**3. PACs: chemical structure, occurrence, and intake**

PACs consist of flavan-3-ol subunits with a degree of polymerization (DP) equal to or greater than 2, mainly linked by (4 ! 8) or (4 ! 6) carbon-carbon bonds (Btype PACs) [45]. In some botanical sources an additional (2 ! 7) ether-linkage also occurs (A-type PACs) [46] (**Figure 2**). Depending on the type of monomers, PACs can be classified into procyanidins, prodelphinidins, and propelargonidins. The most abundant group, procyanidins, consists exclusively of (+)- catechin and (�)- epicatechin monomers [47]. Prodelphinidins and propelargonidins are composed of (�)- gallocatechin/(�)- epigallocatechin and (+)- afzelechin/(�)- epiafzelechin monomers, respectively [45], and have a more limited distribution

*Beneficial Effects of Proanthocyanidins on Intestinal Permeability and Its Relationship… DOI: http://dx.doi.org/10.5772/intechopen.91212*

#### **Figure 2.**

degradation. After pIkB-a degradation, the p65:p50 heterodimers are able to translocate to the nucleus, where they bind to the kB motif found in the promoter

NF-kB target genes include cytokines (e.g., tumor necrosis factor (TNF)-α and interleukins), adhesion molecules, acute phase proteins, and inducible enzymes (inducible nitric oxide synthase (iNOS) and cyclooxygenase 2 (COX2)), among others [11]. All of these genes contain verified NF-kB binding sites in their sequences, providing strong experimental evidence for their direct control by NF-kB [34]. Among all of these genes, the expression of iNOS and COX2 has been widely studied in relation to intestinal inflammation. In this regard, sustained high nitric oxide (NO) production by iNOS plays a role in the pathology of chronic inflammatory bowel disease [35, 36]. During the last decade, it has become increasingly clear that NO overproduction by iNOS is deleterious to intestinal function [37], thus contributing significantly to gastrointestinal immunopathology. Cyclooxygenases are enzymes that are responsible for the metabolism of

arachidonic acid, converting it into prostaglandins. These products influence a wide variety of biological processes, ranging from homeostasis to inflammation [38]. There are two cyclooxygenase isoforms: the constitutive COX1 isoform and the inducible COX2 isoform [38, 39]. As a result of COX2 induction, prostaglandin E2 levels increase at the site of inflammation and can also be detected systemically. Multiple environmental factors have been identified as potential triggers of intestinal inflammatory conditions, including Western dietary habits [40]. It has been described that saturated fats play a direct role in inflammatory signaling. Saturated fatty acids (SFA) such as lauric (C12:0) and palmitic (C16:0) directly induce NF-κB activation, acting as non-microbial TLR2 and TLR4 agonists in macrophages [41]. Data suggest that activation of TLRs by SFA is mediated by TLR dimerization and recruitment into lipid rafts [42]. We have reported mild intestinal

inflammation and increased permeability in rats feeding on a cafeteria diet consisting of high-saturated fat/high-refined sugar food products [43]. This

**3. PACs: chemical structure, occurrence, and intake**

monomers, respectively [45], and have a more limited distribution

enhanced permeability has been shown to favor bacterial LPS and other potentially pro-inflammatory molecules entering the systemic circulation, which is known as

Taken together, these data suggest that HF diet-induced changes in the intestinal microbiota could be responsible for metabolic endotoxemia and for the onset of the corresponding diseases. The causative link between changes in intestinal bacteria populations, endotoxemia, and metabolic disease needs further assessment [44], but the mechanisms likely include altered epithelial permeability, translocation of bacterial products, and upregulation of pro-inflammatory cytokines and hormones produced by gut endocrine cells, mechanisms which might be modulated by PACs.

PACs consist of flavan-3-ol subunits with a degree of polymerization (DP) equal to or greater than 2, mainly linked by (4 ! 8) or (4 ! 6) carbon-carbon bonds (Btype PACs) [45]. In some botanical sources an additional (2 ! 7) ether-linkage also occurs (A-type PACs) [46] (**Figure 2**). Depending on the type of monomers, PACs can be classified into procyanidins, prodelphinidins, and propelargonidins. The most abundant group, procyanidins, consists exclusively of (+)- catechin and (�)- epicatechin monomers [47]. Prodelphinidins and propelargonidins are composed of (�)- gallocatechin/(�)- epigallocatechin and (+)- afzelechin/(�)- epiafzelechin

or enhancer regions of numerous pro-inflammatory genes to induce their

expression [33].

*Weight Management*

metabolic endotoxemia [15].

