α-Carotene, β-Carotene, β-Cryptoxanthin, Lycopene, Lutein, and Zeaxanthin
- Carotenoids are yellow, orange, and red pigments synthesized by plants. The most common carotenoids in North American diets are α-carotene, β-carotene, β-cryptoxanthin, lutein, zeaxanthin, and lycopene. (More information)
- Provitamin A carotenoids, α-carotene, β-carotene, and β-cryptoxanthin, can be converted by the body to retinol (vitamin A). In contrast, no vitamin A activity can be derived from lutein, zeaxanthin, and lycopene. (More information)
- Dietary lutein and zeaxanthin are selectively taken up into the macula of the eye, where they absorb up to 90% of blue light and help maintain optimal visual function. (More information)
- At present, it is unclear whether the biological effects of carotenoids in humans are related to their antioxidant activity and/or other non-antioxidant activities. (More information)
- Although the results of observational studies suggest that diets high in carotenoid-rich fruit and vegetables are associated with reduced risks of cardiovascular disease and some cancers, high-dose β-carotene supplements did not reduce the risk of cardiovascular disease or cancer in large randomized controlled trials. (More information)
- Two randomized controlled trials found that high-dose β-carotene supplements increased the risk of lung cancer in smokers and former asbestos workers. (More information)
- Recent meta-analyses of observational studies reported an inverse association between blood lycopene concentration and risk of developing prostate cancer. To date, most small-scale intervention studies found little-to-no benefit of lycopene supplements in reducing incidence or severity of prostate cancer in high-risk patients. (More information)
- Observational studies have suggested that diets rich in lutein and zeaxanthin may help slow the development of age-related macular degeneration (AMD). Randomized controlled trials found that lutein and zeaxanthin supplements could improve visual acuity and slow the progression to advanced AMD in subjects with AMD. (More information)
- Evidence is lacking to suggest a role for lutein and zeaxanthin in the management of other eye conditions, including cataracts, diabetic retinopathy, and retinopathy of maturity. (More information)
- Carotenoids are best absorbed with fat in a meal. Chopping, puréeing, and cooking carotenoid-containing vegetables in oil generally increase the bioavailability of the carotenoids they contain. (More information)
Carotenoids are a class of more than 750 naturally occurring pigments synthesized by plants, algae, and photosynthetic bacteria (1). 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 carotenoids (1). α-Carotene, β-carotene and β-cryptoxanthin are provitamin A carotenoids, meaning they can be converted by the body to retinol (Figure 1). Lutein, zeaxanthin, and lycopene are nonprovitamin A carotenoids because they cannot be converted to retinol (Figure 2).
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 (1). 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 (2, 3), 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 (reviewed in 4). Because they do not need to be released from the plant matrix, carotenoid supplements (in oil) are more efficiently absorbed than carotenoids in food (3). 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 (5).
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) (Figure 3). 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 (1).
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 (1). 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 (6). 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 (1).
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 (7). 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 (7).
Provitamin A function
Vitamin A is essential for normal growth and development, immune system function, and vision (see the article on Vitamin A). Currently, the only essential function of carotenoids recognized in humans is that of the provitamin A carotenoids, α-carotene, β-carotene, and β-cryptoxanthin, to serve as a source of vitamin A (8).
Retinol activity equivalents (RAEs)
Provitamin A carotenoids are less easily absorbed than preformed vitamin A and must be converted to retinol and other retinoids by the body (Figure 3). The efficiency of conversion of provitamin A carotenoids into retinol is highly variable, depending on factors like food matrix, food preparation, and one’s digestive and absorptive capacities (9).
The most recent international standard of measure for vitamin A is retinol activity equivalent (RAE), which represents vitamin A activity as retinol. It has been determined that 2 micrograms (µg) of β-carotene in oil provided as a supplement could be converted by the body to 1 µg of retinol, giving it an RAE ratio of 2:1. However, 12 µg of β-carotene from food are required to provide the body with 1 µg of retinol, giving dietary β-carotene an RAE ratio of 12:1. Other provitamin A carotenoids in food are less easily absorbed than β-carotene, resulting in RAE ratios of 24:1. RAE ratios are shown in Table 1.
Table 1. Retinol Activity Equivalent (RAE) Ratios for Preformed Vitamin A and Provitamin A Carotenoids
||Quantity Bioconverted to Retinol
|1 µg of dietary or supplemental preformed vitamin A
||1 µg of retinol
|2 µg of supplemental β-carotene
||1 µg of retinol
|12 µg of dietary β-carotene
||1 µg of retinol
|24 µg of dietary α-carotene
||1 µg of retinol
|24 µg of dietary β-cryptoxanthin
||1 µg of retinol
In plants, carotenoids have the important antioxidant function of quenching (deactivating) singlet oxygen, an oxidant formed during photosynthesis (10). Test tube studies indicated that lycopene is one of the most effective quenchers of singlet oxygen among carotenoids (11). They also suggested that carotenoids could inhibit the oxidation of fats (i.e., lipid peroxidation) under certain conditions, but their actions in humans appear to be more complex (12). Although important for plants, the relevance of singlet oxygen quenching to human health is less clear (1).
Nrf2-dependent antioxidant pathway
Some evidence suggests that carotenoids and/or their metabolites may upregulate the expression of antioxidant and detoxifying enzymes via the activation of the nuclear factor E2-related factor 2 (Nrf2)-dependent pathway (reviewed in 13). Briefly, Nrf2 is a transcription factor that is bound to the protein Kelch-like ECH-associated protein 1 (Keap1) in the cytosol. Keap1 responds to oxidative stress signals by freeing Nrf2. Upon release, Nrf2 translocates to the nucleus and binds to the antioxidant response element (ARE) located in the promoter of genes coding for antioxidant/detoxifying enzymes and scavengers. Nrf2/ARE-dependent genes code for numerous mediators of the antioxidant response, including glutamate-cysteine ligase (GCL), glutathione S-transferases (GSTs), thioredoxin, NAD(P)H quinone oxidoreductase 1 (NQO-1), and heme oxygenase 1 (HO-1) (14). A recent study showed an increase in the level of the major antioxidant glutathione and a protection against TNFα-induced oxidative stress in retinal pigment epithelial cells (RPE) following lycopene-mediated Nrf2 activation and GCL induction (15). Nrf2 activation by lycopene also protected RPE against TNFα-mediated proinflammatory signaling involving nuclear factor-κB (NF-κB) activation and intercellular adhesion molecule-1 (ICAM-1) expression (15). Lycopene was shown to trigger Nrf2-mediated antioxidant pathway in various cell types (16-18). At present, evidence from animal and human studies is very limited (13).
