How should we assess the effects of exposure to dietary polyphenols in vitro?

¹ From the Nutrition Division, Institute of Food Research, Norwich, United Kingdom (PAK); the Centre for Nutrition and Food Safety, School of Biomedical and Molecular Sciences, University of Surrey, Guildford, United Kingdom (MNC); the Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, University of Glasgow, United Kingdom (AC); the Procter Department of Food Science, University of Leeds, United Kingdom (AJD); the Laboratory of Drug Disposition and Pharmacogenetics, Medical University of South Carolina, Charleston, SC (JLD); the Unité des Maladies Metaboliques et Micronutriments, INRA de Clermont-Ferand/Theix, St Genes-Champanelle, France (CM); and the Nestlé Research Center, Lausanne, Switzerland (GW)

² Supported by the Biotechnology and Biological Sciences Research Council, United Kingdom.

³ Address reprint requests to PA Kroon, Institute of Food Research, Colney Lane, Norwich NR4 7UA, United Kingdom. E-mail: paul.kroon{at}bbsrc.ac.uk.

ABSTRACT  
Human intervention studies have provided clear evidence that dietary polyphenols (eg, flavonoids—eg, flavonols—and isoflavones) are at least partly absorbed and that they have the potential to exert biological effects. Biological activity of polyphenols is often assessed by using cultured cells as tissue models; in almost all such studies, cells are treated with aglycones or polyphenol-rich extracts (derived from plants and foods), and data are reported at concentrations that elicited a response. There are 2 inherent flaws in such an approach. First, plasma and tissues are not exposed in vivo to polyphenols in these forms. Several human studies have identified the nature of polyphenol conjugates in vivo and have shown that dietary polyphenols undergo extensive modification during first-pass metabolism so that the forms reaching the blood and tissues are, in general, neither aglycones (except for green tea catechins) nor the same as the dietary source. Polyphenols are present as conjugates of glucuronate or sulfate, with or without methylation of the catechol functional group. As a consequence, the polyphenol conjugates are likely to possess different biological properties and distribution patterns within tissues and cells than do polyphenol aglycones. Although deconjugation can potentially occur in vivo to produce aglycone, it occurs only at certain sites. Second, the polyphenol concentrations tested should be of the same order as the maximum plasma concentrations attained after a polyphenol-rich meal, which are in the range of 0.1–10 µmol/L. For correct interpretation of results, future efforts to define biological activities of polyphenols must make use of the available data concerning bioavailability and metabolism in humans.

Key Words: Polyphenols • flavonoids • isoflavones • phytochemicals • plant bioactives • antioxidants • human metabolism • first-pass metabolism • conjugation • quercetin

INTRODUCTION  
Polyphenols have been shown, in both in vitro test systems and small animal models, to induce responses consistent with the protective effects of diets rich in fruit and vegetables against degenerative conditions such as cardiovascular disease (CVD) and cancer (1, 2). In fact many polyphenols, particularly flavonoids (eg, flavonols) and isoflavones, showed potent bioactivity when tested in vitro, which led to clinical trials assessing them with respect to a variety of effects (3, 4). However, because clinical studies are expensive and time-consuming, it is also necessary to optimize the use and interpretation of in vitro experiments.

Human tissues are exposed to polyphenols via the blood, which is the only route through which dietary polyphenols can reach tissues and their cells, except for the cells lining the intestinal tract. Understanding that polyphenols are substantially modified during absorption and identifying the physiologically relevant conjugates are essential to the planning of meaningful in vitro studies of polyphenol activity. Some controversy has attended hypotheses about the nature of circulating conjugates for particular polyphenols, but recent improvements in the analytic methods have resolved many of these questions. In the past few years, studies using physiologic concentrations of polyphenol conjugates helped clarify their specific mechanisms of action in vivo and advanced the field of understanding polyphenols in relation to health. In this article, we briefly discuss the arguments for using physiologic polyphenol conjugates to assess biological responses in vitro, and we define both what is known about polyphenol conjugates in vivo and where the gaps are in our knowledge of this subject.

HOW ARE POLYPHENOLS METABOLIZED?  
The metabolism of several common polyphenols is now reasonably well understood. An important fact is that polyphenols are extensively altered during first-pass metabolism so that, typically, the molecular forms reaching the peripheral circulation and tissues are different from those present in foods (5-10). The term metabolism is used here to describe the typical modifications that occur during or after absorption. In general, the resulting metabolites are conjugates (eg, sulfates and glucuronates) of the parent aglycone or conjugates of methylated parent aglycones. Catabolism of polyphenols in humans usually occurs only as a result of microbial activity in the (large) intestine.

