Phenolic compounds represent a diverse group of phytochemicals whose intake is associated with a wide spectrum of health benefits. As consequence of their low bioavailability, most of them reach the large intestine where, mediated by the action of local microbiota, a series of related microbial metabolites are accumulated. In the present review, gut microbial transformations of non-absorbed phenolic compounds are summarized. Several studies have reached a general consensus that unbalanced diets are associated with undesirable changes in gut metabolism that could be detrimental to intestinal health. In terms of explaining the possible effects of non-absorbed phenolic compounds, we have also gathered information regarded their influence on the local metabolism. For this purpose, a number of issues are discussed. Firstly, we consider the possible implications of phenolic compounds in the metabolism of colonic products, such as short chain fatty acids (SCFA), sterols (cholesterol and bile acids), and microbial products of non-absorbed proteins. Due to their being recognized as affective antioxidant and anti-inflammatory agents, the ability of phenolic compounds to counteract or suppress pro-oxidant and/or pro-inflammatory responses, triggered by bowel diseases, is also presented. The modulation of gut microbiota through dietetic maneuvers including phenolic compounds is also commented on. Although the available data seems to assume positive effects in terms of gut health protection, it is still insufficient for solid conclusions to be extracted, basically due to the lack of human trials to confirm the results obtained by OPEN ACCESSMolecules 2015, 20 17430 the in vitro and animal studies. We consider that more emphasis should be focused on the study of phenolic compounds, particularly in their microbial metabolites, and their power to influence different aspects of gut health.

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Among procyanidins (PC), monomers, such as catechin and epicatechin, have been widely studied, whereas dimer and trimer oligomers have received much less attention, despite their abundance in our diet. Recent studies have showed that as dimers and trimers could be important in determining the biological effects of procyanidin-rich food, understanding their bioavailability and metabolism is fundamental. The purpose of the present work is to study the stability of PC under digestion conditions, the metabolism and the bioavailability by using a combination of in vitro and in vivo models. Simultaneously, the matrix effect of a carbohydrate-rich food on the digestibility and bioavailability of PC is investigated. The results show a high level of stability of PC under gastric and duodenal digestion conditions. However, the pharmacokinetic study revealed limited absorption. Free forms of dimers and trimers have been detected in rat plasma, reaching the maximum concentration 1 h after oral intake of a grape seed extract.Procyanidins: Dimers: Trimers: Digestion: Bioavailability Procyanidins (PC) are found in most plants and in a wide range of foods, such as red wine, cocoa, tea and fruits, and thus they are a part of the human diet. PC belong to the group of flavonoids, and are phenolic compounds mainly formed of (þ)-catechin and (2 )-epicatechin units with C 4 -C 8 and/or C 4 -C 6 bonds. PC are formed from the condensation of monomeric units, between two and five units for oligomers and over five units for polymers (1) .Grape seed procyanidin extracts (GSPE) have shown several bioactivities. They improve antioxidant cell defences (2) and the plasma lipid profile (3,4) , limit adipogenesis (5) and function as insulinomimetic (6) and anti-inflammatory (7) agents. A preliminary study of an in vitro model by the authors has also determined that dimeric and trimeric oligomers are the most powerful PC molecules that mimic the complete GSPE (8) . Thus, to explain these health effects of PC and understand the mechanism by which PC act at the cellular level in vivo, it is essential to determine PC stability during the digestive process as well as their bioavailability.Despite it being clear that monomers are absorbed in human subjects and animals, there are controversies about the bioavailability of oligomeric forms. Different studies, following the ingestion of chocolate (9 -12) , black and green tea (13 -16) , red wine (17,18) and grape seed extract (1) , have shown that during digestion, the oligomers are fragmented into monomeric units of catechin and epicatechin. These are then absorbed, appearing in plasma and urine primarily as glucuronidated, methylated and sulphated metabolites. A study by Rios et al. (19) with six healthy subjects who consumed a rich PC cocoa beverage studied the depolymerisation of PC in the stomach and proved that they were remarkably stable in the stomach environment. Another study by Sano et al. (20) was the first work to detect procyanidin B1 in human serum after oral intake of a GSPE, the ma...

After a single ingestion of olive oil phenolic compounds, these were absorbed, metabolized and distributed through the blood stream to practically all parts of the body, even across the blood-brain barrier.

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This work describes the applicability of an in vitro digestion model for the evaluation of the digestibility and bioaccessibility of cocoa polyphenols (procyanidins, phenolic acids, and flavones) and for the study of the food matrix effect in relation with the fat content. For this purpose, two cocoa samples, cocoa liquor ( approximately 50% fat content) and cocoa powder ( approximately 15% fat content), were used. The results showed an important increase of the concentration of procyanidin (monomers and dimers), probably due to the hydrolysis of procyanidins with a high degree of polymerization (pentamers to nonamers) submitted to the digestion procedure. In relation to flavones, the concentration of aglycone forms remained almost constant after the digestion steps; in contrast, the concentration of the glycoside forms an increase in the digestion mixtures mainly after the duodenal step, probably as a result of the partial digestion of the dietary fiber present in the cocoa. The higher fat content in the cocoa liquor seemed to have a protective effect, probably related with a better micellarization that favors the stability of polyphenols during digestion.

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This study is an exhaustive chemical characterization of the phenolic compounds, triterpenes, and organic and ascorbic acids in red-fleshed apple varieties obtained by different breeding programs and using five traditional and new white-fleshed apple cultivars as reference. To carry out these analyses, solid-liquid extraction (SLE) and ultraperformance liquid chromatography coupled to tandem mass spectrometry (UPLC-MS/MS) were used. The results showed that the red-fleshed apples contained, in either the flesh or peel, higher amounts of phenolic acids (chlorogenic acid), anthocyanins (cyanidin-3-O-galactoside), dihydrochalcones (phloretin xylosyl glucoside), and organic acids (malic acid) but a lower amount of flavan-3-ols than the white-fleshed apples. These quantitative differences could be related to an up-regulation of anthocyanins, dihydrochalcones, and malic acid and a down-regulation of flavan-3-ols (anthocyanin precursors) in both the flesh and peel of the red-fleshed apple varieties. The reported results should be considered preliminary because the complete phytochemical characterization of the red-fleshed apple cultivars will be extended to consecutive harvest seasons.