**272**

*Chemical structures of PACs. Flavan-3-ol monomers differ based on the hydroxylation pattern and their cis- or trans-configuration. Dimers A1/A2 and B1/B2 are shown as example of A- and B-type PACs, respectively.*

Dietary assessment studies have shown that PACs, especially procyanidins are among the most abundant polyphenols in the human diet [6], as they are present in a variety of botanical sources and plant food products such as tea, fruits, nuts, cacao products, legumes, and cereal grains [1, 2]. However, PAC intake varies widely between geographical regions and cultures and is greatly dependent on eating habits, lifestyle behaviors, and socioeconomic status [48]. The daily PAC (dimers to polymers) intakes in adult populations from Korea, the U.S., Mexico, and EU were estimated as 71 [49], 73 [48], 103 [50], and 123–180 mg [51, 52], respectively, but intakes up to 230 mg d<sup>1</sup> have been reported in some regions of Spain and Norway [53].

#### **4. The fate of PACs after ingestion**

Flavan-3-ols are remarkably stable during gastric transit in humans [54]. Monomers such as (+)- catechin and ()- epicatechin are readily absorbed in the upper sections of the small intestine [55, 56], recognized as xenobiotics and then subjected to an extensive phase II metabolism that generates glucuronidated, sulfated, and methylated conjugates [57]. Flavan-3-ol monomers and their conjugated metabolites reach peak plasma concentration 1–4 h after flavan-3-ol-rich food consumption [58–60]. Studies conducted in cultivated epithelial monolayers [61–63], rats [64, 65], and humans [60, 66] indicate that PAC absorption is conversely more

limited and is highly dependent on DP, and that the permeation of larger oligomers (DP > 5) and polymers is negligible. No PAC transporter has been identified in the enterocyte membrane in the small intestine. Thus, dimers to tetramers are passively transported across the intestinal epithelium essentially by paracellular diffusion. Although transcellular passive diffusion is not likely to occur due to the hydrophilic nature of PACs conferred by the multiple hydroxyl groups, uptake might be possible by endocytic mechanisms [62].

In humans, a study assessed the contribution of the ingested cocoa flavan-3-ols and procyanidins to the systemic pool, and found that the plasma (�)- epicatechin came from the orally administered cocoa (�)- epicatechin and not from their oligomers or polymers [67]. This is in agreement with the evidence obtained with rats that suggests that PACs from different sources do not depolymerize to monomers after ingestion [68, 69]. Stalmach et al. [56] conducted a study with ileostomized patients who were administered green tea, and found 70% of the ingested flavan-3-ol in the ileal fluid after 24 h. Altogether, these findings suggest that substantial amounts of ingested flavan-3-ol monomers and PACs remain unabsorbed in the small intestine and reach the colon. There, they are efficiently transformed by the colonic microbiota into low molecular weight phenolic compounds that can be absorbed by colonocytes [57].

In vitro fermentation of purified procyanidin dimers with human fecal microbiota has shown to produce mainly 2-(3<sup>0</sup> ,4<sup>0</sup> -dihydroxyphenyl) acetic acid and 5-(3<sup>0</sup> ,4<sup>0</sup> -dihydroxyphenyl)-γ-valerolactone [70]. In agreement with this, a randomized cross-over study in healthy humans found that a great portion of the ingested (�)- epicatechin and procyanidin B1 was metabolized by the colonic microbiota to produce phenyl-γ-valerolactones as the major microbial metabolites [60]. In this study, microbial degradation of larger procyanidins was substantially lower, possibly to the inhibition of digesting enzymes or to the antibacterial properties exhibited by these compounds. Other human studies analyzing the bioavailability of flavan-3-ols, reported high levels of phenyl-γ-valerolactones in the circulation and urinary excretion after ingestion of a red grape pomace drink [71] and apple juice [72]. In the colonocytes and hepatocytes, these microbial products undergo further metabolism by phase II enzymes to produce conjugated derivatives. Margalef et al. [73] analyzed the tissue distribution of metabolites derived from a grape-seed proanthocyanidin extract (GSPE) 2 h after ingestion by rats. These authors detected a few microbial metabolites (methyl conjugated phenols) at low concentrations in the colon tissue, while most phase II metabolites (glucuronidated and methylglucuronidated forms) were found in the kidneys and liver. In humans, the major contributors to the excretion of phenyl-γ-valerolactones after ingestion of a red grape pomace drink, are sulfated and glucuronidated conjugates of 5-(3<sup>0</sup> ,4<sup>0</sup> dihydroxyphenyl)-γ-valerolactone [71].