Most polyphenol glucosides are deglycosylated by ß-glucosidases in the small intestine, namely, the broad-specificity cytosolic ß-glucosidase and lactase phlorizin hydrolase; this step is requisite for the absorption of many of these polyphenols (see, for example, 11, 12). After absorption, flavonoids are metabolized by the phase II drug–metabolizing enzymes, the uridine-5'-diphosphate glucuronosyl-transferases, sulfotransferases, and catechol-O-methyltransferases. The resulting molecules are glucuronate and sulfate conjugates with or without methylation across the catechol functional group, and many are multiply conjugated (Figure 1). The small intestine appears to be the organ primarily responsible for glucuronidation, but it also has a role in methylation (13, 14). The major products of small-intestine metabolism in the hepatic portal vein are glucuronides and perhaps methylated glucuronides. The conjugates may then gain access to hepatocytes and may be further methylated, glucuronidated, or sulfated therein (13, 15).

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FIGURE 1.. The structure of quercetin (3,3',4,5,7-pentahydroxyflavone). Quercetin contains 5 hydroxyl functional groups that have the potential to be conjugated and that differ in their inherent chemical reactivities (3 > 7 > 3' > 4' >> 5). Quercetin in human plasma is found as sulfate and glucuronate conjugates, and conjugation occurs at positions 3, 3', and 4', but not at position 5 or 7. Methylation of the catechol function (3'/4'-dihydroxy in the B-ring) also occurs, which gives rise to methylated conjugates.

 
It is instructive to compare the fates of dietary polyphenols and of (oral) pharmaceuticals. Most drugs are designed or selected to be relatively slowly metabolized, and they are generally delivered at a dose high enough that most of the dose escapes first-pass metabolism. This ensures that a sufficient amount of the active drug (in the unmetabolized, unconjugated form) is delivered to the appropriate tissues. In contrast, polyphenols delivered through human diets are at low doses, and, in most cases, they do not escape first-pass metabolism (16). The net result of the extensive first-pass metabolism of dietary polyphenols is that, with only a few exceptions, the predominant (and very often exclusive) forms in plasma are conjugates (glucuronates or sulfates, with or without methylation). These conjugates are chemically distinct from their parent compounds, differing in size, polarity, and ionic form. Consequently, their physiologic behavior is likely to be different from that of the native compounds. Glucuronates and sulfates are also negatively charged at physiologic pH. Therefore, to assess in vitro (eg, by using cultured cells) the possible contribution of polyphenols to the overall protection against degenerative diseases afforded by diets rich in fruit and vegetables and to define mechanisms, it is crucial that in vitro studies are designed to use relevant conjugates found in vivo.

Although the processes of glucuronidation, sulfation, and methylation are now well established and accepted, there are numerous sites of possible conjugation, and recent efforts have focused on identifying specific structures that exist in vivo.

IDENTIFYING STRUCTURES OF PLASMA POLYPHENOLS IS KEY TO DEFINING THEIR BIOLOGICAL ACTIVITIES IN HUMANS  
Advances in the understanding of polyphenol metabolism have been made possible by improvements in the analytic methods used, particularly the use of mass spectrometry in combination with high-resolution chromatography systems (especially reversed-phase HPLC) and with detection systems such as mass spectrometry, coulometric electrochemical, and diode array. Whereas most studies up to the middle or late 1990s measured total aglycones in plasma and urine after chemical or enzymatic deconjugation, or both (eg, 17), several studies now report the polyphenol conjugate composition of human plasma or urine samples after the ingestion of polyphenols or polyphenol-rich foods.

The conjugates and approximate concentrations of common dietary polyphenols present in vivo after oral consumption of a physiologically relevant amount of a common dietary source are summarized in Table 1. Although it has been established that products of microbial transformation (eg, ring-fission products) form in humans and that they are excreted in urine, those products have not been included because they (eg, hydroxylated phenylacetic acids and hippuric acid) are common to many polyphenols, and there is little information regarding their biological activity.

