Nature of dietary phenols

To understand the relationship between the intake of polyphenols and a reduced risk of developing diseases, it is essential to know the nature and distribution of phenolic compounds in our diet. Phenolic compounds can be classified into different groups according to the number of phenol rings in the molecule and the substitution groups of the phenol rings. Distinctions are thus made between phenolic acids, flavonoids, stilbenes and lignans⁽ Reference Manach, Scalbert and Morand ^{12 )}. Among these compounds, flavonoids and phenolic acids are the most abundant polyphenols in the human diet⁽ Reference RiceEvans, Miller and Paganga ^{13 –} Reference Perez-Jimenez, Neveu and Vos ^{17 )}. Attention should also be paid to proanthocyanidins which are believed to be ubiquitous in plant foods. Proanthocyanidins are polyhydroxyflavan oligomers or polymers. The most common monomers are the diastereomers (+)-catechin/( − )-epicatechin, ( − )-gallocatechin/( − )-epigallocatechin and (+)-afzelechin/( − )-epiafzelechin, and the respective oligo- and polymers are called procyanidins, prodelphinidins and propelargonidins. It has been suggested that they account for a significant fraction of the polyphenols ingested in a Western diet⁽ Reference Serrano, Puupponen-Pimiä and Dauer ^{18 )}. Phenolic compounds are secondary metabolites of plants and afford protection against UV radiation, pathogens and herbivores⁽ Reference Harborne and Williams ^{19 )}. The anthocyanin co-pigments in flowers attract pollinating insects⁽ Reference Harborne and Williams ^{19 )} and are responsible for the characteristic red and blue colours of berries, wines and certain vegetables⁽ Reference Hammerstone, Lazarus and Schmitz ^{20 –} Reference Reinli and Block ^{26 )}. In addition, polyphenols contribute to the organoleptic properties of plant foods, especially by their astringency, bitter taste and participation in haze formation⁽ Reference Le Bourvellec and Renard ^{27 )}. To play these roles, most polyphenols are concentrated in the peels of many fruits and vegetables. It has been shown that for the jujube (Ziziphus jujuba Mill.), a traditional Chinese fruit, the peel has the highest content of total polyphenols⁽ Reference Zhang, Jiang and Ye ^{28 )}. The results are consistent with the data reported by Tomás-Barberán et al. ⁽ Reference Tomás-Barberán, Gil and Cremin ^{29 )}, who found that the peel tissues of nectarines, peaches and plums contained two-fold more flavonoids than other tissues.

Content of polyphenols in food and their absorption after intake

In the human diet, coffee, tea and red wine are major sources of polyphenols because of these liquids' high polyphenol content and relatively large serving sizes. The polyphenol content of main food sources is shown in Table 1 ⁽ Reference Astill, Birch and Dacombe ^{30 –} Reference Rivero, Perez-Magarino and Gonzalez-Sanjose ^{34 )}. Considering the amounts of total phenols in our diet, it has been reported that a well-balanced diet can provide well over 1000 mg of total phenols per d.

