Carotenoids and Skin
5.3 Carotenoids Bioavailability and Biodistribution to Skin .1 Carotenoids Bioavailability
Because carotenoids are lipophilic molecules, their patterns of absorption and transport are similar to those for other dietary lipids. Bioavailability of carotenoids is defined as a transfer of dietary carotenoids and their metabolites to the lymphatic or portal circulation for distribution to hepatic and extrahepatic tissues for biological functioning, metabolism, or storage. Uptake by intestinal mucosal cells in itself cannot be categorized as bioavailability because mucosal cells may be sloughed off into the lumen before carotenoids or their metabolites can cross the basolateral surface. Therefore, studies of carotenoid absorption are often complex because individuals consume carotenoids as components of meals. Food matrices and the presence of competing molecules in the diet affect the efficiency of the transfer of carotenoids to intestinal absorptive cells, thereby making reliable prediction of absorption difficult. As outlined in Fig. 5.2, absorption of carotenoids from a meal requires several processes, including (1) release of carotenoids from a food matrix, (2) incorporation into lipid droplets, (3) transfer to mixed bile salt micelles in the lumen of the small intestine, (4) uptake of carotenoid molecules across the apical surface of intestinal mucosal cells from bile salt micelles, (5) incorporation of caro- tenoids and their metabolites into chylomicrons, and (6) efflux of chylomicrons across the basolateral membrane into the lymphatic circulation.
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Release of carotenoids from the matrix in which it is embedded begins by disrup- tion during processing and cooking of the food. Reducing the particle size by purée- ing or chopping vegetables results in relatively higher in vitro bioaccessibility and higher potential for improved bioavailability than that in whole or sliced raw vegetables (Ryan et al. 2008; Thakkar et al. 2009). Further disruption of the matrix occurs during mastication and mixing of food with salivary juices and enzymes.
Hydrochloric acid, proteolytic enzymes, gastric lipases, and peristaltic movements of the upper gastrointestinal tract further disrupt the food matrix for release of carotenoids. Release from the food matrix generally results in partitioning of caro- tenoids into lipid droplets available in the gastric lumen. Localization of carotenoids in lipid droplets is dependent on their structure, with polar carotenoids distributed on the surface and the more hydrophobic carotenoids such as carotenes localized in the core (Borel et al. 1996).
The presence of dietary lipid stimulates bile flow from the gallbladder, which facilitates emulsification of lipid droplets and other lipophilic molecules in the small intestine. Pancreatic lipases hydrolyze the lipid droplets to much smaller particles.
This hydrolytic activity leads to conversion of substrates such as triglycerides, phospholipids, cholesterol esters, retinyl esters, and carotenoid esters to free fatty acids monoacylglycerides, lysophospholipids, free cholesterol, retinol, and free carotenoids, respectively. Incorporation of carotenoids in mixed bile salt micelles is obligatory for transfer of carotenoids across the unstirred water layer to the apical surface of absorptive cells. Owing to aforementioned rationales, the presence of dietary lipid in a carotenoid-enriched meal is of utmost importance for its bioavail- ability. Carotenoid uptake into intestinal epithelial cells has been assumed to occur
Fig. 5.2 Carotenoid bioavailability and biodistribution to hepatic and extrahepatic tissues. *CAR carotenoids, Vit A Vitamin A, BCO1 b-carotene 15,15¢-monooxygenase 1; SRB-I scavenger recep- tor type B class I
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passively. However, recent studies from the laboratories of Harrison and Borel suggest facilitated uptake of carotenoids (During et al. 2005; Reboul et al. 2005).
Several apical membrane transporters, including Nieman-Pick C1-Like 1 (NPC1L1), scavenger receptor type B class I (SRB-I), and ATP binding cassette transporter sub- family A (ABCA1), have been hypothesized to participate directly or indirectly in the transport. Among the above-indicated transporters, SRB-I may be of primary interest, yet its role in uptake of carotenoids remains ambiguous.
Provitamin A carotenoids are partly converted by the central cleavage enzyme b-carotene 15,15¢-monooxygenase 1 (BCO1) to vitamin A primarily in the form of retinyl esters in the intestinal mucosal cells. Both carotenoids and retinyl esters are then incorporated into chylomicrons and secreted into lymph for delivery to peripheral tissues. Chylomicrons are modified by lipoprotein lipase in the blood to form chylomicron remnants containing carotenoids that are rapidly taken up by the liver (Yonekura and Nagao 2007). In the liver, carotenoids can be stored, converted to vitamin A, or resecreted into the circulation in very low density (VLDL) and high density (HDL) lipoproteins for transport to peripheral tissues. Carotenoids appear to selectively accumulate in tissues expressing a high density of LDL receptors. These tissues include the adrenals, testes, liver, adipose tissue, kidney, and skin (Meinke et al. 2010; Gerster 1997). An exception may be the selective delivery of lutein and zeaxanthin to the macula region of the retina by HDL (Connor et al. 2007).
