By Matt Gowan, BSC, ND
August 2, 2017

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Astringent herbs – The use of tannins in medicine

By Matt Gowan, BSC, ND
August 2, 2017

Astringent herbs have long been used treat damaged and inflamed tissues.  The term astringent is derived from latin word “adstringere” which means to “to bind together”.  Compounds called tannins are responsible for the astringent effect.  Tannins bind to proteins forming cross-linking that tightens and thickens tissues.  The leather production process uses tannins.  The exposure to tannins converts delicate animal hides into durable leather.  Within medicine, astringent herbs react with tissues to produce a protective layer that can help heal wounds, arrest bleeding and prevent infection.

What conditions can astringent herbs help?

Astringent herbs are primarily used for skin conditions and ailments of the digestive tract including:

  • Wounds (e.g. lacerations, bruises, burns)
  • Leg ulcers (e.g. venous ulcers, diabetic ulcers)
  • Stomach ulcers
  • Hemorrhoids
  • Inflammatory bowel disease (e.g. ulcerative colitis)
  • Infectious diarrhea
  • Esophagitis

Tannins are poorly absorbed and thus are mainly used to alter surface tissues like skin and the internal mucous membranes of the digestive tract. However, some astringent herbs affect internal organs.  Urinary astringents reduce inflammation and bleeding associated with urinary tract infections.  Uterine astringents decrease excess menstrual bleeding.

What are tannins?

Tannins are phenolic compounds rich in hydroxy (-OH) residues that impart their astringent properties. Most plants produce tannins to protect themselves from infections and pests.  Bark and unripe fruit are usually astringent.  Tannins are divided into two classes:Astringent - Hydrolyzable tannins & Condensed tannins

Hydrolyzable tannins

Hydrolyzable tannins consist of numerous simple phenolics (e.g. gallic acid, ellagic acid) attached to a sugar molecule.  These tannins can be broken down by “hydrolysis” using an acid or base to release the phenolics from the sugar molecule. Hydrolyzable tannins are usually dark yellow to brown in colour.  Hydrolyzable tannins are often found in the bark and roots of many plants, including Witch hazel, Oak, Crane’s Bill, and Tea.

 

Hydrolysable tannins - Oak & Witch-hazel

Oak (LEFT) & Witch-hazel (RIGHT)

 

Condensed tannins

Non-hydrolyzable tannins, also called condensed tannins, can not be broken down with acid or base.  They belong to a class of polyphenols called anthocyanins. These compounds give many fruits and berries their red, blue or purple colour.  Examples include grapes, blueberries, bilberries, cranberries, and apples.

Bilberries

How do astringent herbs work?

Astringent herbs have many related medicinal actions associated with the tannins. The hydroxyl groups on the tannins are capable of undergoing several different reactions and interacting with various proteins and enzymes in the body. It is worth noting that hydrolyzable tannins are significantly more astringent than condensed tannins.  The following actions are associated with essentially all tannins.

Antiulcer

Astringent applied topically helps heal ulcers (e.g. venous leg ulcers) and internally for gastrointestinal ulcers (e.g. peptic ulcer, colitis). Tannins react with the damaged tissue to form a protective layer or “second skin” that shields the damaged tissue from both chemical and mechanical irritation.4  (See above image below). In addition, the tannins make the wound more resilient to micro-organisms because they have difficulties attaching and infecting it. In particular, tannins have been shown to have activity against the Helicobacter pylori, the bacteria that causes stomach ulcers.

Leg ulcer treated with tannins

Hemostatic & Styptic

Astringent herbs have long been used as hemostatics to stop bleeding.  Styptics specifically stop bleeding when applied topically to wounds.  Tannins stop bleeding by forming cross-linking with the proteins present in the damaged tissue and this tightens the skin and close wounds.

Anti-inflammatory

Astringent herbs help reduce inflammation by inhibiting a number of enzymes involved in inflammation.  Both hydrolyzable tannins1,2 and condensed tannins (anthocyanins) exert significant anti-inflammatory effects.1–3

Antioxidant

In general, phenolic compounds act as antioxidants.  The hydroxyl groups catch free radicals and stabilize them in the phenolic ring.  Tannins are either made from simple phenolics (hydrolyzable) or polyphenols (non-hydrolyzable) both of with have high antioxidant effects.

Antidiarrheal

In our digestive tract, tannins alter the permeability of the mucous membranes thereby exerting an antisecretory effect.  This reduces the influx of water into the lumen of the bowel and reduces watery stools. By altering the surface of the intestines, toxins and microorganisms have difficulties attaching to the cells. Furthermore, tannins neutralize protein-based exotoxins produced by diarrhea causing micro-organism including Cholera & E. coli.5,6

Antiadherant

Micro-organism must first attach themselves to host cells before they can invade them.  They use their “hands” to grab glycoproteins or  “handles” on the surface of tissues. These ‘hands” consist of proteins that are susceptible to tannins. Exposure to condensed tannins can help prevent and reduce the severity of certain infections including colds and bladder infections.7,8
Crane's Bill - Astringent herbs

Crane’s Bill

Are astringent herbs safe?

Astringent herbs are relatively safe when used as indicated.  However, there are some safety concerns.  Very high amounts of tannins can be toxic if absorbed systemically.  In addition, because tannins alter the structure of proteins they can disrupt the function of enzymes and other proteins in the body.  Taking high amounts of tannins internally can alter the mucous membranes and interfere with digestion and cause abdominal pain and constipation.9  In addition, tannins have an anti-nutritive effect because they can chelate minerals like iron.10–12  However, supplementing with Vitamin C can inhibit the effects that tannins have on iron absorption.13 Milk is often added to black tea and heavy oaky red wine is consumed with meat to bind up the tannins and reduce undesirable effects.  Consuming highly astringent herbs long-term should be used with care.

Summary of Astringent Herbs

Astringent herbs are rich in tannins that bind to proteins.  Tannins exist as hydrolyzable tannins and condensed tannins, with the former being more astringent in nature. Tannins react with surface proteins on exposed tissue to create a protective layer or a “second skin”. This stops bleeding, facilitates healing and prevents infection. Astringent herbs have numerous associated medical actions including antiulcer, anti-inflammatory, hemostatic, styptic, antioxidant, anti-adherence and antidiarrheal.  They are commonly used for wound healing, ulcers, and diarrhea.  Long-term use in not recommend because they can interfere with digestions and also nutrient absorption.

Author & Photographer: Matt Gowan, BSc, ND
All images are copyright of Matt Gowan ©2017 Reference

Disclaimer: This content is subject to change. The information is intended to inform and educate; it does not replace the medical evaluation, advice, diagnosis or treatment by a healthcare professional. www.nhpassist.com © 2017 NDAssist Inc. and/or its affiliates. All rights reserved.

References: Astringent Herbs – The use of tannins in medicine

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