Blue light filtering
The long system of alternating double and single bonds common to all carotenoids allows them to absorb light in the visible range of the spectrum (10). This feature has particular relevance to the eye, where lutein, zeaxanthin, and meso-zeaxanthin (derived from lutein) efficiently absorb blue light. Depending on the carotenoid pigment density at the center of the eye’s retina (macula), up to 90% of blue light can be absorbed by these pigments. Reducing the amount of short-wavelength light that reaches the critical visual structures of the eye may protect them from light-induced oxidative damage (19). Because the only source of these plant pigments in the eye is diet, a number of observational and intervention studies have examined the potential of dietary and supplemental lutein and zeaxanthin to protect against age-related eye diseases (see Age-related macular degeneration and Cataracts). Supplemental lutein, alone or with zeaxanthin, was found to improve contrast sensitivity and protect against visual fatigue in young and/or healthy individuals (20-23). Lutein has also been suggested to improve visual function through stimulating neuronal signaling efficiency in the eye (24).
Carotenoids can facilitate communication between neighboring cells grown in culture by stimulating the synthesis of connexin proteins (25). Connexins form pores (gap junctions) in cell membranes, allowing cells to communicate through the exchange of small molecules. This type of intercellular communication is important for maintaining cells in a differentiated state and is often lost in cancer cells. Carotenoids facilitate intercellular communication by increasing the expression of the gene encoding a connexin protein, an effect that appears unrelated to the vitamin A or antioxidant activities of various carotenoids (26) and involving a retinoic acid receptor (RAR)-independent mechanism (27).
Because vitamin A is essential for normal immune system function, it is difficult to determine whether the effects of provitamin A carotenoids are related to their vitamin A activity or other activities of carotenoids. Although some clinical trials have found that β-carotene supplementation improves several biomarkers of immune function (28-30), increasing intakes of lycopene and lutein — carotenoids without vitamin A activity — have not resulted in similar improvements in biomarkers of immune function (31-33).
Although consumption of provitamin A carotenoids (α-carotene, β-carotene, and β-cryptoxanthin) can prevent vitamin A deficiency (see the article on Vitamin A), no overt deficiency symptoms have been identified in people consuming low-carotenoid diets if they consume adequate vitamin A (8). After reviewing the published scientific research in 2000, the Food and Nutrition Board of the Institute of Medicine concluded that the existing evidence was insufficient to establish a recommended dietary allowance (RDA) or adequate intake (AI) for carotenoids. The Board has set an RDA for vitamin A (see the article on Vitamin A). Recommendations by the National Cancer Institute, American Cancer Society, and American Heart Association to consume a variety of fruit and vegetables daily are aimed, in part, at increasing intakes of carotenoids.
Dietary carotenoids: Several large prospective cohort studies, including the Nurses’ Health Study (NHS) and the Health Professionals Follow-up Study (HPFS), have examined potential associations between carotenoid intake and/or blood concentrations and lung cancer (36). In a meta-analysis of eight prospective cohort studies, including NHS and HPFS, the highest versus lowest quantile of total carotenoid intake was significantly associated with a 21% reduced risk of lung cancer. For the individual carotenoids, the risk of lung cancer was estimated to be 20% and 14% lower with the highest versus lowest intakes of β-cryptoxanthin and lycopene, respectively. In contrast, dietary intakes of β-carotene, α-carotene, and lutein/zeaxanthin were not found to be significantly linked to a reduced risk of developing lung cancer (36). In addition, an analysis of the pooled results of 11 nested case-control and four prospective cohort studies found no association between serum concentrations of total carotenoids, β-carotene, α-lycopene, β-cryptoxanthin, and lutein/zeaxanthin and lung cancer. Only high versus low serum lycopene concentrations could be linked to a 29% lower risk of lung cancer (36). Any protective effect of dietary carotenoids against the development of lung cancer is likely small and not statistically significant (36).
Supplemental β-carotene: The effect of β-carotene supplementation on the risk of developing lung cancer has been examined in three large randomized, placebo-controlled trials. In Finland, the Α-Tocopherol Βeta-Carotene (ATBC) cancer prevention trial evaluated the effects of 20 mg/day of β-carotene and/or 50 mg/day of α-tocopherol on more than 29,000 male smokers (37), and in the United States, the β-Carotene And Retinol Efficacy Trial (CARET) evaluated the effects of a combination of 30 mg/day of β-carotene and 25,000 IU/day of retinol (preformed vitamin A) in 18,314 men and women who were smokers, former smokers, or had a history of occupational asbestos exposure (38). Unexpectedly, the risk of lung cancer in the groups taking β-carotene supplements was increased by 16% after six years in the ATBC participants and by 28% after four years in the CARET participants. In contrast, the Physicians’ Health Study (PHS) examined the effect of β-carotene supplementation (50 mg every other day) on cancer risk in 22,071 male physicians in the United States, of whom only 11% were current smokers (39). In that lower risk population, β-carotene supplementation for more than 12 years was not associated with an increased risk of lung cancer. In addition, in the Linxian General Population trial conducted in about 29,000 Chinese participants, randomization to 15 mg/day of β-carotene, 30 mg/day of α-tocopherol, and 15 µg/day of selenium, was not found to be associated with lung cancer mortality 10 years after the intervention ended (40).
Although the reasons for the increase in lung cancer risk are not yet clear and several mechanisms have been proposed (41). The US Preventive Services Task Force estimated that the risks of high-dose β-carotene supplementation outweigh any potential benefits for cancer prevention and recommended against supplementation, especially in smokers or other high-risk populations (42).