5.3.2 Factors Influencing Carotenoid Bioavailability
The bioavailability of carotenoids is dependent on physiochemical properties of the carotenoid (e.g., crystalline versus solubilized in plant organelles and free versus protein-bound), food matrix (e.g., located in chloroplast versus chromoplast; root versus leaf versus seed as food), type of processing of raw food (e.g., sun-dried, fermented, boiled, fried), presence or absence of compounds that promote or inhibit their absorption (e.g., lipids, fiber, other carotenoids), pathophysiological status of the gut (e.g., malabsorption due to parasites, pancreatic insufficiency, cholestasis), and nutritional status (e.g., deficient or adequate vitamin A) of the individual. These factors are further discussed below.
5.3.2.1 Physicochemical Properties of Carotenoids
Carotenoids can accumulate in plant foods in either crystalline or solubilized form.
Carotenoid solubilized in oil droplets are transferred to the micelle fraction more efficiently than those in crystalline form (Rich et al. 2003a, b). Carotenoid speciation also appears to affect bioavailability. Oxycarotenoids are more hydrophilic owing to the presence of one or more polar functional groups and are located on the surface of oil droplets. Hydrocarbon carotenoids are more hydrophobic in comparison and are embedded in the core. This decreases the efficiency of transfer of the hydrocarbon
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carotenoid to the micelle fraction as compared to oxycarotenoids (Borel et al. 1996;
van het Hof et al. 1999). Xanthophylls in certain foods can also exist as their fatty acid esters. These esters are extremely hydrophobic in nature and require hydrolysis to free carotenoids for efficient partitioning into the micelle fraction and uptake by enterocytes. Small intestinal simulations have demonstrated that hydrolytic cleavage of ester bond by carboxyl ester lipase, a broad-specificity pancreatic enzyme, precedes the preferential uptake of the nonesterified form of carotenoids by small intestinal cells (Chitchumroonchokchai and Failla 2006).
5.3.2.2 Food Matrix and Processing
Carotenoids may be bound to proteins in the food matrix. Food processing alters the matrix by disrupting the cell wall and membrane-bound organelles, loosening the linkages between carotenoids and proteins or fiber, partially dissolving the crystal- line carotenoids in oil, and increasing its surface area. These changes increase access of digestive enzymes and bile salts to promote the partitioning of carotenoids into micelles to enhance their bioavailability (Castenmiller et al. 1999; Edwards et al.
2002; Livny et al. 2003). Stahl and Sies demonstrated that lycopene concentrations in human serum increased only when tomatoes were subjected to 1 h of boiling in the presence of oil (Stahl and Sies 1992). Processing of plant foods also induces isomerization of carotenoids, thus increasing the levels of cis isomers. For example, baking is associated with isomerization and degradation of all-trans b-carotene in sweet potatoes (Chandler and Schwartz 1988). The percent bioavailability of cis analogues have been reported to be higher than their trans counterparts, although it should be noted that quantitatively the isomeric profile in plasma is still a reflection of the profile in food (Stahl and Sies 1992; Stahl et al. 1992). Although cis isomers of b-carotene are also precursors of vitamin A, isomerization of a provitamin A carotenoid reduces the retinol activity equivalence (RAE) as compared to their trans analogues (Deming et al. 2002; National Academy of S and M Institute 2001).
5.3.2.3 Interactions with Other Dietary Components
Consumption of carotenoids in our diet is usually associated with other macrontri- ents and micronutrients in the diet, which may act as promoters or inhibitors of carotenoid bioavailability. The association of carotenoids with dietary proteins has been shown to decrease their absorption in ferrets (Sundaresan et al. 2005). Likewise, dietary fibers such as citrus pectin and wheat bran are likely to bind to bile salts and decrease the formation of micelles and ultimately the absorption of carotenoids (Zanutto et al. 2002). In vivo and in vitro studies have demonstrated that dietary fat promotes carotenoid absorption. For example, Unlu and associates demon- strated that carotenoid absorption from a salad meal was enhanced by addition of avocado or avocado oil (Unlu et al. 2005). Several mechanisms have been proposed to be responsible for this effect. Dietary fat may facilitate the release of carotenoids
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from the food matrix by serving as a sink for hydrophobic compounds, by stimulating the secretion of bile and pancreatic enzymes, and by enhancing micelle formation in the small intestine. Moreover, the synthesis and secretion of chylomicrons by enterocytes is increased by dietary fat (Ribaya-Mercado 2002). Therefore, co- consumption of dietary lipid has been proposed to enhance the bioavailability of carotenoids.
