Polyphenols: food sources and bioavailability

Transepithelial transport of rosmarinic acid in intestinal Caco-2 cell monolayers

The absorption characteristics of rosmarinic acid (RA) were examined by measuring permeation across Caco-2 cell monolayers using an HPLC-electrochemical detector (ECD) fitted with a coulometric detection system. RA exhibited nonsaturable transport even at 30 mM, and the permeation at 5 mM in the apical-to-basolateral direction, J(ap-->bl), was 0.13 nmol/min/mg of protein. This permeation rate is nearly the same as that of 5 mM chlorogenic acid (CLA) and gallic acid, which are paracellularly transported compounds. Almost all of the apically loaded RA was retained on the apical side, and J(ap-->bl) was inversely correlated with paracellular permeability. These results indicate that RA transport was mainly via paracelluar diffusion, and the intestinal absorption efficiency of RA was low. Furthermore, RA appeared to be unsusceptible to hydrolysis by mucosa esterase in Caco-2 cells. These results, together with our previous work (J. Agric. Food Chem., 52, 2518-2526 (2004), J. Agric. Food Chem., 52, 6418-6424 (2004)) suggest that the majority of RA is further metabolized and degraded into m-coumaric and hydroxylated phenylpropionic acids by gut microflora, which are then efficiently absorbed and distributed by the monocarboxylic acid transporter (MCT) within the body. The potential of orally administered RA in vivo will be further investigated.

Comparative effects of food-derived polyphenols on the viability and apoptosis of a human hepatoma cell line (HepG2)

Consumption of fruits and vegetables, which are rich in polyphenols, has been associated with a reduced risk of chronic diseases such as cancer. Dietary polyphenols have antioxidant and antiproliferative properties that might explain their beneficial effect on cancer prevention. The aim of this study was to investigate the effects of different pure polyphenols [quercetin, chlorogenic acid, and (-)-epicatechin] and natural fruit extracts (strawberry and plum) on viability or apoptosis of human hepatoma HepG2 cells. The treatment of cells for 18 h with quercetin and fruit extracts reduced cell viability in a dose-dependent manner; however, chlorogenic acid and (-)-epicatechin had no prominent effects on the cell death rate. Similarly, quercetin and strawberry and plum extracts, rather than chlorogenic acid and (-)-epicatechin, induced apoptosis in HepG2 cells. Moreover, quercetin and fruit extracts arrested the G1 phase in the cell cycle progression prior to apoptosis. Quercetin and strawberry and plum extracts may induce apoptosis and contribute to a reduced cell viability in HepG2 cells.

Dietary polyphenols decrease glucose uptake by human intestinal Caco-2 cells

The effect of different classes of dietary polyphenols on intestinal glucose uptake was investigated using polarised Caco-2 intestinal cells. Glucose uptake into cells under sodium-dependent conditions was inhibited by flavonoid glycosides and non-glycosylated polyphenols whereas aglycones and phenolic acids were without effect. Under sodium-free conditions, aglycones and non-glycosylated polyphenols inhibited glucose uptake whereas glycosides and phenolic acids were ineffective. These data suggest that aglycones inhibit facilitated glucose uptake whereas glycosides inhibit the active transport of glucose. The non-glycosylated dietary polyphenols appear to exert their effects via steric hindrance, and (-)-epigallochatechingallate, (-)-epichatechingallate and (-)-epigallochatechin are effective against both transporters.

Effects of blood orange juice intake on antioxidant bioavailability and on different markers related to oxidative stress

Orange juice is a source of antioxidants that might afford in vivo protection from oxidative stress. To test this hypothesis, we carried out a human intervention study with blood orange juice containing high amounts of vitamin C, anthocyanins, and carotenoids. Sixteen healthy female volunteers were enrolled in a crossover study and were given 600 mL/day of blood orange juice or a diet without juice for 21 days. Before and after each intervention period, plasma vitamin C, cyanidin-3-glucoside, and carotenoids were quantified. Furthermore, plasma antioxidant capacity, malondialdehyde concentration in plasma, 11-dehydrotromboxane B(2) urinary excretion, and lymphocyte DNA damage were evaluated as biomarkers of oxidative stress. Blood orange juice consumption determined a significant increase in plasma vitamin C, cyanidin-3-glucoside, beta-cryptoxanthin, and zeaxanthin. Also, lymphocyte DNA resistance to oxidative stress was improved whereas no effect was observed on the other markers that we analyzed. In turn, these results suggest that blood orange juice is a bioavailable source of antioxidants, which might moderately improve the antioxidant defense system; however, the long-term effects of its consumption are to be further investigated.