Prostate cancer is the most common cancer affecting US men, more prevalent than lung and colorectal cancers (34).
Dietary lycopene: Several early prospective cohort studies have suggested that lycopene-rich diets were associated with significant reductions in the risk of prostate cancer, particularly more aggressive forms (43). However, pooled data analyses of observational studies that examined potential links between dietary intakes and/or circulating concentrations of lycopene and risk of prostate cancer have given mixed results. An early meta-analysis that combined the results of 10 case-control and four prospective studies found that men with the highest intakes of raw tomatoes, cooked tomatoes, or dietary lycopene had a 11 to 19% lower risk of prostate cancer (44). In addition, pooled data from two case-control and five nested case-control studies showed a 26% lower risk of prostate cancer in participants with the highest serum concentrations of lycopene (44). Most recently, a meta-analysis of observational studies found no association of prostate cancer risk with dietary lycopene intakes (10 case-cohort and two prospective cohort studies) but an inverse association with blood lycopene concentrations (two case-control, nine nested case-control, and one cohort studies) (45, 46). Additionally, a meta-analysis of 15 nested case-control studies conducted by the Endogenous Hormones, Nutritional Biomarkers, and Prostate Cancer Collaborative Group showed an inverse association between circulating lycopene concentrations and risk of advanced stage and/or aggressive prostate cancer, while no association was found with risk of non-aggressive or localized disease (47).
While there is considerable scientific interest in the potential for lycopene to help prevent prostate cancer, it is not yet clear whether the prostate cancer risk reduction observed in some observational studies is related to lycopene itself, other compounds in tomatoes, or other factors associated with lycopene-rich diets (48). Of note, the 2014 World Cancer Research Fund International report on Diet, Nutrition, Physical Activity, and Prostate Cancer suggested the need for better designed studies to establish whether consumption of lycopene-containing foods could be linked to a lower risk of prostate cancer (49).
Supplemental lycopene: To date, a few short-term, dietary intervention studies using lycopene in patients with precancerous prostate lesions (high grade prostatic intraepithelial neoplasia; HGPIN) or prostate cancer have been completed. Specifically, two small randomized controlled studies have recently examined the effect of lycopene supplementation for up to six months in men with HGPIN (50, 51). The consumption of 30 to 35 mg/day of supplemental lycopene in the form of tomato extract (50), or together with selenium (55 mg/day) and green tea catechins (600 mg/day) (51), showed no benefit on the rate of progression to prostate cancer at six-month (50, 51) and 37-month follow-ups (51). Earlier small trials in men with HGPIN led to similar conclusions (reviewed in 52). Additionally, a randomized controlled trial in men with localized prostate cancer found that supplementation with 15, 30, or 45 mg of lycopene (until prostatectomy) did not significantly increase plasma lycopene concentration, modify the ratio of steroid hormones in blood, or reduce the concentration of markers of proliferation (i.e., prostate-specific antigen [PSA] and Ki-67) compared to placebo (53). In another trial, 54 patients with metastatic prostate cancer were randomized to orchidectomy alone or orchidectomy plus 4 mg/day of lycopene (54). The proportion of complete clinical response to treatment — assessed by serum PSA and/or bone scan returning to normal — and patient survival rate were found to be significantly higher in patients supplemented with lycopene (54).
Large-scale, controlled clinical trials are needed to further examine the safety and efficacy of long-term use of lycopene supplements for prostate cancer prevention or treatment.
Other types of cancer
In a meta-analysis of 12 prospective cohort studies, no association was found between total and individual carotenoid intake and risk of breast cancer, except with β-carotene for which a 5% reduction in breast cancer risk was estimated for every 5 mg/day increment in consumption (55). In a pooled analysis of 14 nested case-control studies and one follow-up study of a clinical trial, reductions in breast cancer risk were found to be associated with blood concentrations of total carotenoids (-26%), α-carotene (-20%), and lutein (-30%) (55). Another study that recalibrated data for consistency across eight large prospective cohorts before pooled analysis found reduced breast cancer risk to be associated with the highest versus lowest quintile of blood concentrations of total carotenoids (-21%), β-carotene (-17%), and lycopene (-22%) (56). Further analyses found an inverse association between the blood concentrations of β-carotene and α-carotene and risk of estrogen receptor-negative (ER-), but not estrogen receptor-positive (ER+), breast tumors (56). A similar result was recently reported in a case-control study nested within the multicenter, large, European Prospective Investigation into Cancer and Nutrition (EPIC) study (57).
Another case-control study nested within the EPIC study found a 31% lower risk of colorectal cancer with the highest versus lowest quartile of β-carotene intake, while no associations were shown with blood concentrations of other carotenoids or total carotenoids (58). The nested case-control study was included in a recent meta-analysis of 22 observational studies that failed to find associations between carotenoid intakes and colorectal cancer (59).
Pooled data from mostly case-control studies also suggested that higher intakes of individual carotenoids, especially β-cryptoxanthin and lycopene, might be associated with a reduced risk of cancers of the mouth, pharynx, and larynx (60). However, the results of case-control studies are more likely to be distorted by bias than results of prospective studies.
Examining blood concentrations of each carotenoid in relation to cancer subsites may help overcome limitations associated with dietary data and differences in carotenoid absorption.
Age-related macular degeneration
In Western countries, degeneration of the macula, the center of the eye’s retina, is a leading cause of blindness in older adults. Long-term blue light exposure and oxidative damage in the outer segments of photoreceptors may lead to drusen and/or pigment abnormalities in the macula, increasing the risk of age-related macular degeneration (AMD) and central blindness.
Dietary lutein and zeaxanthin: The carotenoids found in the retina are lutein and zeaxanthin, which are both of dietary origin, and meso-zeaxanthin, which is derived from lutein. These three carotenoids are present in high concentrations in the macula (known as macular pigment), where they are efficient absorbers of blue light. They may prevent a substantial amount of the blue light entering the eye from reaching the underlying structures involved in vision and protect against light-induced oxidative damage, which is thought to play a role in the pathology of age-related macular degeneration (reviewed in 19). It is also possible, though not proven, that lutein, zeaxanthin, and meso-zeaxanthin, act directly to neutralize oxidants formed in the retina.