Some, but not all, investigators have reported negative interactions between dietary carotenoids. High doses of pure b-carotene and lutein have been reported to antagonize the absorption of one another (Kostic et al. 1995; van den Berg and van Vliet 1998). The mechanisms for preabsorptive interactions among carotenoids are not well understood. Proposed interactions have been suggested to occur during their incorporation into bile salt micelles, uptake by intestinal epithelial cells, and incorporation into chylomicrons within enterocytes (van den Berg 1999).
5.3.2.4 Health Status of the Host
The digestive health status, and indeed the overall health status of an individual, also influences the absorption of carotenoids. Increased gastric pH appears to decrease b-carotene absorption (Tang et al. 1996). Gastrointestinal conditions that cause fat malabsorption may also lead to decreased carotenoid absorption. Conditions such as cholestasis, biliary cirrhosis, and pancreatic insufficiency may cause a reduction in digestive enzymes and bile release, ultimately affecting overall digestion of carotenoid-rich food and partitioning of lipophilic compounds into micelles (Olson 1999). Parasitemia in general has been negatively associated with plasma concentrations of carotenoids (Metzger et al. 2001). A study with malaria-infested children in Uganda demonstrated that therapeutic reductions in parasitemia resulted in increased concentrations of plasma carotenoids. Also, the presence of parasites in the intestine impaired carotenoid absorption or utilization. Jalal and his co-workers reported enhanced absorption after deworming children infected with Ascaris (Jalal et al. 1998).
5.3.3 Carotenoids Biodistribution to Skin
Once carotenoids clear the intestinal epithelial barrier and reach the systemic circulation via lymphatics, deposition occurs in hepatic and extrahepatic tissues including skin. Carotenoids stored in hepatic tissues can also be mobilized for distribution to extrahepatic tissues in times of need. Alaluf and coworkers (Alaluf et al. 2002) have demonstrated that the yellow component of skin color, quantified by the tristimulus chromameter, may be closely associated with carotenoid levels of the skin at various sites of the body (i.e., the back, forehead, inner forearm, and palm of the hand (Bayerl et al. 2003). Skin derived from subjects undergoing abdominoplasty suggests that apolar carotenoids (e.g., b-carotene, phytoene, lycopene)
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and their isomers constitute approximately 70% of the total pool; xanthophylls such as lutein and zeaxanthin are present at lower concentrations (Hata et al. 2000).
However, it should be noted that dietary intake based on the availability of carotenoid- enriched foods and cultural traditions in different regions of the world may influ- ence this profile.
Although stereoisomers of carotenoids mainly exist as the all-trans configuration in nature, cis isomers are also present in skin (Stahl et al. 1992). A high concen- tration of cis isomers in skin may be due to preferential uptake of cis isomers from the circulation, isomerization in biological systems after absorption, or both.
Recent data from our laboratory suggests that isomerization begins in small-intestinal epithelial cells and may even occur in other tissues (Richelle et al. 2010).
Xanthophylls esters (i.e., lutein, zeaxanthin) are also present in skin as mono- or di-fatty acid esters in picomolar ranges; they may be formed by postprandial reesterification of xanthophylls (Wingerath et al. 1998). Carotenoids exhibit a con- centration gradient in the layers of skin with a higher amount in the dermis (inner layer) and lower levels in the stratum corneum (outer layer). However, it is unclear whether stratum corneum has higher utilization or lower deposition when compared to the deeper layers. Regional variations may also be observed in the skin carote- noid level; indeed, higher concentrations of total carotenoids are measured in skin of the forehead, palm of the hand, and dorsal skin, whereas lower levels are present in the arm and the back of the hand (Bayerl et al. 2003; Stahl et al. 1998).
Occasionally, consumption of diets or supplementation containing more than 30 mg carotenoids per day for more than 4 weeks may result in yellowish discoloration of the skin, called carotenodermia. This condition is reversible, with the usual skin color restored upon cessation of the responsible supplementation or of a carotenoid-enriched diet (Bruch-Gerharz et al. 2001; Micozzi et al. 1988; Dimitrov et al. 1988).