The effects of coffee consumption on lipid peroxidation and plasma total homocysteine concentrations: a clinical trial

Despite extensive research, the cardiovascular effects of coffee consumption in humans remain controversial. Our aim was to investigate the excretion of coffee phenols and the effects of filtered coffee consumption on oxidative stress and plasma homocysteine (tHcy) concentration in humans. The study consisted of a multiple-dose clinical supplementation trial and a single-dose study. In the long-term trial, 43 healthy nonsmoking men optionally consumed daily either no coffee, 3 cups (450 mL), or 6 cups (900 mL) of filtered coffee for 3 weeks, while in the short-term study 35 subjects consumed a single dose of 0, 1 (150 mL), or 2 cups (300 mL) of coffee. Long-term consumption of coffee increased the urinary excretion of caffeic and ferulic acid. The change in the total excretion of phenolic acids in 3 and 6 cups groups represented 3.8 and 2.5% of the amount ingested daily. Plasma tHcy concentrations increased nonsignificantly, but the consumption of coffee had neither short-nor long-term effects on lipid peroxidation or the activity of measured antioxidant enzymes. In conclusion, the consumption of filtered coffee does not have any detectable effects on lipid peroxidation in healthy nonsmoking men. The effect of coffee consumption on tHcy concentrations needs further investigation.

QTc prolongation by grapefruit juice and its potential pharmacological basis: HERG channel blockade by flavonoids

BACKGROUND

A high intake of dietary flavonoids, which are abundant in fruits, vegetables, tea, and wine, is known to reduce cardiovascular mortality. The effects of flavonoids on cardiac electrophysiology, which theoretically may have both antiarrhythmic and proarrhythmic consequences, have not been studied systematically to date.

METHODS AND RESULTS

We screened a broad spectrum of flavonoids for their inhibitory activity on HERG channels by using heterologous expression in Xenopus oocytes. At a concentration of 1 mmol/L, 10 compounds caused a significant inhibition of HERG currents, whereas 11 other flavonoids had no effect. The IC50 value for HERG block by naringenin, the most potent inhibitor, was 102.3 micromol/L in Xenopus oocytes and 36.5 micromol/L in HEK cells. To demonstrate the physiological relevance of these findings, we studied the effects of pink grapefruit juice, which contains large amounts of naringenin glycosides (>1000 micromol/L), in human volunteers. In 10 persons, we observed a peak QTc prolongation of 12.5+/-4.2 ms 5 hours after oral ingestion of 1 L of grapefruit juice. This effect was significant (P=0.02).

CONCLUSIONS

We found a significant QTc prolongation by grapefruit juice in healthy volunteers, probably caused by block of HERG channels by flavonoids. These findings reveal new perspectives on the potential for dietary modification of cardiac electrophysiology.

Transepithelial transport of microbial metabolites of quercetin in intestinal Caco-2 cell monolayers

m-Hydroxyphenylacetic acid (mHPA), 3,4-dihydroxyphenylacetic acid (DHPA), and 4-hydroxy-3-methoxyphenylacetic acid (HMPA) are major microbial metabolites of quercetin. After administration of quercetin to human subjects, these metabolites are readily detected in blood and urine. mHPA, DHPA, and HMPA are thought to exert protective biological activity within the body due to their antioxidant properties. However, very little work has been published concerning their absorption. I have examined the absorption characteristics of the quercetin metabolites in Caco-2 cells by a coulometric detection method using HPLC-ECD. All of them exhibited nonsaturable transport in Caco-2 cells up to 30 mM, whereas HMPA and mHPA also showed proton-coupled polarized absorption. The proton-coupled directional transport of HMPA and mHPA was inhibited by the substrate of the monocarboxylic acid transporter (MCT). A considerable amount of apically loaded HMPA and mHPA was taken up and transported through to the basolateral side, while almost all of the apically loaded DHPA was retained on the apical side. Furthermore, the transepithelial flux of DHPA was inversely correlated with the paracellular permeability of Caco-2 cells, although those of HMPA and mHPA were almost constant. These results indicate that transport of DHPA was mainly via paracellular diffusion, although HMPA and mHPA were absorbed to some extent by the MCT.