View this table:
TABLE 1. Summary of evidence for polyphenol structures in human plasma and urine

 
The flavonoid quercetin (flavonol subclass) is one of the most extensively studied polyphenols. It serves as a good example here because its metabolism in humans is well understood, and many conjugates have been identified. Flavonols are found in foods mainly as glycoside conjugates. The flavonol conjugates that have been identified in plasma and urine from persons fed quercetin-containing foods are not those found in food. For example, plasma samples from volunteers receiving quercetin orally (as an onion meal, buckwheat tea, or pure quercetin, quercetin-4'-glucoside, quercetin-3-glucoside, or quercetin-rutinoside supplements) contained conjugated forms of quercetin but not quercetin glucosides, quercetin rutinoside, or quercetin aglycone (6, 7, 18, 52, 53). Day et al (6) showed that the plasma of volunteers fed fried onions (containing quercetin-4'-glucoside and quercetin-3,4'-di-glucoside) contained a mixture of glucuronidated and sulfated conjugates of quercetin and methylquercetin. A total of 12 discrete quercetin conjugates were detected, and several were identified by using a combination of retention time, ultraviolet spectra, shift reagents, mass spectrometry, and comparison with authentic standards. Three of the 4 major conjugates of quercetin in plasma were identified positively as quercetin-3-glucuronide, 3'-methylquercetin-3-glucuronide, and quercetin-3'-sulfate, respectively. The fourth major conjugate was identified as one of several isomeric quercetin diglucuronides. Evidence was also obtained for the presence in human plasma of lower concentrations of quercetin-3'-glucuronide and 3'-methylquercetin-4'-glucuronide.

The metabolisms of most other dietary polyphenols in humans are comparable in several ways: (1) glycosides are generally not found in plasma or urine in the form ingested, (2) the major forms in plasma and urine are sulfate and glucuronate conjugates of the parent aglycones, (3) methylation may occur on polyphenols that contain orthohydroxy functional groups, and (4) aglycones are absent or constitute only a very small proportion of the total amount of polyphenols present, except in green tea catechins, of which aglycones can constitute a significant proportion of the total amount in plasma (54). Furthermore, although isoflavones are usually glycosylated in foods (the exceptions are fermented soy products such as tempeh), a small but significant proportion (7%) exists in the plasma as aglycones (25), and the remainder is present as sulfate and glucuronate conjugates (10, 25, 26). Details are presented in Table 1. Originally, anthocyanins were thought to be an exception to item 2 above, because anthocyanin glucosides have been identified in human plasma and urine (see Table 1; 40-47), albeit at low concentrations (pmol/L–nmol/L range). In all but 2 of these reports (46, 47), the anthocyanin glycosides were the only form present in plasma or urine or both, and the urinary yield was extremely low (<0.05%). However, a recent report using improved methods and describing the conjugate profile of human urine after the ingestion of strawberries that contained pelargonidin glycosides showed that glucuronate and sulfate conjugates were the predominant structures (98% of total) and indicated a urinary yield of 2% (46). It is clear that the results from the earlier studies must be viewed with some caution because it is unlikely that pelargonidin would differ so dramatically from the closely related anthocyanins in the extent of its absorption and susceptibility to phase II metabolism.

EFFECT OF POLYPHENOL METABOLISM ON BIOLOGICAL ACTIVITY  
What, then, are the effects of metabolism on the biological activities of quercetin? It has been shown that some conjugates of quercetin retain antioxidant properties and the ability to inhibit lipoxygenase and xanthine oxidase in vitro (55). Furthermore, quercetin glucuronides were shown to inhibit the N-acetylation of 2-aminofluroene (an arylamine carcinogen) by human acute myeloid leukemia HL-60 cells (56). Further studies show that quercetin-3-glucuronide (a major human conjugate of dietary quercetin) is able to prevent angiotensin-II–induced vascular smooth muscle cell hypertrophy in cultured rat aortic smooth muscle cells through its inhibitory effects on the JNK and AP-1 signaling pathways (57), possesses a substantial antioxidant effect on copper ion–induced oxidation of human plasma LDL as well as on 1,1-diphenyl-2-picrylhydrazyl radical-scavenging activity (58), and suppresses the peroxynitrite-induced consumption of lipophilic antioxidants in human plasma LDL (59). In general, the responses to quercetin conjugates were weaker than those to the aglycone. For example, the antioxidant activity of quercetin conjugates is, on average, about half that of aglycone, but there is significant variation according to the position of conjugation (55). Quercetin-3-glucuronide, one of the 3 major plasma quercetin conjugates, significantly delayed the Cu(II)-induced oxidation of human LDL ex vivo, but 2 other major conjugates (quercetin-3'-sulfate and 3'-methylquercetin-3-glucuronide) were largely ineffective (60). In contrast, the inhibition of JNK and AP-1 signaling pathways in rat aortic smooth muscle cells by quercetin-3-glucuronide occurred at concentrations similar to those of the aglycone (57).

