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NutrInsight • Do we need dietary polyphenols for health?
1 TyPES, FOOD SOURCES, CONSUMPTION AND 1 TyPES, FOOD SOURCES, CONSUMPTION AND
BIOAVAILABILITy OF DIETARy POLyPHENOLS BIOAVAILABILITy OF DIETARy POLyPHENOLS
by Prof Francisco A. Tomás-Barberán
This chapter will describe what polyphenols are, where they are found in our diet and how much we eat. It will deal also with the level of bioavailability of polyphenols and their metabolites pointing out potential effectiveness on the body.
1.1 Types and food sources
Polyphenols, a complex group of phytochemicals
Polyphenols include a chemically complex and large family of phytochemicals. They are, by far, the main secondary metabolites in plants, and therefore are present in all foods of plant origin. The common structural feature of all polyphenols is the presence of phenolic hydroxyl group(s). Simple monomeric phenolics, such as benzoic acid derivatives and hydroxycinnamic acid derivatives, usually co-exist in plants with oligomeric and polymeric derivatives (tannins and lignans etc). Figure 1 shows some of the most common groups of polyphenols (phenolic acids, lignans, flavonoids and stilbenes) with some molecules of interest and examples of their main food sources.
Flavonoids, a well-studied class, form a large polyphenol subgroup with a carbon skeleton built of 2 phenyl rings (C6) bridged by a chain of 3 carbon atoms (C3) forming a heterocyclic 6-membered ring with oxygen and 2 carbon atoms from an adjacent phenyl ring. These include flavones and flavanones (found in citrus fruits); flavonols (found in tea, onions, apples) and flavan-3-ols (catechins and epi-catechins and proanthocyanidins) which are found in tea, red wine, cocoa/chocolate, apples. Other anthocyanins are found in fruits, vegetables and nuts including coloured berries and aubergine. Isoflavones (found in soy) are another important class of flavonoids.
There is also much interest in stilbenes such as resveratrol (in red wine) and other phenolics such as the hydroxycinnamates - in coffee (chlorogenic acids), in whole grain (ferulic acids). Minor phenolics such as hydroxytyrosol, found in olives, should not be overlooked. Currently, olive oil polyphenols characterised by their content of hydroxytyrosol and its derivatives are the only polyphenols so far to have achieved a positive opinion for a health claim in Europe [EFSA,2011]. The health claim approved by EFSA states that consumption of olive oil polyphenols contributes to the protection of blood lipids.
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