Table 1 Main sources of polyphenols in the human diet

After the intake of polyphenols, aglycones can be absorbed from the small intestine, and the maximal plasma concentrations often reach 1–2 h after ingestion⁽ Reference Aziz, Edwards and Lean ^{35 –} Reference Kivits, van der Sman and Tijburg ^{37 )}. However, most polyphenols in food are in the form of esters, glycosides or polymers that cannot be absorbed in the native form. These substances must be hydrolysed by intestinal enzymes before the substances can be absorbed⁽ Reference Manach, Scalbert and Morand ^{12 )}. In the case of quercetin glucosides, it has been suggested that these compounds could be hydrolysed inside cells by a cytosolic β-glucosidase⁽ Reference Day, DuPont and Ridley ^{38 )}. Another pathway involves lactase phloridzine hydrolase, a glucosidase of the brush-border membrane of the small intestine that catalyses the extracellular hydrolysis of certain glucosides, which is followed by the diffusion of aglycones across the brush border⁽ Reference Day, Cañada and Díaz ^{39 )}. Certain polyphenols with special structures cannot be hydrolysed by intestinal enzymes and thus cannot be absorbed until reaching the colon. Unlike intestinal enzymes, colonic microflora catalyse the breakdown of polyphenols into more simple compounds, such as phenolic acids. For example, when quercetin-3-O-rhamnoside was incubated anaerobically with human intestinal bacteria, quercetin, 3,4-dihydroxyphenylacetic acid and 4-hydroxybenzoic acid were found to be metabolites⁽ Reference Scalbert and Williamson ^{31 )}. Proanthocyanidins are a subclass of polyphenols. Since they are associated with the cell wall in plants by non-covalent or covalent bindings, it is believed that they undergo two main processes in the intestinal tract after intake: (1) partial depolymerisation into their constituent units⁽ Reference Spencer, Chaudry and Pannala ^{40 )}, mainly by the gastric milieu coupled with absorption of the free monomers and small oligomers (dimers, trimers) in the small intestine⁽ Reference Mateos-Martín, Pérez-Jiménez and Fuguet ^{41 )}; and (2) direct fermentation by the intestinal microbiota into smaller metabolites such as phenolic acids and their subsequent absorption⁽ Reference Deprez, Brezillon and Rabot ^{42 ,} Reference Rios, Gonthier and Rémésy ^{43 )}. It has been suggested that the chain length of proanthocyanidins has a more crucial role in their microbial metabolism than the dose. Short-chain proanthocyanidins exhibited a strong metabolite formation and a higher concentration of proanthocyanidins could be tolerated without inhibition of metabolite formation⁽ Reference Bazzocco, Mattila and Guyot ^{44 )}. On the other hand, proanthocyanidins with high polymerisation were able to inhibit the conversions by the intestinal microbiota⁽ Reference Aura, Mattila and Hyötyläinen ^{45 )}. The total yields of metabolites decrease significantly with increasing polymerisation⁽ Reference Rios, Gonthier and Rémésy ^{43 ,} Reference Bazzocco, Mattila and Guyot ^{44 ,} Reference Gonthier, Donovan and Texier ^{46 )}. It is possible for proanthocyanidins to pass through the entire gastrointestinal digestion largely intact, especially if their molecular weight is large⁽ Reference Serrano, Puupponen-Pimiä and Dauer ^{18 )}. Thus, there can be some intact parent compounds⁽ Reference Choy, Jaggers and Oteiza ^{47 )} and non-absorbable metabolites (probably mid-molecular-weight tannins)⁽ Reference Serrano, Puupponen-Pimiä and Dauer ^{18 )} after the metabolism of proanthocyanidins that remain in the colonic lumen where they may contribute to the health of the gastrointestinal tract.

After the hydrolysis of polyphenols into free aglycones and before passage into the bloodstream, polyphenols are conjugated by methylation, sulfation or glucuronidation, or a combination⁽ Reference Walle ^{48 ,} Reference Karakaya ^{49 )}. This conjugation is a metabolic detoxification process common to many xenobiotics. The conjugation is particularly important for increasing the molecular weight and solubility of phenolic aglycones, which reduces compounds' potential toxic effects and enhances their elimination ability⁽ Reference Day, Bao and Morgan ^{50 )}. The relative contribution of different types of conjugation (methylation, sulfation and glucuronidation) appears to vary according to the nature of the substrate and the dose ingested. Sulfation is generally a higher-affinity, lower-capacity pathway than glucuronidation, so that when the ingested dose increases, a shift from sulfation toward glucuronidation occurs⁽ Reference Koster, Halsema and Scholtens ^{51 ,} Reference Piskula ^{52 )}. The balance between the sulfation and the glucuronidation of polyphenols also seems to vary with species, sex and food deprivation⁽ Reference Piskula ^{52 )}.

Health benefits of polyphenols

Interaction of plant phenols with proteins

The interaction of polyphenols with proteins is an old but continuous research topic. Principally, two types of interactions can be classified between phenolic compounds and proteins: covalent and non-covalent binding⁽ Reference Kroll, Rawel and Rohn ^{95 ,} Reference Harbourne, Jacquier and O'Riordan ^{96 )}. The formation of covalent bonds between proteins and polyphenols is an irreversible reaction⁽ Reference Sastry and Rao ^{97 )}, involving the oxidation of phenolic compounds and the formation of o-quinones or o-semi-quinones⁽ Reference Kalyanaraman, Felix and Sealy ^{98 –} Reference Nicolas, Richard-Forget and Goupy ^{100 )} or the cleavage of proanthocyanidin interflavanic bonds in an acid medium with the formation of carbocations⁽ Reference Beart, Lilley and Haslam ^{101 )}. Physiologically and technologically relevant data from the literature suggest that the majority of protein–polyphenol interactions may occur by non-covalent binding⁽ Reference Rawel, Meidtner and Kroll ^{102 )}, although both covalent and non-covalent binding are likely to occur simultaneously, as shown for the binding of chlorogenic acid to proteins⁽ Reference Prigent, Gruppen and Visser ^{103 )}. Three types of non-covalent interactions have been suggested: hydrogen, hydrophobic and ionic bonding⁽ Reference Rawel and Rohn ^{104 )}.