Polyphenols and prevention of cardiovascular diseases

PURPOSE OF REVIEW

Polyphenols are the most abundant dietary antioxidants and research on their role in the prevention of degenerative diseases has developed quickly over these last few years. This paper reviews the recent studies on the prevention of cardiovascular diseases by polyphenols, focusing on human studies.

RECENT FINDINGS

A large number of recent intervention studies have shown that several biomarkers of cardiovascular risk are influenced by the consumption of polyphenol-rich foods. Effects on biomarkers of oxidative stress, lipemia and inflammation appear so far inconclusive. More consistent effects have been observed on endothelial function and haemostasis and support a reduction of risk by polyphenols in agreement with the few epidemiological studies already published. All clinical studies have used foods or beverages containing a mixture of different polyphenols and the exact nature of the most active compounds remains largely unknown. Absorption, metabolism and elimination vary widely between polyphenols. These data on bioavailability should be taken into account to improve the experimental design and the interpretation of the observed effects.

SUMMARY

Future intervention studies should include a detailed assessment of the bioavailability of polyphenols. Beyond clinical trials carried out with polyphenol-rich foods, more studies with pure polyphenols will also be needed to establish their role in the prevention of cardiovascular diseases.

Dietary proanthocyanidins: Occurrence, dietary intake, bioavailability, and protection against cardiovascular disease

The French have one of the lowest incidences of coronary heart disease in the Western world despite a diet with a relatively high fat content. This phenomenon that has puzzled researchers worldwide for more than a decade is known as the 'French paradox' and has been linked to the high consumption of red wine in France. Red wine is rich in the complex polyphenols, the proanthocyanidins, and these compounds have recently attracted attention as potential cardiac-protective compounds. The present review summarizes the literature on proanthocyanidins with focus on their chemical structure, the occurrence, the daily intake from foods, the bioavailability and metabolism, and the evidence for a protective effect against cardiovascular diseases.

Metabolism of antioxidant and chemopreventive ellagitannins from strawberries, raspberries, walnuts, and oak-aged wine in humans: identification of biomarkers and individual variability

Ellagitannins (ETs) are dietary polyphenols, containing ellagic acid (EA) subunits, with antioxidant and cancer chemopreventive activities that might contribute to health benefits in humans. However, little is known about their metabolic fate. We investigate here the metabolism of different dietary ETs and EA derivatives in humans. Forty healthy volunteers were distributed in four groups. Each group consumed, in a single dose, a different ET-containing foodstuff, i.e., strawberries (250 g), red raspberries (225 g), walnuts (35 g), and oak-aged red wine (300 mL). After the intake, five urine fractions (F) were collected at 8 (F1), 16 (F2), 32 (F3), 40 (F4), and 56 (F5) h. Neither ETs nor EA were detected in urine after LC-MS/MS analysis. However, the microbial metabolite 3,8-dihydroxy-6H-dibenzo[b,d]pyran-6-one (urolithin B) conjugated with glucuronic acid was detected along the fractions F3-F5 in all of the subjects, independently of the consumed foodstuff. The mean percentage of metabolite excretion ranged from 2.8 (strawberries) to 16.6% (walnuts) regarding the ingested ETs. Considerable interindividual differences were noted, identifying "high and low metabolite excreters" in each group, which supported the involvement of the colonic microflora in ET metabolism. These results indicate that urolithin B (a previously described antiangiogenic and hyaluronidase inhibitor compound) is a biomarker of human exposure to dietary ETs and may be useful in intervention studies with ET-containing products. The antioxidant and anticarcinogenic effects of dietary ETs and EA should be considered in the gastrointestinal tract whereas the study of potential systemic activities should be focused on the bioavailable urolithin B derivatives.