In the aglycone form, quercetin is a powerful antioxidant in vitro (61). Indeed, in vitro studies using quercetin aglycone have shown its potential as an agent for the prevention or treatment (or both) of various cancers, CVDs, inflammation, dementia, and cataract. However, quercetin is highly unstable and is reactive at physiologic pH values (62, 63). Damaging effects, especially on kidney, were observed after very high doses of quercetin were given to volunteers intravenously (3), thus bypassing the protection afforded by the gastrointestinal epithelium and phase II conjugation. Such phase II conjugations disrupt the electron delocalization of the quercetin ring structure so that quercetin conjugates have a reduced tendency to undergo redox cycling, although their ability to function as antioxidants is not complete abolished. Fortunately, quercetin is present in foods almost exclusively as glycosides; onions, tea, and apples are the most important dietary sources (17, 54, 64). Very few foods contain significant amounts of quercetin as aglycone; some red wines are a notable exception (65). Hence, dietary quercetin is unlikely to have a damaging effect on the body.

Although we have focused on quercetin as an example, it is worth mentioning the findings from some similar studies with other dietary polyphenols. Daidzein and genistein monoglucuronides are the major isoflavone conjugates in human plasma after ingestion of soy (66), and they possess some estrogenic properties but are weaker than those of their corresponding aglycones. With the exception of the gallate esters in green tea, orally delivered catechins appear in plasma predominantly (>98%) as conjugated forms (sulfated or glucuronidated conjugates, or both, with or without methylation; 31, 33-36, 67-69), and the major conjugates of (–)-epicatechin have been identified (see Table 1). It is noteworthy that the plasma conjugates (predominantly glucuronides and sulfates of (+)-catechin and methylated (+)-catechin) obtained after oral administration of pure (+)-catechin to rats effectively inhibited both the generation of reactive oxygen species and the binding of U937 monocyte cells to interleukin 1ß–stimulated human aortic endothelial cells, whereas (+)-catechin did not do so (70).

IN VIVO POLYPHENOL CONJUGATES: FACT AND FICTION  
The past few years have seen very significant advances in our understanding of polyphenol metabolism. However, controversy remains concerning the nature and properties of flavonoid conjugates in vivo, and that uncertainty hampers progress toward understanding the real contribution of flavonoids as dietary protective agents against cancer, CVD, and other diseases. The combination of very complex conjugate profiles, the difficulty in obtaining in vivo conjugates, and the seemingly endless variety of possible endpoints for the demonstration of biological effects provides significant challenges to those working in this scientific field. High-quality scientific data in this area are therefore extremely valuable. Conversely, conclusions drawn from poorly designed studies have the potential to be misleading.

Many reports describe in vitro bioactivity studies that used polyphenol aglycones, food, or herbal extracts. Some of these reports produced confusing or difficult-to-interpret results, especially if the reports claim to have identified a polyphenol as the much-searched-for chemopreventive agent for cancer, CVD, and other such illnesses or to have identified a health risk associated with polyphenols. The confusion and difficulty in interpreting results are more widespread when the reports are published in high-profile journals.

Also of concern are the growing numbers of reports in which the authors' claims of using physiologic conjugates are not supported by their own or any literature evidence. For example, Spencer et al (71) stated that their recent study used the major in vivo human conjugates of quercetin in cell culture to investigate the potential uptake of quercetin and assess cytotoxicity and cytoprotection in dermal fibroblasts. The authors stated, without evidence, that the major reported in vivo human conjugates of quercetin are quercetin-7-glucuronide and the aglycones 3'-methylquercetin (isorhamnetin) and 4'-methylquercetin (tamarixetin). We are not aware of any reports in the scientific literature that support this statement. Whereas the use of these conjugates for in vitro studies may be a step in the right direction, the biological activity of conjugates also may differ significantly among the positional isomers (55, 58, 72). It is critical that all future studies attempting to use in vitro models to assess the effects of polyphenols in humans use physiologic conjugates at appropriate concentrations.

CONCLUSIONS AND IMPLICATIONS FOR FUTURE RESEARCH  
Identification and measurement of the physiologic polyphenol conjugates are key prerequisites to an understanding of the role of dietary polyphenols in human health. Acquiring such data will permit more reliable investigation of many phenomena by using cost-effective in vitro models. In the long term, the application of advanced metabolomic approaches and nanotechnologies has the potential to significantly advance our understanding in this area.

We strongly recommend that all experiments using in vitro models to study biological responses to dietary polyphenols use only physiologically relevant flavonoids and their conjugates at appropriate concentrations, provide evidence to support their use, and justify any conclusions generated. When authors fail to do this, referees and editors must act to ensure that data obtained in vitro are relevant to what might occur in vivo.

ACKNOWLEDGMENTS  
All authors contributed to the writing of the manuscript. All contributors read and commented on the manuscript.

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