Plasma proteins are the most studied binding targets, especially albumin. Albumin is the primary protein that interacts with polyphenols in the blood. The incubation of quercetin with human plasma revealed that quercetin extensively binds to plasma proteins⁽ Reference Boulton, Walle and Walle ^{105 )}. Smith et al. ⁽ Reference Smith, Halliwell and Aruoma ^{106 )} found that the binding of polyphenols, such as quercetin, myricetin and morin, to albumin reduces polyphenols' antioxidant activity. Similarly, Rohn et al. ⁽ Reference Rohn, Rawel and Kroll ^{107 )} showed that the quercetin–albumin interaction decreases total antioxidant activity in comparison with an equivalent amount of free quercetin.

In addition to plasma proteins, interactions may occur between polyphenols and food proteins. In many countries, phenolic-rich foods, such as coffee and tea, are usually consumed with milk. It was observed that the radical-scavenging activity of black tea alone was significantly higher than with milk⁽ Reference Sharma, Kumar and Rao ^{108 )}. Lorenz et al. ⁽ Reference Lorenz, Jochmann and vonKrosigk ^{109 )} showed that the addition of milk to black tea blunted the tea's beneficial vascular effects, and milk caseins were identified as interfering agents via interaction with tea catechins and the subsequent formation of complexes. Similarly, Hasni et al. ⁽ Reference Hasni, Bourassa and Hamdani ^{110 )} reported preferred binding sites in α- and β-caseins for tea catechins and suggested that the binding involves hydrogen bonding and hydrophobic interactions. For coffee–milk beverages, an immediate decrease of in vitro bioaccessible polyphenols was found when milk was mixed with coffee, which was also due to the binding of polyphenols to milk proteins⁽ Reference Tagliazucchi, Helal and Verzelloni ^{111 )}. In addition, soya proteins behave in a similar manner to milk caseins, resulting in protein–polyphenol binding and a net loss of antioxidant activity⁽ Reference Rawel, Czajka and Rohn ^{112 )}. This is reasonable considering the structural similarities of soya proteins and milk caseins. A cross-reactivity of milk caseins with soyabean glycinin and β-conglycinin has been described⁽ Reference Rozenfeld, Docena and Añón ^{113 )}. Further support is that polyphenols bind to the proline-rich regions of proteins, with subsequent complexation⁽ Reference Luck, Liao and Murray ^{114 )}, and both soyabean glycinin and β-conglycinin contain proline-rich regions⁽ Reference Maruyama, Maruyama and Mikami ^{115 )}.

In addition to food proteins, polyphenols have been reported to bind to enzymes. Phenolic extracts from a number of plants were found to be effective inhibitors of intestinal α-glucosidase/maltase activity⁽ Reference Matsui, Ueda and Oki ^{116 )}. The inhibitory effect is generally attributed to the ability of polyphenols to bind to the enzymes⁽ Reference Funke and Melzig ^{117 –} Reference Shobana, Sreerama and Malleshi ^{119 )}. Similarly, α-glucosidase activity in vitro was significantly inhibited by anthocyanin-rich extracts of blueberry and blackcurrant⁽ Reference McDougall, Shpiro and Dobson ^{120 )}. Extracts of Salacia reticulata are used to prevent diabetes and obesity in Japan and exhibit pancreatic lipase inhibition, which has been partly attributed to ( − )-epigallocatechin, ( − )-epigallocatechin dimers, and a ‘tannin’ fraction⁽ Reference Yoshikawa, Shimoda and Nishida ^{121 )}. A comprehensive study on the inhibition of pancreatic lipase by tea polyphenols suggested that epigallocatechin 3-O-gallate was an effective inhibitor⁽ Reference Nakai, Fukui and Asami ^{122 )}. Proteases have also been shown to be inhibited by polyphenols. Trypsin was inhibited by phenolic-rich extracts of cocoa, pears and lentils⁽ Reference Quesada, Bartolomé and Nieto ^{123 )}.

