Flavonoids are a group of plant antioxidants with phenolic ring structures (here). Flavonoids not attached to a sugar are referred to as aglycones. The predominant forms of flavonoids in plants are flavonoid glycosides (figure 1). Except for catechins, flavonoids do not usually appear in plants as aglycones. In foods for example, the flavonol quercetin is present as glycosylated forms, mainly as β-glycosides. A flavonoid glycoside is a flavonoid molecule with sugar moiety. Glycosylation of the flavonoid increases the polarity of the flavonoid molecule, which is necessary for its storage in plant cell vacuoles. In plants flavonoids are relatively resistant to heat, oxygen, dryness and acidity, with the photo-stability of the flavonoid depending on the nature of the hydroxyl group attached to 3-position of ring C. The absence or glycosylation of this hydroxyl group results in high photo-stability of the molecule.
Figure 1. Quercetin-3-O-glucoside (left) and quercetin aglycone (right).
The human gut forms a selectively permeable barrier to limit the absorption of certain nutrients and xenobiotics. In order for flavonoids to pass into the circulation and have a biological effect on target tissue, they must cross this barrier. Flavonoids in the aglycone form are generally hydrophobic and can passively diffuse through biological membranes. However, linkage of a flavonoid to a sugar or organic acid as occurs naturally in most plant sources, increases water solubility and severely limits passive diffusion. It is still unclear as to the precise mechanism by which flavonoids pass from the gut to the human circulation. A number of possible routes of absorption are known for flavonoid and have been described in research (figure 2). It is at this point still unclear as to which is quantitatively the most important absorptive route.
Exogenous deglycosylation of flavonoids by bacteria in the large intestine was once thought to be quantitatively the most important and possibly the only route for the absorption of these molecules. The diverse microflora residing in the intestine can release enzymes (such as β-glucosidase) to gradually hydrolyze flavonoid glycosides into aglycones, a portion of which may be subsequently absorbed across the epithelial lining of the large intestine. The aglycones that are not absorbed in the intestine can be further degraded by colonic microflora, for example into phenolic acids. However, the theory that bacterial degradation provided the only route for the absorption of flavonoid glycosides was challenged when researchers reported that absorption of flavonoids occurs in healthy ileostomy patients. This research demonstrated for the first time that the small intestine must provide a possible route for the absorption of dietary flavonoids.
Paracellular diffusion is a process by which chemicals move through the gaps between adjacent cells. Paracellular diffusion can occur through the tight junctions of the epithelial cells for certain dietary constituents, including flavonoids. For example, research has shown that catechins are absorbed by paracellular diffusion in cell culture models of the gut. Catechins from tea are also thought to be absorbed by paracellular diffusion in the oral cavity of human subjects. The fact that anthocyanin glucosides are absorbed from the stomach of rats, and have been observed in human plasma and urine also suggests that passive diffusion of these flavonoids is possible as the stomach possesses no active transport mechanisms. However, it is not clear as to the importance of paracellular diffusion as a route for flavonoid absorption and it is difficult to quantify.
Research has demonstrate a higher absorption of quercetin derived from quercetin glucosides as present in onions, compared with free aglycone. This is good evidence for the involvement of active transport of flavonoid glycosides in the small intestine. Some subsequent research highlighted the sodium-dependent glucose transporter-1 (SGLT-1) as playing a possible role in the absorption of certain flavonoid glucosides. Evidence supporting a role for SGLT-1 in the uptake of dietary flavonoids has since come from a number of studies. For example, research has demonstrated that cells with higher levels of SGLT have high levels of flavonoid uptake and that flavonoids are able to inhibit uptake of certain SGLT sugar substrates. This may suggest that flavonoids can inhibit the absorption of certain sugars in the diet if the both use the same SGLT-1 transport system.
The enzyme complex, lactase phlorizin hydrolase (LPH), is located in the brush border of the small intestine and is capable of hydrolysing phlorizin to the aglycone phloretin. Since phlorizin is closely related to the flavonoid glycosides it is possible that LPH also has similar activity on the flavonoid glycosides. For example, researchers have shown that intestinal uptake of quercetin-3-glucoside in rats may involve hydrolysis by lactase phlorizin hydrolase. LPH therefore may be responsible for the hydrolysis of quercetin 3-glucoside observed in small intestine cell free extract. If LPH hydrolyses certain flavonol glycosides, then it could act on the apical side of the brush border, releasing the aglycone into the intestinal lumen. The close proximity of the released aglycone to the membrane may increase the ability of the flavonol to passively diffuse into the enterocyte.
Some evidence suggests that other active transporters are involved in the absorption of flavonoids. A number of other hexose transporters and non-hexose transporters are present in the human gut including the GLUT transporters (GLUT 1-8), the sodium-dependent glucose transporter (SGLT1 and SGLT2) and the sodium-dependent vitamin C transporters (SVTC1 and SVCT2). Some of these transporters have been implicated in the active transport of flavonoids across the luminal surface of the enterocytes. For example, some evidence supports an interaction of quercetin with the sodium-dependent vitamin C transporter 1 (SVCT1) as well as glucose transporter isoform 2 (GLUT2). GLUT transporters have been shown to transport flavonoids into rat adiposite cells in vitro, and so it is possible that they play a role in the absorption of flavonoids from the gut. However, evidence for the involvement of these transporters is weak.
Evidence suggests that extensive metabolism of flavonoids occurs once inside the small intestinal cells. Hence it is likely to be flavonoid conjugates and not the parent glucosides or aglycone that act to elicit biological responses in the body, beyond the gut. Within epithelial cells, cytosolic β-glucosidase is able to deglycoslate certain flavonoid glycosides to form the aglycone, the substrate specificity for which has been investigated. Extensive conjugation of the flavonoid aglycone could then occur in the epithelial cell to form mainly sulfate and glucuronide phase II conjugates. Research has shown that metabolism of quercetin compounds occurs in the small intestine epithelial cells, such that no aglycone or glucoside compounds are detected in the plasma after ingestion of onions. It would appear likely therefore that when we talk about the health benefits of flavonoids, we really mean flavonoid conjugates.
Figure 2. Schematic diagram summarising the research to date regarding the absorption of flavonoids in the human gut. Flavonoids may be absorbed as glycosides and then cleaved to the aglycone form with cytosolic glucosidase. Alternatively, they may be deglycosylated in the lumen (by LPH or bacteria) and then the flavonoid aglycone passively diffuses into the enterocyte. Flavonoid aglycones within enterocytes are rapidly and completely metabolised to flavonoid glucuronides and flavonoid sulfates via UDP-glucuronosyltransferase or sulfotransferase. Some flavonoids may be absorbed by paracellular diffusion.
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