Flavonoids and related compounds: new data on dietary sources, metabolism and availability in humans

Abstract

The present doctoral thesis deals with dietary polyphenols from several viewpoints. After a brief introduction to the topic, the work starts with an investigation into the polyphenolic composition of hazelnut skin. Hazelnuts are typically consumed whole (raw - with skin, or roasted - without skin) or used as ingredient in a variety of processed foods, especially in bakery and confectionery products. Hazelnut skin, hazelnut hard shell and hazelnut green leafy cover, as well as hazelnut tree leaf, are by-products of roasting, cracking, shelling/hulling, and harvesting processes, respectively and are now investigated for their composition in the attempt to add economic value to waste from the hazelnut industry. In this first work, skins from different hazelnut samples were characterised for total polyphenol content, total antioxidant capacity (TAC) and for their content in specific polyphenolic compounds. The second investigation deals with a source of peculiar polyphenols: the milk thistle. This study evaluated the absorption and metabolism of milk thistle flavonolignans (silychristin, silydianin, silybin and isosilybin isomers - all together known as silymarin) in humans. Fourteen volunteers consumed an extract of milk thistle and urine was collected up to 48 hours after consumption. Thirty-one metabolites were identified in urine by means of HPLC-MS/MS, monoglucuronides being the most common excreted form, followed by sulphate-glucuronides and diglucuronides, respectively. The third study aimed to investigate green tea flavan-3-ol catabolism, plasma pharmacokinetic and urinary excretion up to 24h from tea ingestion in humans, demonstrating that green tea catechins are more bioavailable than previously observed when colonic ring fission metabolites are taken into consideration. Regular consumption of green tea containing flavan-3-ols allows a non marginal exposition of the human body to their catabolites, somehow justifying the numerous beneficial actions described as linked to green tea intake. The data obtained in this last study along with those provided by other researchers, allowed to review the most recent findings on green tea flavan-3-ol bioavailability in humans in the fourth work presented in this thesis. The work also reviewed the bioavailability of coffee chlorogenic acids. The fifth study represents an update on the bioavailability of green tea flavan-3-ols. Novel information was obtained by monitoring urinary excretion of flavan-3-ol metabolites up to 48h from tea consumption by human volunteers. The estimated bioavailability was close to the 62% of the total ingested flavan-3-ols and the main reasons of this increment were the longer urinary collection period, allowing an almost complete excretion of flavan-3-ol metabolites, and the identification and quantification of novel molecules derived by microbial modification of flavan-3-ols. In conclusion, this doctoral thesis updates the knowledge about polyphenols by providing new information about their sources, their human and microbial metabolism and their bioavailability. All these aspects are paramount when the use of polyphenols as functional ingredients is suggested, as the defined identification and quantification of these molecules in food items, as well as their absorption and modifications occurring within the human body, are fundamental steps to unravel their putative beneficial effects toward human health.