Factors influencing the in vitro interactions of polyphenols with proteins are related to the physical and chemical properties of both proteins and polyphenols. It has been suggested that high-molecular-weight polyphenols bind to proteins more effectively⁽ Reference Charlton, Baxter and Khan ^{124 ,} Reference Siebert ^{125 )}. In contrast, when the conformation of a polyphenol is constrained, its capacity to interact with proteins is dramatically reduced regardless of the polyphenol's molecular weight⁽ Reference Frazier, Papadopoulou and Green ^{126 –} Reference Deaville, Green and Mueller-Harvey ^{129 )}. In addition, hydrophobicity favours a strong association between proteins and polyphenols⁽ Reference Poncet-Legrand, Edelmann and Putaux ^{130 )}. The ability of proteins to bind to polyphenols also depends on protein size, secondary/tertiary structure and amino acid composition. In general, proteins that interact strongly with polyphenols have a high basic-residue content and a high proline content, are relatively large and hydrophobic, and have a conformationally open and flexible structure⁽ Reference Lu and Bennick ^{131 –} Reference Richard, Vitrac and Merillon ^{138 )}. More details on the structure–affinity relationship of the interactions between polyphenols and proteins can be found in the review by Xiao & Kai⁽ Reference Xiao and Kai ^{139 )}.

Interaction of plant phenols with carbohydrates

A study of a phenolic acid–starch model system indicated that the interactions of phenolic acid with starch significantly contributed to the inhibitory effect of starch hydrolysis⁽ Reference Kandil, Li and Vasanthan ^{140 )}. The carboxyl and hydroxyl groups of phenolic acids are capable of binding to starch and other polysaccharides through hydrogen bonds, chelation or covalent bonds, forming bridges or cross-links⁽ Reference Chi, Chang, Mou, Ho, Lee and Huang ^{141 )}. The effect of tannic acid and catechin on legume starch hydrolysis⁽ Reference Deshpande and Salunkhe ^{142 )} and the interference of gallic and chlorogenic acids with the starch–iodine reaction have been reported⁽ Reference Sharma, Sharma and Kakkar ^{143 )}. In addition to phenolic acids, various studies have shown that proanthocyanidins can interact with the different polysaccharides⁽ Reference Nitta, Fang and Takemasa ^{144 ,} Reference Nishinari, Kim and Fang ^{145 )}. Oligomeric and polymeric procyanidins have the ability to bind non-covalently on apple cell walls by simple incubation in aqueous buffer⁽ Reference LeBourvellec, Guyot and Renard ^{146 –} Reference Le Bourvellec, Bouchet and Renard ^{148 )}. Moreover, the amount of procyanidins bound to the cell wall increases with procyanidin concentrations, degree of polymerisation and percentage of galloylation⁽ Reference LeBourvellec, Guyot and Renard ^{146 –} Reference Le Bourvellec, Bouchet and Renard ^{148 )}. In addition, the apparent affinity constants between procyanidins and polysaccharides were found to decrease as follows: pectins > xyloglucan > starch > cellulose⁽ Reference Le Bourvellec, Bouchet and Renard ^{148 )}. Higher affinities are obtained with pectin, a polysaccharide having the ability to develop a gel-like network, forming hydrophobic pockets to encapsulate procyanidins. It has been reported that complex formation between soluble pectin and tannins reduces the astringency of both fresh juice and a solution of tannins purified from persimmon fruit⁽ Reference Taira, Ono and Matsumoto ^{149 )}. Filamentous and globular polysaccharides, like cellulose and xyloglucan, bind procyanidins weakly. The results are consistent with the results of Renard et al. ⁽ Reference Renard, Baron and Guyot ^{150 )} which showed that xyloglucan has no marked effect for the procyanidins with a high degree of polymerisation. It was reported that nearly 35 % of the polyphenols in red wine are bound to soluble dietary fibre⁽ Reference Saura-Calixto and Díaz-Rubio ^{151 )}. It has also been reported that polyphenol–pectin complexes show no superoxide-scavenging capacity, have reduced hydroxyl radical-scavenging activity, and have high 2,2-diphenyl-1-picrylhydrazyl radical-scavenging activity, indicating potential changes in functionality because of complex formation⁽ Reference Jacob and Paliyath ^{152 )}. Another cellular study showed that when the extract was ingested along with carbohydrate-rich food, an enhanced uptake of polyphenols was observed⁽ Reference Serra, Macià and Romero ^{153 )}. This finding is in agreement with the results of Schramm et al. ⁽ Reference Schramm, Karim and Schrader ^{154 )}, who reported an enhanced activity of carbohydrate-rich foods and suggested that this effect may be mediated by a yet-unidentified carbohydrate–flavanol transporter.

