activity in UVB-irradiated guinea pigs, and melanogenesis in UVB-irradiated mice. Studies have also found that oral administration of fucoxanthin decreases the mRNA levels of proteins linked to melanogenesis in skin cells. This indicates that fucoxanthin can negatively regulate melanogenesis factors at the transcriptional level ¹³³⁾. In addition, fucoxanthin has the ability to counteract oxidative stresses caused by UV radiation, due to which it is currently used in cosmeceuticals ¹³⁴⁾. Another important carotenoid exhibiting strong antioxidant activity is β-carotene, which helps in preventing the formation of free radicals that can cause premature aging in skin cells. In the epidermal and dermal layers of skin, the carotenoid lutein has been shown to protect against UV-induced oxidative damage, especially in combination with other antioxidant systems and immunoprotective substances ¹³⁵⁾.

A study conducted by Darvin et al. ¹³⁶⁾, which compared skin roughness with age in a cohort of women aged 40–50 years, indicated that there was no significant correlation between the two parameters. However, skin roughness was clearly correlated with the concentration of lycopene present in skin. Individuals with higher levels of antioxidants in their skin showed fewer furrows and wrinkles than those with lower levels of antioxidants ¹³⁷⁾. UV radiation from the sun is one of the major causes of premature skin aging. UV radiation from sun rays destroys elastin and collagen fibers in the skin ¹³⁸⁾. High concentrations of antioxidants such as carotenoids can efficiently neutralize free radicals before they can cause damage. These studies confirm the results of a study conducted by Heinrich et al. ¹³⁹⁾, which showed that a significant reduction in skin roughness could be achieved with supplements of antioxidant micronutrients such as lycopene.