Flavonoid absorption and metabolism

In general, flavonoids are bound in plant cells through hydroxyl (OH) groups as O-glycosides via O-glycosidic bonds or as C-glycosides through carbon-carbon (C-glycosidic) bonds [100,101]. In C-glycosides, the glycosidic bond is directly attached to the flavonoid backbone, typically at the C-6 or C-8 positions of the A-ring, resulting in compounds that are more resistant to hydrolysis and consequently exhibit altered biological activity. Hydrophilic glycoside forms can be absorbed in the small intestine mainly through passive diffusion after conversion to aglycones [102].

The physicochemical properties of flavonoids can influence the oral absorption of dietary flavonoids; these properties include molecular size and structure, solubility, lipophilicity and acidity [103]. Therefore, the flavonoid content in the food is less important than the fraction of the compound that is orally bioavailable [104]. Before absorption, flavonoids must be released from the food matrix through mastication, enzymatic digestion in the gastrointestinal tract and microbial metabolism in the intestine [105]. Flavonoid glycosides that are not absorbed in the small intestine are converted into aglycones in the cecum and colon by intestinal microbiota via methylation, sulfation and glucuronidation [106]. Lactase-phlorizin hydrolase (LPH) hydrolyses flavonoid glycosides into aglycones, which are subsequently absorbed by passive diffusion. LPH is a mammalian β-glycosidase located on the brush-border membrane of the intestine and is specific for flavonoid O-β-D-glucosides [107]. In vitro studies using LPH purified from sheep small intestine demonstrated that flavonoid glycosides serve as substrates for this enzyme [108]. The specificity of LPH substrates varies depending on the glycoside type (glucoside, galactoside, arabinoside, xyloside, rhamnoside, or rutinoside) and the flavonoid subclass, including flavones, isoflavones, flavonols, flavanones and anthocyanins [109]. In individuals with lactase deficiency, for example, reaches 87 % in three major ethnic groups in Malaysia, plasma isoflavone levels are initially low during early absorption but later become comparable to those of lactase-sufficient individuals, likely due to compensatory microbial hydrolysis in the gut [110,111]. Deglycosylation of flavonols is highly efficient, as glycoside forms are undetectable in plasma after administration of quercetin glycosides [112]. In contrast, anthocyanins can resist deglycosylation, as evidenced by their presence in urine [113].

Another enzyme involved in glycoside hydrolysis is cytosolic β-glucosidase (CBG), which is specific for 7-O-glucosides [114]. In vitro studies using human liver and small intestine tissues demonstrated that CBG acts on isoflavone, flavone and flavanone-7-glucosides but not on naringin, which contains a 7-rhamnoglucoside moiety [115]. Hydrolysis products are subsequently transported by sodium-glucose transporter 1 (SGLT1) and distributed via passive transport into the portal vein or metabolized through phase I and II reactions [116]. Together with GLUT2, SGLT1, widely expressed in intestinal epithelium, is involved in the detection and absorption of flavonoid glycosides from the food matrix [117].

Glycosides that are not substrates for LPH or CBG reach the colon, where they are hydrolysed and degraded into aglycones [118]. For example, the bioavailability of quercetin-3-β-rutinoside in human volunteers was only 20% compared to quercetin-4′-β-glucoside, indicating that sugar moieties strongly influence absorption and bioavailability. Food processing methods, such as fermentation and autolysis, can cleave flavonoid glycosidic bonds [119]. Fermented soybean products containing flavonoid aglycones are more readily absorbed than non-fermented soybeans, due to the transformation of flavonoids into glucosides, sulfonyl conjugates and glucuronides by the microbes [120].

Flavonoids in aglycone form are directly absorbed in the small intestine via passive diffusion due to increased lipophilicity and are transported to the hepatic vein, influenced by lipid solubility and molecular interactions; thus, dietary fat content affects flavonoid bioavailability [116]. In rat studies using lymph duct cannulation, administration of quercetin with long-chain fatty acids significantly increased hepatic quercetin levels compared to combinations with glucose or medium-chain fatty acids [121]. These findings suggest that long-chain fatty acids enhance quercetin bioavailability by promoting lymphatic transport and bypassing hepatic phase I metabolism. The addition of milk to black tea does not alter the absorption curves of flavonols and catechins, indicating that proteins do not affect flavonoid absorption [102]. In contrast, alcohol significantly enhances flavonoid absorption. Studies using rat intestinal sac models showed a threefold increase in quercetin absorption and a 1.5-fold increase in quercetin-3-O-glucoside absorption when combined with alcohol [122]. In human volunteers consuming alcohol with catechins, plasma catechin levels increased threefold within one hour and were present as distinct metabolites [123].

After intestinal absorption, flavonoids undergo extensive first-pass metabolism mainly in the liver and colon. In the liver, Phase I reactions, primarily oxidation or O-demethylation mediated by cytochrome P450 enzymes, are followed by Phase II conjugation catalysed by UGTs, SULTs and COMT, yielding glucuronidated, sulphated, or methylated metabolites [124,125]. Unabsorbed flavonoids and conjugated metabolites excreted via bile enter the colon, where gut microbial enzymes deconjugate them into aglycones that may be reabsorbed through enterohepatic circulation or further degraded [126]. The metabolic fate of flavonoids is strongly influenced by structural features, as highly hydroxylated compounds are more susceptible to microbial degradation, whereas O-methylation enhances metabolic stability [127]. In systemic circulation, flavonoids predominantly bind to serum albumin, with binding affinity governed by hydroxylation patterns and glycosylation [128]. Transport into tissues occurs through regulated membrane transporters and passive diffusion, depending on molecular size and hydrophobicity, while highly hydrophobic aglycones preferentially enter the intestinal lymphatic system, facilitating their distribution to peripheral tissues [129-131].