Interaction of plant phenols with fats

Only a few studies have focused on the in vitro interactions between fats and polyphenols. A positive relationship has been observed between fat concentration and bioaccessibility of polyphenols after in vitro gastropancreatic digestion⁽ Reference Tagliazucchi, Helal and Verzelloni ^{111 )}. Results have indicated that a high fat content in food seems to have a protective effect, as hydrophobic interactions between fats and polyphenols increase the stability of polyphenols during digestion⁽ Reference Ortega, Reguant and Romero ^{155 )}. Similarly, the lipid fraction (rich in PUFA) showed a protective effect on the recovery of phenolic compounds during duodenal digestion. Disruption of the natural matrix under digestive conditions led to the release of PUFA, which could establish interactions with phenolic compounds, enhancing the compounds' recovery and stability during digestion⁽ Reference Ortega, Macià and Romero ^{156 )}. Certain indirect evidence has been provided by Langley-Evans⁽ Reference Langley-Evans ^{157 )}, who found that the addition of soya milk or cows' milk appeared to diminish the antioxidant potential of black tea preparations. This effect was greatest when whole milk was used and appeared to be primarily related to the fat content of the added milk⁽ Reference Langley-Evans ^{158 )}.

Interaction of plant phenols with proteins

Bioavailability refers to the extent to which and rate at which a substance becomes available to target tissue after administration. The clarification of polyphenol bioavailability is important because the polyphenol content and biological activity observed in vitro have little nutritional relevance unless the active metabolites of polyphenols can gain access to appropriate sites within the body⁽ Reference Canon, Pate and Meudec ^{159 )}. So far, few studies have investigated the effect of proteins alone on the bioavailability of phenolic compounds. Most researchers used foodstuffs directly. Milk is the most often chosen food because phenolic-rich foods are usually consumed with milk. Serafini et al. ⁽ Reference Serafini, Ghiselli and Ferro-Luzzi ^{61 )} found that compared with tea consumed without milk, the antioxidant potential of tea with milk decreased. Mullen et al. ⁽ Reference Mullen, Borges and Donovan ^{160 )} reported that milk significantly lowered the excretion of urinary flavan-3-ol metabolites. The overall 0–24 h excretion of metabolites corresponded to 18·3 % of intake after ingestion of a cocoa–water beverage compared with 10·5 % after ingestion of a cocoa–milk beverage. Moreover, they emphasised that, with a lower flavan-3-ol content, which is typical of commercial cocoas that are available to the general public, milk does have the capacity to interfere with absorption. Lorenz et al. ⁽ Reference Lorenz, Jochmann and vonKrosigk ^{109 )} showed that the addition of milk to black tea blunted the tea's beneficial vascular effects. Similarly, yoghurt was also reported to have a marked impact on the urinary excretion of phenolic acids. The overall 0–24 h excretion of phenolic acids was 62 μmol after drinking orange juice alone and this fell markedly to 9·3 μmol when the orange juice was ingested with yoghurt⁽ Reference Roowi, Mullen and Edwards ^{161 )}. There were also several other studies that showed that the absorption of phenolic compounds or increased plasma antioxidant capacity is suppressed when the compounds are consumed with milk⁽ Reference Serafini, Testa and Villano ^{162 ,} Reference Duarte and Farah ^{163 )}. Most studies have attributed this inhibitory effect of milk to milk proteins because phenolic compounds can bind to milk proteins through hydrogen bonds and hydrophobic interactions⁽ Reference Charlton, Baxter and Khan ^{124 ,} Reference Spencer, Cai and Martin ^{164 )}. However, this speculation is based on in vitro studies. In addition to proteins, there are other macronutrients, such as carbohydrates and fats, in whole milk. These nutrients' possible effects cannot be excluded. In our recent study, after excluding the effects of other macronutrients, it was found that milk proteins led to significant changes in the plasma kinetics profile of polyphenols but did not affect the overall absorption of polyphenols in rats⁽ Reference Zhang, Zheng and Liu ^{9 )}. Similarly, it was also found that milk proteins extended the time needed to reach a maximal plasma polyphenol concentration in human subjects⁽ Reference Zhang, Jiang and Guo ^{10 )}. Casein, which is a major component of milk protein, can form curds in the stomach that delay casein's absorption compared with other proteins⁽ Reference Barth, Pfeuffer and Scholtissek ^{165 )}. It has been suggested that polyphenols can bind to milk proteins and subsequently form complexes⁽ Reference Rawel, Kroll and Hohl ^{166 ,} Reference Dupas, Marsset-Baglieri and Ordonaud ^{167 )}. Therefore, the in vivo results may be explained by the formation of protein–polyphenol complexes that remain in the stomach for an extended period of time, thereby delaying the appearance of polyphenols in the blood. Additionally, the absence of an effect on overall polyphenol absorption indicated that polyphenols could be released from the complexes and be absorbed later. Certainly, more studies are needed to evaluate the effect of different types of proteins on the bioavailability and in vivo bioactivity of polyphenols.