Capillary electrophoresis of highly sulfated flavanoids and flavonoids

Flavanoids and flavonoids are natural products present in our diet and known to possess multiple biological activities. Sulfated species of these natural products represent highly charged water-soluble organic molecules that possess unique biochemical properties. We describe here the first studies on capillary electrophoresis of these highly charged molecules. Fully sulfated flavanoids and flavonoids can be electrophoresed and resolved under reverse polarity at pH 3.5 using 5-10 kV in less than 20 min. In contrast, at high pH under normal polarity these species can be electrophoresed only if a pressurized capillary is employed. (+/-)-Catechin sulfate, a racemic sulfated flavanoid, was resolved into its enantiomers using 15% beta-cyclodextrin, a chiral selector, but not with alpha- or gamma-cyclodextrins. Yet, the high charge density of these molecules challenges the resolving capability of capillary electrophoresis as diastereomers (-)-epicatechin sulfate and (+)-catechin sulfate do not resolve, even in the presence of cyclodextrins or chiral positively charged amino acids. Overall, capillary electrophoresis of highly sulfated flavanoids and flavonoids is expected to be useful in rapid structure analysis of sulfated flavonoids, either synthetic or natural.

Polyphenols: antioxidants and beyond

Research on the effects of dietary polyphenols on human health has developed considerably in the past 10 y. It strongly supports a role for polyphenols in the prevention of degenerative diseases, particularly cardiovascular diseases and cancers. The antioxidant properties of polyphenols have been widely studied, but it has become clear that the mechanisms of action of polyphenols go beyond the modulation of oxidative stress. This supplemental issue of The American Journal of Clinical Nutrition, published on the occasion of the 1st International Conference on Polyphenols and Health, offers an overview of the experimental, clinical, and epidemiologic evidence of the effects of polyphenols on health.

Analytical approaches to the determination of simple biophenols in forest trees such as Acer(maple), Betula(birch), Coniferus, Eucalyptus, Juniperus(cedar), Picea(spruce) and Quercus(oak)

Analytical methods are reviewed for the determination of simple biophenols in forest trees such as Acer (maple), Betula (birch), Coniferus, Eucalyptus, Juniperus (cedar), Picea (spruce) and Quercus (oak). Data are limited but nevertheless clearly establish the critical importance of sample preparation and pre-treatment in the analysis. For example, drying methods invariably reduce the recovery of biophenols and this is illustrated by data for birch leaves where flavonoid glycosides were determined as 12.3 +/- 0.44 mg g(-1) in fresh leaves but 9.7 +/- 0.35 mg g(-1) in air-dried samples (data expressed as dry weight). Diverse sample handling procedures have been employed for recovery of biophenols. The range of biophenols and diversity of sample types precludes general procedural recommendations. Caution is necessary in selecting appropriate procedures as the high reactivity of these compounds complicates their analysis. Moreover, our experience suggests that their reactivity is very dependent on the matrix. The actual measurement is less contentious and high performance separation methods particularly liquid chromatography dominate analyses whilst coupled techniques involving electrospray ionization are becoming routine particularly for qualitative applications. Quantitative data are still the exception and are summarized for representative species that dominate the forest canopy of various habitats. Reported concentrations for simple phenols range from trace level (<0.1 microg g(-1)) to in excess of 500 microg g(-1) depending on a range of factors. Plant tissue is one of these variables but various biotic and abiotic processes such as stress are also important considerations.

Application of liquid chromatography/electrospray ionization tandem mass spectrometry to the analysis of polyphenolic compounds from an infusion of Byrsonima crassa Niedenzu

A fast and reliable method, based on high-performance liquid chromatography coupled to electrospray ionization ion trap tandem mass spectrometry (HPLC/ESI-ITMS), was developed to investigate the infusion prepared from the leaves of Byrsonima crassa Niedenzu (Malpighiaceae), a native plant used in Brazil against gastric disorders. The use of on-line reverse-phase HPLC/ESI-ITMS allowed separation of three major classes of compounds and identification of over 20 very polar compounds characterized as galloylquinic acids, proanthocyanidins, and flavonoid glycosides, as well as the dimeric flavonoid amentoflavone and minor amounts of galloyl hexose and galloyl saccharose. This approach provided data that will allow establishment of a method for a future standardization of the infusion.