Increasing dietary consumption of lutein and zeaxanthin was shown to raise their serum concentration and macular pigment density (61, 62). Some, but not all, observational studies have provided evidence that higher intakes of lutein and zeaxanthin are associated with lower risk of age-related macular degeneration (AMD) (63). While cross-sectional and retrospective case-control studies found that higher levels of lutein and zeaxanthin in the diet (64-66), blood (67, 68), and retina (69, 70) were associated with a lower incidence of AMD, several prospective cohort studies found no relationship between baseline dietary intakes or serum concentrations of lutein and zeaxanthin and the risk of developing AMD over time (71-74). A recent report examined the association between the incidence of AMD and calculated dietary intakes and predicted plasma concentrations of lutein and zeaxanthin in older adults (≥50 years) from two large prospective cohorts, the Nurses’ Health Study (NHS; 63,443 women) and the Health Professionals Follow-up Study (HPFS; 38,603 men), followed for 26 years and 24 years, respectively (75). The highest versus lowest quintile of predicted plasma lutein and zeaxanthin scores was associated with a 41% lower risk of advanced AMD, yet no association was found with intermediate AMD. The evidence also suggested that the consumption of about 6 mg/day of lutein and zeaxanthin from fruit and vegetables (compared with less than 2 mg/day) may decrease the risk of advanced AMD (75).
Supplemental lutein and zeaxanthin: The first randomized controlled trial in patients with atrophic AMD found that supplementation with 10 mg/day of lutein slightly improved visual acuity after one year compared to a placebo (76). The effects of long-term lutein supplementation on atrophic AMD were further investigated in combination with antioxidant vitamins and minerals in the Age-Related Eye Disease Study 2 (AREDS2), a multicenter, randomized, double-blind, placebo-controlled trial. In AREDS, oral supplementation with β-carotene (15 mg/day), vitamin C (500 mg/day), vitamin E (400 IU/day), zinc (80 mg/day as zinc oxide), and copper (2 mg/day as cupric oxide) for five years was shown to reduce the risk of developing advanced AMD by 25% (77). In the AREDS2 study conducted in 4,203 participants at risk for developing late AMD, supplemental lutein (10 mg/day) and zeaxanthin (2 mg/day), in combination with β-carotene, vitamin C, vitamin E, zinc, and copper (the ‘AREDS’ formulation), did not slow the progression to advanced AMD, although subgroup analyses revealed a benefit in those with the lowest dietary intakes of lutein + zeaxanthin (77). A total of 3,036 subjects were further randomized to various combinations of carotenoids; supplementation with lutein and zeaxanthin significantly reduced the risk of progression to late AMD and to neovascular AMD compared to supplementation with β-carotene (78).
Several smaller (n=30-433) randomized controlled trials also suggested that supplementation with xanthophyll carotenoids would be beneficial in the management of AMD (reviewed in 79). A meta-analysis of eight trials that examined the effect of supplemental lutein (6 to 20 mg/day) or/and zeaxanthin (0 to 10 mg/day) in 1,176 AMD subjects for up to 36 months found improvements in visual acuity and contrast sensitivity with increased levels of xanthophyll carotenoids (80). Finally, a recent randomized, placebo-controlled trial found daily consumption of a buttermilk drink with egg yolks enriched with lutein (1.4 mg), zeaxanthin (0.2 mg), and an omega-3 polyunsaturated fatty acid (DHA; 160 mg) for one year improved visual acuity and macular pigment optical density in subjects with drusen and/or retinal pigment abnormalities (about half of them being classified as having early AMD) (81).
Supplemental β-carotene: The first randomized controlled trial designed to examine the effect of a carotenoid supplement on AMD used β-carotene in combination with vitamin C, vitamin E, and zinc because lutein and zeaxanthin were not commercially available as supplements at the time the trial began (82). Although the combination of antioxidants and zinc lowered the risk of developing advanced macular degeneration in individuals with signs of moderate-to-severe macular degeneration in at least one eye, it is unlikely that the benefit was related to β-carotene since it is not present in the retina. Supplementation of male smokers in Finland with 20 mg/day of β-carotene for six years did not decrease the risk of AMD compared to placebo (83). A placebo-controlled trial in a cohort of 22,071 healthy US men found that β-carotene supplementation (50 mg every other day) had no effect on the incidence of age-related maculopathy — an early stage of AMD (84). Recent systematic reviews of randomized controlled trials have concluded that there is no evidence that β-carotene supplementation prevents or delays the onset of AMD (85, 86).
Retinopathy of prematurity
Preterm infants have an immature retinal vascular system that places them at risk of developing retinopathy of prematurity (ROP). Basically, preterm birth halts the normal development of retinal vascular system, which results in a retina that is poorly vascularized and highly susceptible to hyperoxia. In order to meet metabolic demand, the hypoxic retina induces the production of proangiogenic factors like VEGF (vascular endothelial growth factor). These factors stimulate the development of new blood vessels (angiogenesis), causing aberrant vessels sprouting from the retina into the vitreous. It is thought that an imbalance between the production of reactive oxygen species (ROS) and the reduced levels of antioxidants in preterm infants contributes to ROP pathogenesis and causes additional damage to the retina (reviewed in 87).