Interaction of plant phenols with carbohydrates

Consistent with the in vitro results, it has been shown that sugar and bread increase flavanol AUC values to 140 % of controls. The ability of treated meals to affect AUC values has been found to be positively correlated with carbohydrate content⁽ Reference Schramm, Karim and Schrader ^{154 )} (Table 2). Moreover, Neilson et al. ⁽ Reference Neilson, George and Janle ^{168 )} reported that the presence of sucrose in chocolate may positively influence the bioavailability of flavan-3-ols compounds. Several studies have attempted to clarify the mechanism associated with this increase in polyphenol bioavailability. The positive effects observed for carbohydrate-rich foods (bread, sucrose and grapefruit juice) may have been mediated by a carbohydrate-specific effect on gastrointestinal motility and/or secretion⁽ Reference Schramm, Karim and Schrader ^{154 )}. Along with an increase in the absorption of cocoa flavanols, a faster rate of flavanol elimination following sugar treatment has been observed. This observation is consistent with the concept of an increased tubular secretion of xenobiotics with increased solute filtration. Increased solute filtration can result from higher blood solute (xenobiotic) concentrations and an increased glomerular filtration rate⁽ Reference Schramm, Karim and Schrader ^{154 )}. In addition, an indigestible saccharide, difructose anhydride III, has been demonstrated to promote rutin absorption after a single bolus administration using portally and duodenally cannulated rats⁽ Reference Matsumoto, Matsukawa and Chiji ^{169 )}. This disaccharide is an enhancer of paracellular transport in the small intestine⁽ Reference Mineo, Hara and Shigematsu ^{170 )}. Furthermore, difructose anhydride III is fermentable in the large intestine⁽ Reference Afsana, Shiga and Ishizuka ^{171 )}, and it has been suggested that the bacterial fermentation of difructose anhydride III affects the bioavailability of flavonoid glycosides⁽ Reference Tamura, Nishimukai and Shigematsu ^{172 )}. It has also been suggested that fructo-oligosaccharides, which are the most popular non-digestible oligosaccharides and are well established to be readily fermentable⁽ Reference Bomba, Nemcova and Gancarcikova ^{173 )}, increase isoflavone glycoside absorption with the promotion of intestinal fermentation⁽ Reference Uehara, Ohta and Sakai ^{174 )}. The results strongly suggest that increased fermentation by oligosaccharides in the large intestine enhances polyphenol bioavailability. Moreover, the fermentation of oligosaccharides enlarges the intestinal mucosa⁽ Reference Shiga, Nishimukai and Tomita ^{175 )}, suppresses the bacterial degradation of polyphenols in the caecum⁽ Reference Matsukawa, Matsumoto and Shinoki ^{176 )} and stimulates mucosal blood flow⁽ Reference Younes, Alphonse and Hadj-Abdelkader ^{177 )}, which may also contribute to polyphenol absorption in the large intestine. In addition, the cell walls are major components in fruits and vegetables and their impact cannot be ignored. Proanthocyanidins are associated with cell walls through either weak (hydrophilic/hydrophobic)⁽ Reference Le Bourvellec and Renard ^{147 ,} Reference Le Bourvellec, Bouchet and Renard ^{148 )} or strong (covalent) interactions⁽ Reference Pérez-Jiménez, Arranz and Saura-Calixto ^{178 )}, which may influence their absorption⁽ Reference Touriño, Pérez-Jiménez and Mateos-Martín ^{179 )}. Early studies suggested that condensed tannins were not absorbed from the digestive tract of chicken and sheep⁽ Reference Jimenez-Ramsey, Rogler and Housley ^{180 ,} Reference Terrill, Waghorn and Woolley ^{181 )}. Recent rat and human studies suggested that proanthocyanidins with more than three subunits could be more resistant to degradation due to their chemical structure and, therefore, higher levels are observed in the colon⁽ Reference Choy, Jaggers and Oteiza ^{47 )}. Similar results were observed for hydroxycinnamic acid derivatives. In wheat, ferulic acid is ester-linked to position O-5 of the arabinosyl side chains of cell wall arabinoxylans. In fact, the presence of ester-linked monomeric and dimeric phenolics leads to reduced biodegradability of the cell wall polysaccharide, and limits the release of ferulic acid. Diferulates have a dramatic effect on both the rate and extent of polysaccharide degradation. Cross-linking through diferulates may inhibit the binding of endoxylanases, thus limiting the extent of arabinoxylan degradation and consequently the release of esterified ferulic acid⁽ Reference Grabber, Hatfield and Ralph ^{182 )}. Furthermore, cross-linking may prevent localised areas from swelling, excluding key enzymes for dietary fibre degradation⁽ Reference Bunzel, Ralph and Marita ^{183 )}. Likewise, Aprikian et al. ⁽ Reference Aprikian, Duclos and Guyot ^{184 )} showed that procyanidins with high polymerisation inhibit enzymic degradation of cell walls in apples. Finally, covalently bound phenolics are released only after extensive microbial digestion in the colon⁽ Reference Chesson, Provan and Russell ^{185 )}.

