Polyphenols - Tannins Flashcards
What are tannins?
**The largest group of Polyphenols **- give colour to the leaves in autumn.
Properties of tannins
* Used in production of leathers.
* Water molecules form hydrogen bonds with proteins.
* Tannins contain a high number of phenolic groups, capable of binding strongly to proteins at several sites and locking the proteins together.
What are the medicinal uses of tannins?
Medicinal actions of tannins
* Tannins haven an astringent effect on
mucous membranes - they cross link proteins in the mucous.
* Astringency offers increased protection from microorganisms (tighten things up) and an anti-secretory, drying effect.
* Inhibitory effect on many enzymes.
* Precipitates with polysaccharides and
alkaloids: do not use tannin and alkaloid rich herbs together.
What are tannin types and bioavailabilty?
Types and bioavailability
* Tannins have very little bioavailability and are usually very local.
* Decomposition products of tannins
and monomers of condensed tannins
can be absorbed and have systemic effects however.
Tannins: two main groups
* Condensed tannins found in many foods - they are called procyanidins or proanthocyanidins.
* Hydrolysable tannins are less common in foods, but one example is pomegranate
How do tannins act - astringency?
Astringent (local on skin and gut)
* Prevents bleeding and forms protective
layer on wounds, antimicrobial, constrict blood vessels – ‘wound herbs’.
* Protects inflamed mucous membranes from microorganisms and chemicals.
* Contracts tissue, diminishes exudation.
* Local effect on diarrhoea - can reduce peristalsis.
* Anti-secretory effect.
High doses over a long time can lead to constipation and other side effects such as irritation so take care; also take iron supplements seperately to tannin rich teas otherwise it affects absorption. Should not be used topically long term on damaged skin as it can lead to excessive scar formation.
Reducing menstrual bleeding
* Traditionally some of those tannin rich herbs were used reduce uterine bleeding, antihaemorrhagic.
What are hydrolysable tannins?
Hydrolysable tannins
* Made of several hydroxyl-rich phenols joined to central sugar.
* Readily hydrolysed hence the name.
* Gallotannins: found in witch hazel bark.
* Ellagitannins: astringent, antimicrobial,
antioxidant, anti-inflammatory and
anticarcinogenic.
* Ellagitannins: found in raspberry leaf, pomegranate fruit, grapes, green and black teas, cranesbill, oak bark (also contains condensed tannins), meadowsweet, walnuts and strawberries.
They decompose quickly once hydrolysed however.
What about Pomegranate?
Pomegranate
* Pomegranate has Ellagitannins and metabolites punicalin, punicalagin, tellimagrandin and corilagin.
* Found to be anticarcinogenic (in vitro and in vivo), also pomegranate peel extract.
* Topical pomegranate peel promoted wound
healing in vivo.
* Ellagic acid suppresses acid secretion,
pomegranate prevents experimentally induced
gastric ulcers.
* Clinical study for prostate cancer
Explain the metabolism of ellagitannins?
There have been studies into the metabolism of ellagitannins
* In small intestine: hydrolysis and release of ellagic acid.
* A little intact ellagitannin and ellagic acid may be absorbed.
* Gut flora: metabolised to urolithins
* Intestinal lining and liver: glucuronidated.
* Enterohepatic circulation:
urolithins and metabolites
detected in plasma and urine.”
What are some cautions with use of tannins?
Adverse reactions and toxicology
* Chronic intake of tannins inhibits digestive enzymes.
* High doses can lead to constipation.
* Excessive astringency on mucous membranes can produce an irritating effect. Use cautiously in inflamed or ulcerated conditions of the gastrointestinal tract.
* One study found that when tea and iron are consumed separately, iron absorption is
not affected, although tannins react with thiamine and decrease its absorption.
* Hydrolysable tannins absorbed into the bloodstream can cause hepatotoxicity.
* Best to avoid the use of high doses of highly astringent herbs in patients with very
damaged gastrointestinal tracts.
* Caution with long term topical use on damaged skin.
What are condensed tannins?
Condensed tannins - proanthocyanidins, and oligomeric proanthocyanidins (OPCs)
- Condensed tannins are readily oxidised over time (colour changes to purplish pink). Reddish colour of many barks and roots.
