QUALITIES AND ACTION
The original Latin derivation of the word astringent, “adstringere”, means “to bind together” and gives a clue to this herbal category’s action (Gowan, 2017).  Astringent herbs promote the tightening of tissues mainly due to the presence of tannins, a type of flavonol which has two main classes; condensed and hydrolyzable tannins (Hoffman, 2003).
Hydrolyzable tannins are phenolic compounds attached to a sugar molecule, they are soluble in water and alcohol and are broken down by acid, alkali or certain enzymes (Hoffman, 2003). Generally speaking, these are more astringent in character and are commonly found in the bark and roots of plants. They are more likely to be brown or yellow in nature and examples include Hamamelis virginiana and Quercus robur (Gowan, 2017) and Alchemilla vulgaris (Duckstein et. al, 2012).

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Condensed tannins, also known as proanthocyanidins, are a common constituent in many plants (Hoffman, 2003). This group of flavolans are the precursors to anthocyanidins and can be widely found in fruit bearing plants and members of the rosaceae family such as Rubus idaeus (Busia, 2016). However, it is important to note, that both types of tannin can in fact occur together in the same plant (Hoffman, 2003).
Tannins work by forming complexes with proteins, an action that has been used for hundreds of years in the practice of leather tanning (Hoffman, 2003). Acting on the body’s mucous membranes and mucosa epithelial cells, they can create a protective layer, dry up secretions and cause cell membranes to contract (Bruneton, 2008). In addition, they also reduce inflammation and swelling, reduce irritation, stop bleeding (internally and externally) and create a barrier to prevent infection (Hoffman, 2003). It is this wide variety of different actions that often mean many herbal astringents are also referred to as styptics and vulneraries (Hoffman, 2003).
However, not all astringents work directly via tannins, Aesculus hippocastanum and Equisetum arvense both being examples of this. A. hippocastanum instead is higher in aescin, a triterpenoid saponin which is responsible for its astringing action. It is commonly indicated for its ability to bring tone to veins which may have become too relaxed or distended such as in the case of varicose veins, piles, oedema and chronic venous insufficiency (Bone & Mills, 2013).  A Cochrane review of seventeen different randomised controlled trials into chronic venous insufficiency found that overall patients experienced an improvement in the symptoms of leg pain, oedema and pruritus when A. hippocastanum capsules were taken over a two to 16-week period (Pittler and Ernst, 2012).
E. arvense on the other hand contains a large amount of silicic acid and silicates and it is the plant’s water-soluble silica content, that gives way to its therapeutic action (Chevallier, 2000).  It has a particular affinity for genitourinary conditions such as cystitis, prostate issues, incontinence and other bladder issues, and as a haemostatic, it also assists wound healing (Evans, 2009) indicating its use for nosebleeds and reducing the coughing up of blood (Chevallier, 2000). 
INDICATIONS, PAITENT TYPE & BODY SYSTEM AFFINITY
Regardless of their differing constituents what ties the astringents together is their indication for damp flowing and relaxed tissue states (Wood, 2004). According to Wood, this is any condition where tissues are prolapsed or unable to hold onto secretions such as diarrhoea, excessive sweating, digestive secretions, menorrhagia and urination issues. In the case of the vascular system becoming overly relaxed, it can present as passive venous congestion whereby blood collects causing haemorrhoids and varicose veins as previously mentioned (2004).
Whilst they have a wide and varied application for many health concerns astringents really come into their own when addressing digestive issues and wound healing. However, when it comes to interchangeability of herbs within the category, Hoffman explains that some have more of an affinity to certain body systems than others, and this should therefore be considered in the clinical setting (Hoffman, 2003).
A. hippocastanum has already been discussed for its merits to the cardiovascular system but another astringent that could be used instead of, or alongside, is that of H. virginiana. H. virginiana, like some other astringents, can be used as a styptic – it is especially indicated in the treatment of haemorrhoids and varicose veins (Hoffman, 2003), with research suggesting that it exerts local styptic and vasoconstrictive effects to the affected areas (Shenefelt, 2011).
In the digestive system astringents are indicated for the treatment of diarrhoea. Filipendula ulmaria and Agrimonia eupatoria are the gentler treatment options and help to improve digestive tone. The latter of which is especially indicated when an alternating bowel pattern, such as that associated with irritable bowel syndrome, exists (Hoffman, 2003). Q. robur is also indicated for diarrhoea but only in chronic conditions. It has a very high content of astringent tannins and as a result might be considered too potent for certain situations, therefore, it might not be a suitable substitute in the clinic environment (Hoffman, 2003). Since diarrhoea can be as a result of bacterial infection from Campylobacter or Salmonella (Hoffman, 2003), Hydrastis canadensis is another herbal consideration, not only will it astringe the mucous membranes in the gut, but as an antibacterial it will also counter infection (Chevallier, 2000). 
