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NutrInsight • Do we need dietary polyphenols for health?
1.3 Bioavailability and metabolism
Although the daily intake of phenolics can be quite high, the absorption of these phytochemicals can range widely according to their structure. Whatever their bioavailability, these compounds can already have a direct effect on digestive tract. Moreover, their metabolism can lead to active metabolites that can be bioavailable and active in the body.
Biological effects of polyphenols in the gastro-intestinal (GI) tract
Before being absorbed into the bloodstream and metabolized, the polyphenols go through the GI tract where they can already have some useful effects. These include anti-inflammatory effects [Larrosa et al., 2010], prebiotic effects on the microbiota [Selma et al., 2009; Tzounis et al., 2011; Larrosa et al., 2010], interaction with other nutrients in the gut (such as lipid peroxidation) [Ligumsky et al., 2008; Kanner et al., 2011] and interaction with enzymes such as alpha-glucosidases, alpha-amylases and pancreatic lipases [Frei et al., 2011]. There is also evidence of in vitro effects with gut cell lines which could indicate anti-tumoral properties [González-Sarrías et al., 2009].
Pharmacokinetics
The pharmacokinetics of polyphenols can give clues to where they, and their metabolites, are absorbed, their distribution in tissues and also about bioavailability. Animal model studies and analysis of human biopsies and tissues after surgery show the metabolites and concentrations that reach the tissues. These are usually in the nM range [González-Sarrías et al., 2010; Azorín-Ortuño et al., 2011].
Stalmach and colleagues [Stalmach et al., 2009] have analysed the circulating hydroxycinnamates compounds and their metabolites after ingestion of coffee polyphenols. They found that some compounds, like caffeic acid, are absorbed fairly quickly and the peak plasma concentrations can be reached within 1 hour. On the other hand, other compounds or metabolites reach their highest concentrations in plasma in 4-5 hours indicating that the absorption of these polyphenols take place in the colon and probably need to be metabolized prior to be absorbed.
Factors affecting the bioavailability of polyphenols
Understanding the bioavailability, metabolism and tissue distribution is essential to evaluate the health effects of the polyphenols. Bioavailability of polyphenols is affected by three main factors (Figure 4):
• Foodmatrixandotherconstituentsofthedietexertingtheireffectsintheintestinallumen,
• Geneticaspectsofindividualsaffectinguptakeintogutcellsandintothebloodstreamviaasystemof
enzymes and transporters,
• Microbiota metabolism affecting the formation of metabolites in the colon and their subsequent uptake
into the bloodstream.
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