Supplemental lutein and zeaxanthin: A randomized controlled trial in 62 preterm infants (≤32 weeks of gestational age) failed to observe any benefits regarding ROP incidence and severity with lutein (0.5 mg/kg/day) and zeaxanthin (0.02 mg/kg/day) supplemented from the seventh day post birth until about 10 weeks of age (88). In two other multicenter, placebo-controlled trials in a total of 343 preterm infants, a daily oral dose of 0.14 mg of lutein and 0.006 mg of zeaxanthin administered from the first week after birth did not significantly reduce ROP incidence or the rate of progression from early to more advanced stages of ROP occurrence (89, 90). A fourth multicenter, randomized controlled trial in 203 preterm infants found that administration of a formula containing carotenoids (lutein/zeaxanthin, lycopene, and β-carotene) had no effect on ROP incidence but limited the progression to severe ROP stages in infants with mild ROP compared to a carotenoid-free formula (91). Compared with the control formula, preterm infants free of ROP fed the carotenoid-containing formula had significantly increased plasma carotenoid concentrations, which were correlated with greater rod photoreceptor sensitivity (91). More research is needed to examine whether carotenoid supplementation might promote normal photoreceptor development and prevent ROP in preterm infants.
Supplementation with lutein and zeaxanthin has been shown to help preserve retina integrity in diabetic rodents by reducing oxidative stress and inflammatory mediators (87). A higher ratio of plasma nonprovitamin A carotenoids (lutein, zeaxanthin, and lycopene) to vitamin A carotenoids (β-carotene, β-carotene, β-cryptoxanthin) was associated with a reduced risk of diabetic retinopathy in a cross-sectional analysis in 111 individuals with type 2 diabetes mellitus (92). Recent data from 1,430 individuals participating in the Atherosclerosis Risk In Communities (ARIC) study found no association between lutein intake and diabetic retinopathy after adjustment for confounding variables (93). To date, the role of carotenoids in the prevention of diabetic retinopathy has not been examined in clinical studies.
The primary function of the eye’s lens is to collect and focus light on the retina. Ultraviolet light and oxidants can damage proteins in the lens, causing structural changes that result in the formation of opacities known as cataracts. As people age, cumulative damage to lens proteins often results in cataracts that are large enough to interfere with vision (10). Risk factors other than aging and sunlight exposure include smoking, diabetes mellitus, higher body mass index, and use of estrogen replacement therapy. An estimated 50 million US adults are projected to be affected by age-related cataracts by 2050 (94).
Dietary lutein and zeaxanthin: The observation that lutein and zeaxanthin are the only carotenoids in the human lens has stimulated interest in the potential for increased intakes of lutein and zeaxanthin to prevent or slow the progression of cataracts (reviewed in 79). Large prospective cohort studies have found that men and women with the highest intakes of foods rich in lutein and zeaxanthin, particularly spinach, kale, and broccoli, were 18%-50% less likely to require cataract extraction (95, 96) or develop cataracts (97-99). Moreover, plasma concentrations of lutein and zeaxanthin have been found to be inversely associated to the progression of nuclear cataract. Additional research is required to determine whether these findings are related specifically to lutein and zeaxanthin intake or to other factors associated with diets high in carotenoid-rich foods (63).
Supplemental lutein and zeaxanthin: The Age-related Eye Disease Study 2 (AREDS2) failed to show an effect of supplemental lutein (10 mg/day) and zeaxanthin (2 mg/day; supplementation for a median of 4.7 years) on the risk of developing cataract, on the progression to severe cataract or to cataract surgery, and on visual acuity (100). However, the results might have been confounded by the fact that most participants were better nourished than the general population and/or used multivitamins that have been found to decrease the risk of developing cataract (101). Additional limitations to consider in interpreting the results have been reviewed elsewhere (100). Additional interventions are required to study whether supplemental lutein and zeaxanthin might be helpful in the prevention of cataract.
Supplemental β-carotene: Evidence from observational studies that cataracts were less prevalent in people with high dietary intakes and blood concentrations of carotenoids led to the inclusion of β-carotene supplements in several large randomized controlled trials of antioxidants. The results of those trials have been somewhat conflicting. β-Carotene supplementation (20 mg/day) for more than six years did not affect the prevalence of cataracts or the frequency of cataract surgery in male smokers living in Finland (83). In contrast, a 12-year study of male physicians in the US found that β-carotene supplementation (50 mg every other day) decreased the risk of cataracts in smokers but not in nonsmokers (102). Note that use of β-carotene supplements have been shown to increase the risk of lung cancer in smokers (see Supplemental β-carotene in lung cancer). Three randomized controlled trials examined the effect of an antioxidant combination that included β-carotene, vitamin C, and vitamin E on the progression of cataracts. Two trials found no benefit after supplementation for five years (103) or more than six years (104), but one trial found a small decrease in the progression of cataracts after three years of supplementation (105). Overall, the results of randomized controlled trials suggest that the benefit of β-carotene supplementation in slowing the progression of age-related cataracts does not outweigh the potential risks.
Dietary carotenoids: Because carotenoids are very soluble in fat and very insoluble in water, they circulate in lipoproteins, along with cholesterol and other fats. Evidence that low-density lipoprotein (LDL) oxidation plays a role in the development of atherosclerosis led scientists to investigate the role of antioxidant compounds like carotenoids in the prevention of cardiovascular disease (106). The thickness of the inner layers of the carotid arteries can be measured noninvasively using ultrasound technology. This measurement of carotid intima-media thickness is considered a reliable marker of atherosclerosis (107). A number of case-control and cross-sectional studies have found higher blood concentrations of carotenoids to be associated with significantly lower measures of carotid artery intima-media thickness (108-113). Additionally, higher plasma carotenoids at baseline have been associated with significant reductions in risk of cardiovascular disease in some prospective cohort studies (114-118) but not in others (119-122).
More recently, an analysis of data from the 2003-2006 US National Health And Nutrition Examination Survey (NHANES) in a sample of 2,856 adults found that serum total carotenoid concentration was inversely associated to blood concentrations of two cardiovascular risk factors, C-reactive protein (CRP) and total homocysteine (123). HDL-cholesterol concentration was found to be positively associated with α-carotene, β-cryptoxanthin, and lutein/zeaxanthin concentrations, but only the latter was inversely associated with LDL-cholesterol (123). Finally, a recent meta-analysis of observational studies reported lower risks of coronary heart disease (-12%) and stroke (-18%) in individuals in the highest versus lowest tertile of blood lutein concentration (124).