Table 2 In vivo studies assessing the effects of food macronutrients on the bioavailability and bioactivity of polyphenols

↑ , Increase.

In addition to the effect on absorption, carbohydrates have been observed to change the plasma kinetics profile of polyphenols. Bub et al. ⁽ Reference Bub, Watzl and Heeb ^{186 )} reported that the mean time required to achieve a maximal plasma concentration of malvidin 3-glucoside was extended after the ingestion of red grape juice compared with red wine in human subjects. The researchers concluded that the delaying effect of the sugar present in the red grape juice could result from the competition of glucose and malvidin 3-glucoside for the Na-dependent GLUT, as reported for quercetin 3-glucoside⁽ Reference Walgren, Lin and Kinne ^{187 ,} Reference Arts, Sesink and Hollman ^{188 )}. The viscosities of certain types of carbohydrates also cannot be ignored. Anthocyanins were absorbed and excreted more slowly when administered in an acidified oatmeal solution than in an acidified water solution⁽ Reference Walton, Hendriks and Broomfield ^{189 )}. The addition of sucrose resulted in a slight delay in reaching a maximal plasma concentration. This delay may be partly due to the increase in the viscosity of sucrose⁽ Reference Peters, Green and Janle ^{11 )}. Sucrose formulations were observed to have viscosities approximately six times higher than non-sucrose-containing formulations. Increased viscosity is known to have an impact on gastric emptying and can influence the kinetics of absorption for phenolic compounds in the small intestine⁽ Reference Marciani, Gowland and Spiller ^{190 )}. As non-extractable proanthocyanidins are associated with the food matrix, particularly with insoluble polymers such as dietary fibre, their absorption may be slower compared with that of extractable proanthocyanidins. This deferred release would make non-extractable proanthocyanidin metabolites bioavailable for a long time after intake and may result in long-term health effects⁽ Reference Mateos-Martín, Pérez-Jiménez and Fuguet ^{41 )}. There is supporting evidence that the presence of ferulic acid–arabinoxylans bonds in the food matrix increases the occurrence time of ferulic acid in the organism. The phenomenon may be explained by a slower hydrolysis rate of phenolics–cell wall bonds by intestinal enzymes⁽ Reference Andreasen, Kroon and Williamson ^{191 )}.

Interaction of plant phenols with fats

Many investigations have shown that the co-ingestion of lipids could affect the bioavailability of polyphenols (Table 2)⁽ Reference Lesser, Cermak and Wolffram ^{192 –} Reference Murkovic, Toplak and Adam ^{197 )}. In a pig study, the bioavailability of quercetin was significantly higher in a 17 % fat diet compared with a 3 % fat diet⁽ Reference Lesser, Cermak and Wolffram ^{192 )}. Azuma et al. ⁽ Reference Azuma, Ippoushi and Ito ^{198 )} observed that the co-administration of lipids (soyabean oil or lecithin) and emulsifiers was able to enhance the intestinal absorption of quercetin in rats. In another rat study, the absorption of catechin was enhanced when green tea was administered as a phospholipid complex rather than as free catechins⁽ Reference Pietta, Simonetti and Gardana ^{199 )}. It has also been shown that more than 4·6 % (w/w) of soyabean oil in diets significantly enhanced the accumulation of quercetin metabolites in the plasma. Fish oil and beef tallow increased this accumulation to an extent similar to that of soyabean oil⁽ Reference Azuma, Ippoushi and Ito ^{193 )}. In addition to fat concentration, the effects of different fat types on the bioavailability of polyphenols have also been assessed. For example, the chain length of fatty acids has been reported to influence the micellarisation of phenolic compounds during digestion. Bioavailability was enhanced by 38 % and 12 % after intake along with medium-chain fatty acid and long-chain fatty acid diets, respectively, compared with the standard diet⁽ Reference Lesser, Cermak and Wolffram ^{194 )}. Moreover, the degree of saturation may affect the bioavailability of polyphenols. Goltz et al. ⁽ Reference Goltz, Campbell and Chitchumroonchokchai ^{200 )} observed the increased absorption of carotenoids from a vegetable salad with higher levels of lipids and found that unsaturated fat promoted carotenoid absorption to a greater extent than saturated fat. For the mechanism associated with this increase in polyphenol bioavailability, it has been suggested that dietary fat can probably promote the solubility of polyphenols⁽ Reference Lesser, Cermak and Wolffram ^{192 ,} Reference Mateo Anson, van den Berg and Havenaar ^{201 )}.

