Apple fruit: Bioavailability of phytochemicals

As the link between diet and chronic disease grows stronger, many are working to understand how phytochemicals may provide health benefits. An important question to be asked is: Are these phytochemicals bioavailable? Concentrations and bioavailability of phytochemicals are important issues to evaluate when characterizing the effects of dietary phytochemicals on human health. To this date, little literature exists that addresses the bioavailability of phytochemicals from whole foods, including the apple. One of the few studies addressing bioavailability from apples or apple products looks at the bioavailability of polyphenolic compounds from alcoholic apple cider in humans [60]. After drinking 1.1 liters of apple cider, no quercetin was found in the volunteers' plasma. Instead, low levels of 3'-methyl quercetin and 4'-methyl quercetin were seen within 60 minutes following consumption of the cider. Caffeic acid was rapidly absorbed, but within 90 minutes the caffeic levels in the plasma were undetectable. Catechin, epicatechin, and phlorizin were not seen in the plasma, possibly because the concentration in the cider was too low. Hippuric acid and phloretin were both increased in the subjects' urine following the consumption of the cider, but there was no evidence of quercetin, catechin, or epicatechin in the urine [60].In another study involving human subjects, quercetin bioavailability from apples was only 30% of the bioavailability of quercetin from onions [61]. In this study, quercetin levels reached a peak after 2.5 hours in the plasma, however the compounds were hydrolyzed prior to analysis, so the extent of quercetin conjugation in the plasma is unknown. The bioavailability differences between apples and onions most likely are from the differences in quercetin conjugates in the different foods. Onions contain more quercetin aglycone and more quercetin glucosides, whereas apples tend to contain more quercetin monoglycosides and quercetin rutinoside, which may be less bioavailable. Our lab has examined the bioavailability of both quercetin and quercetin-3-glucoside from apple peel extracts and onion extracts in Caco-2 cells. Apple peel extracts contained no free quercetin, and no quercetin accumulation was seen in the Caco-2 cells following incubation with apple peel extract. Low amounts of quercetin-3-glucoside were absorbed by the cells (4%). However, onions contain some free quercetin and greater amounts of quercetin glucosides, and the absorption of quercetin into the Caco-2 cells from onion extracts was much greater than from apple extracts.The above results can be explained by recent research examining quercetin and quercetin glycoside bioavailability. In a study by Walle et al. [62], it was found that, in the ileostomy fluid, quercetin primarily existed as the aglycone form. The group hypothesized that β-glucosidases hydrolyzed quercetin glucosides to quercetin, which could be then passively transported [62]. In support of this theory, Day et al. [63] determined that quercetin glycosides were mainly deglycosylated by lactase phlorizin hydrolase before the aglycone then passed into the cell. Some intact glycoside transport by SGLT1 occurred and the glucosides were deglycosylated within the cell by cytosolic β-glucosidase. Quercetin-3-glucoside appeared to utilize only the lactase phlorizin hydrolase pathway, not the SLGT1 transporter, but quercetin-4-glucoside used both pathways [63]. Apples contain some quercetin-3-glucoside that, following hydrolysis by LPH, would be available for uptake by intestinal cells. However, apples also contain other conjugates such as quercetin rhamnosides, quercetin xylosides, and quercetin galactosides that are not easily hydrolyzed by lactase phlorizin hydrolase, and most likely are not readily absorbed by small intestinal cells. In comparison, the quercetin in onions is almost all in the form of quercetin glucosides and free quercetin, making it more bioavailable to small intestinal cells.Some bacterial degradation of quercetin conjugates most likely occurs in the human intestinal tract. Enterococcus casseliflavis and Eubacterium ramulus, microorganisms isolated from human feces, were both found to degrade quercetin-3-glucoside as a carbon and energy source [64]. Enterococcus casseliflavis utilized only the sugar moiety of the glucoside, whereas Eubacterium ramulus was also capable of degrading the aromatic ring system with phloroglucinol produced as an intermediate [64].Phloridzin, the glucoside conjugate of phloretin, is the major dihydrochalcone found in apples. Similarly to quercetin glucosides, phloridzin is thought to be hydrolyzed by lactase phloridzin hydrolase, and phloretin aglycone is taken up by the intestinal cells. When rats were fed phloridzin and phloretin, their plasma contained glucuronidated and sulfated phloretin but no phloridzin [65]. This supports the theory that phloridzin is hydrolyzed prior to uptake and further glucuronidation by intestinal epithelial cells. Phloridzin is known to be a potent SGLT1 inhibitor, but recently it has been discovered that phloridzin is also transported by SGLT1 [66]. However, phloridzin, as well as other flavonoid glucosides such as quercetin glucoside, is also effluxed by the multi-drug resistance protein (MRP1) [66].In human ileostomy subjects, chlorogenic acid absorption was approximately 33%, and only traces of chlorogenic acid was found in the urine [67]. The majority of chlorogenic acid will reach the large intestine and may be metabolized by the gut microflora. Gonthier et al (2003) found that rats fed chlorogenic acid excrete very little chlorogenic acid in their urine, but instead they excrete mainly microbial produced metabolites of chlorogenic acid, such as hippuric acid and m-coumaric acid [68]. A more recent study by Olthof et al (2003) involving human subjects showed that half of the ingested chlorogenic acid was converted to hippuric acid in the colon, most likely by microbial metabolism [69].Catechin and epicatechin are both absorbed by small intestinal epithelial cells [70]. In contrast to quercetin, epicatechin was not glucuronidated by human liver microsomes, nor was it glucuronidated by human small intestinal or large intestinal tissue [71]. Both liver and intestinal tissues contain UDP-glucuronosyltransferases (UGT) that are involved in the glucuronidation of various other flavonoids. Epicatechin was found to be sulfated by the human liver and intestinal cytosols, indicating that sulfation is the major metabolic pathway for epicatechin metabolism [71].

The mechanisms concerning the bioavailability of specific apple phytochemicals are becoming clearer as bioavailability research increases. In general, many flavonoid aglycones tend to pass through the intestinal epithelial cells where they are further conjugated. The flavonoid glycosides may be absorbed in small amounts, but most absorption seems to occur following hydrolysis by intestinal hydrolases such as lactase phloridzin hydrolase. Upon absorption these compounds are also conjugated. More research is still needed to understand the bioavailability of compounds from whole foods. The effects of the food matrix, interactions between compounds, digestion and processing on bioavailability of apple phytochemicals are still unknown.