While the results of several prospective studies indicate that people with higher intakes of carotenoid-rich fruit and vegetables are at lower risk of cardiovascular disease (122, 125-127), it is not yet clear whether this effect is a result of carotenoids or other factors associated with diets high in carotenoid-rich fruit and vegetables.
Supplemental β-carotene: In contrast to the results of observational studies suggesting that high dietary intakes of carotenoid-rich fruit and vegetables may decrease cardiovascular disease risk, four randomized controlled trials found no evidence that β-carotene supplements in doses ranging from 20 to 50 mg/day were effective in preventing cardiovascular disease (39, 128-130). Based on the results of these randomized controlled trials, the US Preventive Health Services Task Force found good evidence to suggest that β-carotene supplements provided no benefit in the prevention of cardiovascular disease in healthy adults (42). Thus, although diets rich in β-carotene have generally been associated with reduced cardiovascular disease risk in observational studies, there is no evidence that β-carotene supplementation reduces cardiovascular disease risk (131).
A recent experimental study found that the administration of β-cryptoxanthin to ovariectomized mice limited bone resorption by inhibiting osteoclast differentiation but had no effect on osteoblast-driven bone formation (132). Yet, evidence of a protective role of β-cryptoxanthin — and other individual carotenoids — against bone loss in humans is scarce. In the prospective Framingham Osteoporosis Study, data analysis of 603 participants found no association between β-cryptoxanthin intake and changes in bone mineral density (BMD) over a four-year period (133). In contrast, intakes of total and individual carotenoids (β-carotene, lycopene, and lutein/zeaxanthin) were positively associated with proximal femur BMD in men over a four-year period, while lycopene intake was positively linked to lumbar spine BMD in women (133). The 17-year follow-up of participants in the Framingham Osteoporosis Study (946 participants) showed those in the highest tertile of total carotenoid intake (median intake: 23.7 mg/day) had a 51% lower risk of hip fracture compared to those in the lowest tertile (median intake: 7.3 mg/day) (134). In a much larger prospective study — the Singapore Chinese Health Study — in 63,257 men and women followed for 10 years, the highest versus lowest quartile of total carotenoid intake was associated with a 37% lower risk of hip fracture in men but not in women (135). Dietary intakes of α- and β-carotene — but not of β-cryptoxanthin, lycopene, and lutein/zeaxanthin — were inversely associated with hip fracture risk in men (135). Nevertheless, in a small, four-year, Japanese prospective cohort study in 457 adults, the risk of osteoporosis was 93% lower in postmenopausal women in the highest versus lowest tertile of serum β-cryptoxanthin concentration (136). No such association was reported with serum concentrations of other individual carotenoids. Whether carotenoid supplementation may help prevent bone loss and reduce the risk of osteoporosis in older individuals is currently unknown.
Observational studies have suggested that dietary lutein may be of benefit in maintaining cognitive health (137, 138). As stated above, among the carotenoids, lutein and its isomer zeaxanthin are the only two that cross the blood-retina barrier to form macular pigment in the eye. Lutein also preferentially accumulates in the brain (139, 140). Recent studies suggested that lutein and zeaxanthin concentrations in the macula were correlated with brain lutein and zeaxanthin status and might be used as a biomarker to assess cognitive health (140-142). Additionally, in the Georgia Centenarian Study, the analysis of cross-sectional data from 47 centenarian decedents showed a positive association between post-mortem measures of brain lutein concentrations and pre-mortem measures of cognitive function (139). Brain lutein concentrations were found to be significantly lower in individuals with mild cognitive impairment compared to those with normal cognitive function (139). In a small, four-month, double-blind, placebo-controlled study in older women (ages, 60 to 80 years) without cognitive impairment, supplementation with lutein (12 mg/day) and zeaxanthin (~0.5 mg/day) significantly improved cognitive test performance (143). Nevertheless, the Age-related Eye Disease Study 2 (AREDS2) failed to show an effect of supplemental lutein (10 mg/day) and zeaxanthin (2 mg/day; supplementation for a median of 4.7 years) on the cognitive test scores of 3,741 participants (mean age, 72.7 years) (144).
The most prevalent carotenoids in the human diet are α-carotene, β-carotene, β-cryptoxanthin, lycopene, lutein, and zeaxanthin (8). Most carotenoids in foods are found in the all-trans form (see Figure 1 and Figure 2 above), although cooking may result in the formation of other isomers. The relatively low bioavailability of carotenoids from most foods compared to supplements is partly due to the fact that they are associated with proteins in the plant matrix (145). Chopping, homogenizing, and cooking disrupt the plant matrix, increasing the bioavailability of carotenoids (3). For example, the bioavailability of lycopene from tomatoes is substantially improved by heating tomatoes in oil (146, 147).
α-Carotene and β-carotene
α-Carotene and β-carotene are provitamin A carotenoids, meaning they can be converted in the body to vitamin A. The vitamin A activity of β-carotene in food is 1⁄12 that of retinol (preformed vitamin A). Thus, it would take 12 µg of β-carotene from food to provide the equivalent of 1 µg (0.001 mg) of retinol. The vitamin A activity of α-carotene from foods is 1⁄24 that of retinol, so it would take 24 µg of α-carotene from food to provide the equivalent of 1 µg of retinol. Orange and yellow vegetables like carrots and winter squash are rich sources of α- and β-carotene. Spinach is also a rich source of β-carotene, although the chlorophyll in spinach leaves hides the yellow-orange pigment. Some foods that are good sources of α-carotene and β-carotene are listed in Table 2 and Table 3 (148).
Table 2. α-Carotene Content of Selected Foods
|Carrot juice, canned
||1 cup (8 fl oz)
|Mixed vegetables, frozen, cooked
|Winter squash, baked
|Collards, frozen, cooked
|Peas, edible-podded, frozen, cooked
Table 3. β-Carotene Content of Selected Foods
|Carrot juice, canned
||1 cup (8 fl oz)
|Spinach, frozen, cooked
|Sweet potato, baked
|Collards, frozen, cooked
|Kale, frozen, cooked
|Turnip greens, frozen, cooked
|Winter squash, cooked
|Dandelion greens, cooked
Like α- and β-carotene, β-cryptoxanthin is a provitamin A carotenoid. The vitamin A activity of β-cryptoxanthin from food was estimated to be 1⁄24 that of retinol, so it would take 24 µg of β-cryptoxanthin from food to provide the equivalent of 1 µg of retinol. Orange and red fruit and vegetables like sweet red peppers and oranges are particularly rich sources of β-cryptoxanthin. Some foods that are good sources of β-cryptoxanthin are listed in Table 4 (148).
