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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).
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, 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).
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).
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.
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).
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).
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).
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.
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).
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). 041b061a72