In addition, fat has been reported to affect the absorption kinetics of polyphenols. After the ingestion of oil-enriched tomato sauces, naringenin metabolites showed a biphasic profile of absorption in the plasma, suggesting that the lipid matrix added to the sauce may stimulate reabsorption events via the enterohepatic circulation, thus potentially affecting the apparent plasma half-life of the flavanone before excretion⁽ Reference Tulipani, Martinez Huelamo and Rotches Ribalta ^{195 )}. Additionally, a delay in attaining a maximal plasma concentration was observed in subjects consuming strawberries with cream⁽ Reference Mullen, Edwards and Serafini ^{196 )}. These observations are consistent with an earlier study showing that the time needed to reach a maximal serum level was significantly extended when anthocyanins were consumed together with a high-fat meal⁽ Reference Murkovic, Toplak and Adam ^{197 )}. It was suggested that the fat present in the cream slowed the transit of anthocyanins due to duodenal and ileal brakes activated by fat⁽ Reference Mullen, Edwards and Serafini ^{196 ,} Reference Welch, Bruce and Hill ^{202 )}. It has also been shown that a maximal plasma concentration of quercetin was reached significantly later for a medium-chain fatty acid diet than for a long-chain fatty acid or a standard diet⁽ Reference Lesser, Cermak and Wolffram ^{194 )}.

Recently, a novel effect of fat on the bioavailability and antioxidant activity of polyphenols was found. At a relatively low content (3·8 % (w/v) in milk), milk fat alone did not interact with polyphenols and did not have an effect on the bioavailability and antioxidant activity of polyphenols in rats and human subjects⁽ Reference Zhang, Zheng and Liu ^{9 ,} Reference Zhang, Jiang and Guo ^{10 )}. Similarly, milk proteins did not affect the absorption and antioxidant activity of polyphenols. However, the combination of milk proteins and fats showed the highest degree of inhibition for both the bioavailability and the antioxidant capacity of polyphenols. To clarify the exact mechanism of this phenomenon, further in vitro and in vivo experiments were performed. The results showed that no significant difference was found in the median diameters of the emulsions between the consumption of polyphenols with milk fat and the consumption of milk fat alone, in accordance with the in vivo results, which did not show any interaction between milk fats and polyphenols. The median diameter of the mixture was significantly higher when polyphenols were consumed along with milk proteins than when milk proteins alone were consumed. The findings suggested the formation of milk protein–polyphenol complexes. However, during digestion, the decreasing trend of the median diameter indicated the breakdown of the milk protein–polyphenol complexes. Nevertheless, when polyphenols, milk proteins and fats were ingested together, a significant increase in the median diameter of the emulsion was found during digestion⁽ Reference Zhang, Zheng and Liu ^{9 )}. Considering that polyphenols can bind to milk proteins⁽ Reference Luck, Liao and Murray ^{114 )} and that proteins can be adsorbed to the surface of lipid droplets⁽ Reference Ye, Cui and Singh ^{203 )}, the data evidenced the participation of milk fat in the interactions between milk proteins and polyphenols during digestion, resulting in remarkable aggregation. Unlike polyphenol–protein complexes, the complexes formed with proteins and fats did not completely break during digestion and thus impaired the bioavailability and antioxidant activity of the polyphenols. One possible reason is that the aggregation of the fat globules affects the way that gastrointestinal enzymes access substrates⁽ Reference Michalski, Briard and Desage ^{204 )}. Another explanation is that lipase activity is inhibited by phenolic compounds⁽ Reference Ikeda, Tsuda and Suzuki ^{205 )} and long-chain fatty acids generated during digestion⁽ Reference Borel, Armand and Ythier ^{206 –} Reference Gargouri, Pieroni and Riviere ^{208 )}. The findings show that there are synergisms between food macronutrients in affecting the bioavailability and bioactivity of polyphenols. Based on these results, we propose a hypothesis (Fig. 1) that allows the prediction of the effects of different food macronutrients on the bioavailability and bioactivity of polyphenols.

Fig. 1 A simplified scheme of the physico-chemical processes occurring before and after digestion when different food macronutrients are added to polyphenols. (A colour version of this figure can be found online at http://www.journals.cambridge.org/nrr)