Table 4. β-Cryptoxanthin Content of Selected Foods
|Sweet red peppers, cooked
|Sweet red peppers, raw
|Orange juice, fresh
||1 cup (8 fl oz)
|Carrots, frozen, cooked
|Yellow corn, frozen, cooked
||1 wedge (1⁄16 of a melon that is 15 inches long x 7.5 inches in diameter)
Lycopene gives tomatoes, pink grapefruit, watermelon, and guava their red color. It has been estimated that 80% of the lycopene in the US diet comes from tomatoes and tomato products like tomato sauce, tomato paste, and ketchup (catsup) (149). Lycopene is not a provitamin A carotenoid because it cannot be converted to retinol. Some foods that are good sources of lycopene are listed in Table 5 (148).
Table 5. Lycopene Content of Selected Foods
|Tomato paste, canned
|Tomato purée, canned
|Tomato soup, canned, condensed
|Vegetable juice cocktail, canned
|Tomato juice, canned
||1 wedge (1⁄16 of a melon that is 15 inches long x 7.5 inches in diameter)
|Pink grapefruit, raw
|Baked beans, canned
Lutein and zeaxanthin
Although lutein and zeaxanthin are different compounds, they are both classified as xanthophylls and nonprovitamin A carotenoids (see Figure 2 above). Some methods used to quantify lutein and zeaxanthin in food do not separate the two compounds, so they are typically reported as lutein and zeaxanthin or lutein + zeaxanthin. Both pigments are present in a variety of fruit and vegetables. Dark green leafy vegetables like spinach and kale are particularly rich sources of lutein but poor sources of zeaxanthin (150). Although relatively low in lutein, egg yolks and avocados are highly bioavailable sources of lutein. Good sources of dietary zeaxanthin include yellow corn, orange pepper, orange juice, honeydew (melon), and mango (150). Some foods containing lutein and zeaxanthin are listed in Table 6 (148).
Table 6. Lutein + Zeaxanthin Content of Selected Foods
||Lutein + Zeaxanthin (mg)
|Spinach, frozen, cooked
|Kale, frozen, cooked
|Turnip greens, frozen, cooked
|Collards, frozen, cooked
|Dandelion greens, cooked
|Mustard greens, cooked
|Summer squash, cooked
|Peas, frozen, cooked
|Winter squash, baked
|Brussel sprouts, frozen, cooked
|Broccoli, frozen, cooked
|Sweet yellow corn, boiled
|Egg yolk, raw
For more information on the carotenoid content of certain foods, search the USDA National Nutrient Database.
Dietary supplements providing purified carotenoids and combinations of carotenoids are commercially available in the US without a prescription. Carotenoids are best absorbed when taken with a meal containing fat.
Supplements containing a mixture of carotenoids may include α-carotene. As a provitamin A carotenoid, supplemental α-carotene can contribute to fulfill vitamin A requirements. It is not known whether the relative bioavailability of supplemental α-carotene is greater than that of dietary α-carotene.
Supplements containing a mixture of carotenoids may include α-cryptoxanthin. As a provitamin A carotenoid, supplemental α-cryptoxanthin can contribute to fulfill vitamin A requirements. It is not known whether the relative bioavailability of supplemental α-cryptoxanthin is greater than that of dietary α-cryptoxanthin.
β-Carotene is sold as individual supplements and also found in supplements marketed to promote visual health (151). Commercially available β-carotene supplements usually contain between 1.5 mg and 15 mg of either synthetic β-carotene or natural β-carotene (mainly from the algae Dunaliella salina) per softgel capsule (150). As a provitamin A carotenoid, β-carotene may be used to provide all or part of the vitamin A in multivitamin supplements. The provitamin A activity of β-carotene from supplements is much higher than that of β-carotene from food: it takes only 2 micrograms [µg] (0.002 mg) of β-carotene from supplements to provide 1 µg of retinol (preformed vitamin A) compared to 12 µg of dietary β-carotene.
Of note, the β-carotene content of supplements is often listed in international units (IU) rather than µg: 3,000 µg (3 mg) of supplemental β-carotene provides 5,000 IU of vitamin A.
Lycopene has no provitamin A activity. Synthetic lycopene and lycopene from natural sources, mainly tomatoes, are available as nutritional supplements containing up to 15 mg of lycopene per softgel capsule (150).
Lutein and zeaxanthin
Lutein and zeaxanthin are not provitamin A carotenoids. Lutein and zeaxanthin supplements are available as free carotenoids (non-esterified) or as esters (esterified to fatty acids). Both forms appear to have comparable bioavailability (152). Many commercially available lutein and zeaxanthin supplements have much higher amounts of lutein than zeaxanthin (150). Supplements containing only lutein or zeaxanthin are also available.
Although β-carotene can be converted to vitamin A, the conversion of β-carotene to vitamin A decreases when body stores of vitamin A are high (see Absorption, Metabolism, and Bioavailability). This may explain why high doses of β-carotene have never been found to cause vitamin A toxicity. High doses of β-carotene (up to 180 mg/day) have been used to treat erythropoietic protoporphyria, a photosensitivity disorder, without toxic side effects (8).
Lycopene, lutein, and zeaxanthin
No toxicities have been reported (153).
Increased lung cancer risk: Two randomized controlled trials in smokers and former asbestos workers found that supplementation with 20 to 30 mg/day of β-carotene for 4 to 6 years was associated with significant 16%-28% increases in the risk of lung cancer compared to placebo (see Supplemental β-carotene in lung cancer). Although the reasons for these findings are not yet clear, the potential risk of lung cancer in smokers and other high-risk groups supplemented with high-dose β-carotene outweigh any possible benefits for chronic disease prevention (42). It should be noted that there is no evidence that β-carotene supplementation may harm nonsmokers.