Due to their styptic and vulnerary nature some astringents are especially useful in conditions of the digestive system where local bleeding may occur. One such condition is gastris, here F. ulmaria is especially indicated since it also has the ability work as an antacid (Hoffman, 2003). This also indicates F. ulmaria’s use in ulcerative conditions such as peptic ulcers which occur due to increased secretions of stomach acid, pepsin and also the bacteria Helicobacter pylori (Hoffman, 2003). Hoffman goes on to explain that this could be why H. canadensis is also an effective anti-ulcer treatment, especially since bitters are normally contraindicated due to their ability to increase gastric secretions (2003). Interestingly enough, a recent research study into the effectiveness of herbal medicine in the treatment of gastric diseases found that out of 21 herbal extracts examined A. eupatoria, H. canadensis, F. ulmaria and Salvia officinalis were the most active herbal extracts in inhibiting the growth of H. pylori when compared to antibiotics, this therefore presents a number of clinical alternatives in the treatment of ulcerative and gastric conditions (Cwikla et al., 2009).
H. canadensis also works in a similar way on the mucous membranes of the respiratory system and is indicated as an anticatarrhal, especially when accompanied with inflammation (Bone & Mills, 2013). Plantago lanceolata, can also be used to treat respiratory congestion and catarrh and as it is more widely available, it is a cheaper and more sustainable option. This makes it a viable alternative to H. canadensis unless the yellow or green discharge, associated with a chronic condition, is presenting (Bone & Mills, 2013).
The styptic and vulnerary qualities of astringents are of course useful externally on the skin (Hoffman, 2003).  Achillea millefolium is one such herb that works in this way and has had a long history of use for its ability to staunch bleeding, as has Capsella bursa-pastoris. C. bursa-pastoris in fact being the go-to during the first world war when H. canadensis was not available (Chevallier, 2000).  Another herb that offers alternatives in this application is P. lanceolata it too can staunch blood effectively and may be used as an alternative to, or in conjunction with Symphytum officinale to aid tissue repair and broken bones (Chevallier, 2000). 
In terms of the reproductive system, unsurprisingly astringents have a place in treating menorrhagia and the antihemorrhagic qualities of C. bursa-pastoris, A. vulgaris and A. millefolium indicate them for the condition (Bone & Mills, 2013).
CONTRAINDICATIONS & SAFTEY
As a group of herbs, they are generally safe when used as indicated. Tannins, having a large molecule and along with their ability to bind to proteins, means they are not easily absorbed into systemic circulation. It is in fact the metabolites of the tannins which are produced once they are consumed, that are absorbed (Bruneton, 2008). That said, because of the way they work in the body, habitual use should be avoided as there is some concern that high amounts can cause excessive astringency and lead to irritation of the gut and in some cases abdominal pain and constipation (Smeriglio et al., 2017). In addition to which, overuse may also cause inhibition of food absorption across the gut wall and so care must be taken with patients who are malnourished (Hoffman, 2003). Iron deficiency is also a consideration, some sources cite that tannins can chelate iron and other minerals (Bruneton, 2008) but a more recent research review suggested that changes in iron levels may not be related to tannin intake and that further research should be concluded in this area (Delimont, 2017). Finally as herbal medicine practitioners, consideration should be given when prescribing, tannin rich herbs can precipitate with alkaloids and this may reduce the absorption and therefore limit some of the action sought by herbs high in alkaloids (Bruneton, 2008).
SUSTAINABILITY & AFFORDABILITY
Despite the fact that Britain is one of the largest importers of medicinal herbs, the majority of the astringents discussed here can be found in our local hedgerows (Seal and Seal, 2018). This therefore avoids negative environmental impacts in the form of airmiles as well as any sustainability issues as a result of overharvesting due to excessive overseas demand (Seal and Seal, 2018). The one exception to this, is H. canadensis, which according to United Plant Savers, is in decline and listed on their ‘At Risk’ plant list (2018). However, as discussed earlier, pertinent research which would suggest that there are alternatives to using this herb. Since expensive doesn’t necessarily equate to better, just simply rarer, other equally effective solutions are beneficial to both the environment and to patients in terms of economic cost and affordability (Hoffman, 2003).
Reference List

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