- Found in pine bark and grapeseed extracts, witch hazel, green tea, hawthorn and ginkgo.
- Antioxidant, anti-inflammatory, anticancer, cardiotonic, astringent and antimicrobial.
- Resist microbial degradation
- A study found the tannins were able to bind to and inactivate the hypersecretory activity of cholera toxin.
- Antiulcer activity: e.g. black tea extract.
Condensed tannins
* Mostly made up of monomers which are catechins and epicatechins.
* However if two monomeres - Dimers - three are Trimers, Oligomers (a few monomers) and eventually Polymers (many monomers; condensed tannin).
* Not readily hydrolysable hence called condensed
* Small OPCs soluble in hot water, buut larger tannins are not.
* Absorption: dimers low, polymers not at all.
* Protein binding in proportion to polymerisation.
* Pine bark and grapeseed extracts also contain monomers, OPCs in witch hazel, green tea, hawthorn and ginkgo.
* Grape seed, pine bark: monomers could be largely responsible for the therapeutic effects.
What are OPCs?
- OPCs and their monomers are similar
to flavonoids. - Condensed tannins: when arrive in the gut the gut flora produce smaller bioavailable phenolic compounds.
- Condensed tannins are more resistant
to decomposition into their monomeric
components. - OPCs are Antioxidant, anti-inflammatory
anticancer, cardiotonic, astringent and
antimicrobial.
Why is green and black tea useful?
Green and black tea
* Green tea polyphenols: Epigallocatechin (EGC) and epigallocatechin gallate (EGCG).
* Black tea is fermented green tea.
* Human studies found:
* Antioxidant activity of plasma increased by
green tea more than black.
* Adding milk destroys this effect.
* Polyphenols: following decomposition in the gut, the smaller antioxidant molecules are absorbed. Adding milk causes protein binding and inhibits this decomposition.
Theaflavins and thearubigins in black tea
* Fermentation causes oxidation of some OPCs into theaflavins (flavan-3-ol dimers) and thearubigins
* Theaflavins: orange antioxidant pigments, may help to prevent cardiovascular disease, cancer and premature aging.
* Antimicrobial.
* Black tes shown to have antiulcer activity (black tea): ellagic acid suppresses acid secretion.
Also found in Rheum palmatum or rhubarb
* Gallotannin, catechin and proanthocyanidins affect bowel flora composition.
* Also contains anthraquinone glycosides.
* Rhubarb and other tannin-containing herbs reduced levels of uraemic toxins in vivo with renal failure.
* Rhubarb has been used in China to treat
renal failure.
Why are cranberries useful?
OPCs in cranberry
* In vitro: proanthocyanins block pathogen
adhesion to target tissues.
* In vivo: OPCs in cranberry discourages bacterial adherence to the bladder wall. Proanthocyanidins known to be important, though too large to be excreted in the urine. Perhaps due to decomposition products.
* Addition of cranberry to triple therapy
(omeprazole, amoxicillin and clarithromycin)
improves the rate of H. pylori eradication.
* Beneficial effects on LDL oxidation, platelet
aggregation, inflammation, thrombosis and blood pressure regulation.
What about hawthorn?
Crataegus spp
* Flowers rich in flavonoids (quercetin glycosides), leaves rich in OPCs.
* Antioxidant.
* Increase coronary blood flow, improve revascularisation (flavonoids, OPCs).
* Inotropic (flavonoids, OPCs: increasing the force of the contraction) and negatively chronotropic (decreases the heartrate).
Why is bilberry useful?
Bilberry and Tormentil
Vaccinium myrtillus berries: Anthocyanins
(0.5%), catechin, epicatechin, condensed
tannins, OPCs, flavonoids, phenolic acids
and pectins.
Potentilla erecta root: Hydrolysable and
condensed tannins. Clinical trial
effectively treated rotavirus infection in
children and reduced duration of
diarrhoea from 5 to 3 day
Why is Meadowsweet useful?
Filipendula ulmaria (meadowsweet)
* Tannins in meadowsweet are traditionally regarded as beneficial in mild peptic ulceration and inflammation.
* Bind to exposed proteins at the ulcer base, and the barrier protects the ulcer from gastric contents.