Carotenodermia: High doses of β-carotene supplements (≥30 mg/day) and the consumption of large amounts of carotene-rich food have resulted in a yellow discoloration of the skin (xanthoderma) known as carotenodermia. Carotenodermia is not associated with any underlying health problems and resolves when supplementation with β-carotene is discontinued or dietary carotene intake is reduced.
Lycopenodermia: High intakes of lycopene-rich food or supplements may result in a deep orange discoloration of the skin known as lycopenodermia. Because lycopene is more intensely colored than the carotenes, lycopenodermia may occur at lower doses than carotenodermia (8).
Lutein and zeaxanthin
Adverse effects of lutein and zeaxanthin have not been reported (150).
Safety in pregnancy and lactation
Unlike vitamin A, high doses of β-carotene taken by pregnant women have not been associated with increased risk of birth defects (8). However, the safety of high-dose β-carotene supplements in pregnancy and lactation has not been well studied. Although there is no reason to limit dietary β-carotene intake, pregnant and breast-feeding women should avoid consuming more than 3 mg/day (1,500 µg RAE/day; 5,000 IU/day) of β-carotene from supplements unless they prescribed under medical supervision.
The safety of carotenoid supplements other than β-carotene in pregnancy and lactation has not been established, so pregnant and breast-feeding women should obtain carotenoids from food rather than supplements. There is no reason to limit the consumption of carotenoid-rich fruit and vegetables during pregnancy (150).
The cholesterol-lowering agents, cholestyramine (Questran) and colestipol (Colestid), can reduce absorption of fat-soluble vitamins and carotenoids, as can mineral oil and Orlistat (Xenical), a drug used to treat obesity (150). Colchicine, a drug used to treat gout, can cause intestinal malabsorption. However, long-term use of 1 to 2 mg/day of colchicine did not affect serum β-carotene concentrations in one study (154). Increasing gastric pH through the use of proton-pump inhibitors (Omeprazole, Lansoprazole) may decrease the absorption of a single dose of a β-carotene supplement, but the effect is unlikely to be clinically significant (155).
Antioxidant supplements and statins (3-hydroxy-3-methylglutaryl-coenzyme A [HMG-CoA] reductase inhibitors)
A three-year randomized controlled trial in 160 patients with documented coronary heart disease (CHD) and low serum high density lipoprotein (HDL) concentrations found that a combination of simvastatin (Zocor) and niacin increased HDL2 levels, inhibited the progression of coronary artery stenosis, and decreased the frequency of cardiovascular events, including myocardial infarction and stroke (156). Surprisingly, when an antioxidant combination of 1,000 mg of vitamin C, 536 mg of RRR-α-tocopherol (vitamin E), 100 µg of selenium, and 25 mg of β-carotene daily was taken with the simvastatin-niacin combination, the protective effects were diminished. Since the antioxidants were taken together in this trial, the individual contribution of β-carotene cannot be determined. In contrast, a much larger randomized controlled trial of simvastatin and an antioxidant combination of 297 mg of RRR-α-tocopherol, 250 mg of vitamin C, and 20 mg of β-carotene daily in more than 20,000 men and women with CHD or diabetes mellitus found that the antioxidant combination did not diminish the cardioprotective effects of simvastatin therapy over a five-year period (157). These contradictory findings indicate that further research is needed on potential interactions between antioxidant supplements and cholesterol-lowering agents, such as niacin and statins.
Interactions with food
Fat substitute Olestra
In a controlled feeding study, consumption of 18 g/day of the fat substitute Olestra (sucrose polyester; Olean) resulted in a 27% decrease in serum carotenoid concentrations after three weeks (158). Studies in people before and after the introduction of Olestra-containing snacks to the marketplace found that total serum carotenoid concentrations decreased by 15% in those who reported consuming at least 2 g/day of Olestra (159). One study in adults found that those who consumed more than 4.4 g of Olestra weekly experienced a 9.7% decline in total serum carotenoids compared to those not consuming Olestra (160).
Plant sterol- or stanol-containing foods
Some studies found that the regular use of plant sterol-containing spreads resulted in modest, 10%-20% decreases in the plasma concentrations of some carotenoids, particularly α-carotene, β-carotene, and lycopene (see the article on Phytosterols) (161, 162). However, advising people who use plant sterol- or stanol-containing margarines to consume an extra serving of carotenoid-rich fruit or vegetables daily prevented decreases in plasma carotenoid concentrations (163, 164).
The relationships between alcohol consumption and carotenoid metabolism are not well understood. There is some evidence that regular alcohol consumption inhibits the conversion of β-carotene to retinol (165). Increases in lung cancer risk associated with high-dose β-carotene supplementation in two randomized controlled trials were enhanced in those with higher alcohol intakes (38, 166).
Interactions among carotenoids
The results of metabolic studies suggested that high doses of β-carotene competed with lutein and lycopene for absorption when consumed at the same time (167-169). However, the consumption of high-dose β-carotene supplements did not adversely affect serum carotenoid concentrations in long-term clinical trials (170-173).
Authors and Reviewers
Originally written in 2004 by:
Jane Higdon, Ph.D.
Linus Pauling Institute
Updated in December 2005 by:
Jane Higdon, Ph.D.
Linus Pauling Institute
Oregon State University
Updated in May 2009 by:
Victoria J. Drake, Ph.D.
Linus Pauling Institute
Oregon State University
Updated in July 2016 by:
Barbara Delage, Ph.D.
Linus Pauling Institute
Oregon State University
Reviewed in August 2016 by:
Elizabeth J. Johnson, Ph.D., Research Scientist
Antioxidants Research Laboratory
Jean Mayer USDA Human Nutrition Research Center on Aging
Assistant Professor, Friedman School of Nutrition Science
Copyright 2004-2016 Linus Pauling Institute
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