Risks and safety of polyphenol consumption

This article gives an overview of the potential hazards of polyphenol consumption, as reported during the round-table discussion at the 1st International Conference on Polyphenols and Health, held in Vichy, France, November 2003. Adverse effects of polyphenols have been evaluated primarily in experimental studies. It is known, for example, that certain polyphenols may have carcinogenic/genotoxic effects or may interfere with thyroid hormone biosynthesis. Isoflavones are of particular interest because of their estrogenic activity, for which beneficial as well as detrimental effects have been observed. Furthermore, consumption of polyphenols inhibits nonheme iron absorption and may lead to iron depletion in populations with marginal iron stores. Finally, polyphenols may interact with certain pharmaceutical agents and enhance their biologic effects. It is important to consider the doses at which these effects occur, in relation to the concentrations that naturally occur in the human body. Future studies evaluating either beneficial or adverse effects should therefore include relevant forms and doses of polyphenols and, before the development of fortified foods or supplements with pharmacologic doses, safety assessments of the applied doses should be performed.

Bioavailability and bioefficacy of polyphenols in humans. I. Review of 97 bioavailability studies

Polyphenols are abundant micronutrients in our diet, and evidence for their role in the prevention of degenerative diseases is emerging. Bioavailability differs greatly from one polyphenol to another, so that the most abundant polyphenols in our diet are not necessarily those leading to the highest concentrations of active metabolites in target tissues. Mean values for the maximal plasma concentration, the time to reach the maximal plasma concentration, the area under the plasma concentration-time curve, the elimination half-life, and the relative urinary excretion were calculated for 18 major polyphenols. We used data from 97 studies that investigated the kinetics and extent of polyphenol absorption among adults, after ingestion of a single dose of polyphenol provided as pure compound, plant extract, or whole food/beverage. The metabolites present in blood, resulting from digestive and hepatic activity, usually differ from the native compounds. The nature of the known metabolites is described when data are available. The plasma concentrations of total metabolites ranged from 0 to 4 mumol/L with an intake of 50 mg aglycone equivalents, and the relative urinary excretion ranged from 0.3% to 43% of the ingested dose, depending on the polyphenol. Gallic acid and isoflavones are the most well-absorbed polyphenols, followed by catechins, flavanones, and quercetin glucosides, but with different kinetics. The least well-absorbed polyphenols are the proanthocyanidins, the galloylated tea catechins, and the anthocyanins. Data are still too limited for assessment of hydroxycinnamic acids and other polyphenols. These data may be useful for the design and interpretation of intervention studies investigating the health effects of polyphenols.

FRUITS AND THE PREVENTION OF LIFESTYLE-RELATED DISEASES

1. Fruits contain a variety of compounds that may slow or prevent chronic diseases through a variety of possible mechanisms. Components in fruits thought to be associated with the reduction of these conditions include soluble and insoluble dietary fibre, anti-oxidant nutrients (vitamins C, E, selenium, beta-carotene) and phytonutrients (bioactive plant compounds that impart colour, flavour and other functional properties to foods). 2. The present paper briefly reviews selected studies on the role of some fruits in lifestyle conditions such as cardiovascular disease, some types of cancer and conditions that accompany ageing.

Vitamin E and breast cancer

Vitamin E is a term that describes a group of compounds with similar yet unique chemical structures and biological activities. One interesting property possessed by certain vitamin E compounds-namely, delta-tocotrienol, RRR-alpha-tocopheryl succinate [vitamin E succinate (VES), a hydrolyzable ester-linked succinic acid analogue of RRR-alpha-tocopherol], and a novel vitamin E analogue referred to as alpha-TEA (alpha-tocopherol ether linked acetic acid analogue, which is a stable nonhydrolyzable analogue of RRR-alpha-tocopherol)-is their ability to induce cancer cells but not normal cells to undergo a form of cell death called apoptosis. In contrast, the parent compound, RRR-alpha-tocopherol, also referred to as natural or authentic vitamin E and known for its antioxidant properties, does not induce cancer-cell apoptosis. Efforts to understand how select vitamin E forms can induce cancer cells to undergo apoptosis have identified several nonantioxidant biological functions, including restoration of pro-death transforming growth factor-beta and Fas signaling pathways. Recent studies with alpha-TEA show it to be a potent inducer of apoptosis in a wide variety of epithelial cancer cell types, including breast, prostate, lung, colon, ovarian, cervical, and endometrial in cell culture, and to be effective in significantly reducing tumor burden and metastasis in a syngeneic mouse mammary tumor model, as well as xenografts of human breast cancer cells. Studies also show that alpha-TEA, in combination with the cyclooxygenase-2 inhibitor celecoxib and the chemotherapeutic drug 9-nitro-camptothecin decreases breast cancer animal model tumor burden and inhibits metastasis significantly better than do single-agent treatments.