**5.3 Intestinal absorption**

Polyphenols are not completely absorbed in the small intestine (5–10%). More than 90% enter the large intestine and are fermented by the human colon microbiota interacting with microorganisms (10–14 bacterial cells) and enzymes (α-Lrhamnosidase and β-D-glucosidase). Fermentation facilitates the liberation and absorption of insoluble bound phenolics involved in colorectal cancer prevention. The degradation of phenolic acids by enteric bacterial or chemical conversions may produce other metabolites, including protocatechuic acid, syringic acid, vanillic acid, phloroglucinol aldehyde, phloroglucinol acid, and gallic acid [3, 9, 15, 20, 21, 24, 30, 63, 69].

Phenolic compounds are catabolized by the gut microbiota, originate common phenolic (e.g., daidzein to equol, flavan-3-ols to valerolactones, and ellagitannins to urolithins) intermediates as in phenylpropionic, phenylacetic, and benzoic acids with different degrees of hydroxylation [17]. The flavonoids present in bound form (glycosides) and in free form (aglycones) are changed during digestion; a low fraction of these glycosides can be enzymatically hydrolyzed to aglycones in the small intestine, causing these aglycones to be more hydrophobic compared to the original glycosides. Aglycones are absorbed by epithelial cells through passive diffusion and then transported to the liver, where they will be metabolized. However, flavonoid glycosides are hardly absorbed in the small intestine due to their hydrophilic nature and reach the large intestine intact, where they will be metabolized by the gut microbiota [24].

During intestinal digestion, polyphenols such as anthocyanins, phenolic acids, catechin, quercetin, resveratrol, and rutin are unstable in the alkaline environment intestinal fluid (pH 6.8–8.0) due to their degradation [29]. The main phenolic acids absorbed in the small intestine are gallic, caffeic, and ferulic acids [63, 68]. Luzardo-Ocampo et al. [68] observed that phenolic acids in beans, like ferulic, chlorogenic, and vanillin, do not change their content during their passage through the small intestine, for 60–120 min, indicating their resistance to the intestinal enzymes and allowing them to arrive at the large intestine for fermentation. Milán-Noris et al. [12] observed that chickpea cooking increased intestinal absorption of the existent isoflavones. On the other hand, Cárdenas-Castro et al. [54] evaluated the bioaccessibility and *in vitro* release kinetics of phenolic compounds from two varieties of beans (Azufrado and Negro Jamapa). These authors reported that in cooked beans, the phenolic compounds showed 50% bioaccessibility, and 30% in cooked-fried beans, indicating that cooking did not modify the release kinetics of phenolic compounds during the first 60 min, being kaempferol-3-*O*-glucoside, quercetin-3-*O*-glucoside, and chlorogenic acid the main compounds released.
