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Polyphenols are the most abundant and ubiquitous secondary metabolites present in the plant kingdom with more than 8000 phenolic structures currently known. These compounds play an important role in plant growth and reproduction, providing protection against biotic and abiotic stress such as pathogen and insect attack, ultra violet (UV) radiation and wounding.
2. Potential Effect Against Hypertension
In order to evaluate the in vivo effect of grape pomace extracts and their potential effect on hypertension, rats were fed with different grape pomace EA70 extracts at a dose of 21 mg/kg/day, equivalent to a daily dose of 70 kg human consumption of 0.5 L of wine. The study was conducted over six weeks including three weeks of treatments, one week of treatment resumption followed again by two weeks of treatment.
Higher levels of O-methyl-(epi)catechin-O-glucuronide relative to (epi)catechin-O-glucuronide were found in every urine sample. Sulphate derivatives of 5-(hydroxyphenyl)-γ-valerolactone were excreted in greater amounts than glucuronides. A decrease of 5-(hydroxyphenyl)-4-hydroxyvaleric acid-O-sulfate from 0–8 h to 8–24 h at day 1 was observed while an increase of 5-(phenyl)-4hydroxyvaleric acid-O-sulphate occurred at 8–24 h compared to 0–8 h in all rat urines at day 1 and day 7. A previous report on a human almond skin polyphenol bioavailability  suggested that partial dihydroxylation reactions occur gradually as colonic metabolism progresses. These authors observed a change in the hydroxylation pattern of the phenyl ring from di- to mono- and unhydroxylated forms. Dihydroxylated derivatives were found 6–10 h after the intake, monohydroxylated forms were observed at 6–24 h and unhydroxylated derivatives were found 10–24 h after the intake.
At day 1, excretion as a percentage of intake varied from 1.02% to 1.57%. Highest recoveries were observed in the SHR5 group (1.57%, over 0–24 h). At day 7, an increase of intake was observed for the two collection periods and, as a consequence, over 0–24 h. The recoveries ranged from 1.43% in SHR6 to 4.41% in SHR5. A large increase in metabolite excretion was observed in SHR5 group with 4.41% of intake at day 7 compared to 1.57% at day 1. These observations suggested that over time, SHR rats may be able to ingest higher doses of polyphenols especially those contained in MOU (EA70) skin pomace extract.
Substantial levels of polyphenols after the winemaking process, remain in pomace in quantities sufficient to exert anti-hypertensive effects. In addition, according to the extract used and its composition, it is feasible to modulate anti-hypertensive effects by amplifying or decreasing polyphenols absorption. Therefore, it will be interesting to elucidate the exact mechanisms and compounds involved in this phenomenon in order to have a better control on blood pressure regulation and facilitate the choice of effective grape pomace extracts for further experiments. Moreover, it will be useful to investigate the effect of different flavan-3-ol fractions (i.e., oligomeric, monomeric) and anthocyanin fractions (i.e., glucosides, acetylated glucosides and coumarylic glucosides) in order to identify whether anti-hypertensive effects are linked to a particular compound or to the extracts as a whole.
As SHR represents a good model to investigate hypertension, studies could be extended to human clinical trials. For clinical tests, different parameters have to be taken into account such as the subjects (i.e., pre-hypertensive or hypertensive subjects), the dose used, the diet, biological fluid collections and biological markers to be quantified. In addition, different processes will have to be considered such as the election of grape pomace varieties and their parts (seeds/skins), the extraction processes which will be used, the dosage and the galenic formulation used in order to provide great stabilisation of the active substances.
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