Tannins are water-soluble polyphenols that are present in many plant foods and affect the nutritional value of foods or beverages because they bind to proteins. Most plants contain tannins however algae, fungi and mosses contain very little. While generally safe for humans, in animals, tannins can impair the digestibility of food. They can contribute an astringent taste to the foods in which they are found, but also play a role in the pigment of flowers and leaves. They are known to possess antibacterial, antifungal, anti-inflammatory, antiviral and anti-haemorrhagic (prevent easy bleeding) properties.
The growth of many fungi, yeasts, bacteria, and viruses are inhibited by natural food tannins but eating too much of them without vitamin C can cause some health issues.
With the exception of tea, long-term or excessive use of herbs containing high concentrations of tannins is not recommended. A correlation has been made between the above symptoms and nasal, oesophageal and throat cancer in humans and regular consumption of certain medicinal herbs with high tannin concentrations. All the following herbs should not be used over long periods on a daily basis.
Herbs and spices that contain high levels of tannins
St. John's wort
If ingested in excessive quantities, tannins also inhibit the absorption of minerals such as iron, which may, if prolonged, lead to anaemia. In order to prevent these problems, it is advised to drink tea between meals, not during. Foods rich in vitamin C help neutralise tannin's effects on iron absorption. Adding lemon juice to tea will reduce the negative effect of tannins in iron absorption as well.
Many foods contain added artificial tannins as a yellow colouring (annatto) or have been produced with tannin leaching from the wood during storage which have been shown to be detrimental to the health.
Food and drinks that may contain artificial tannins
Carob bean products
Wood-aged beer, cider, red and white wines or spirits
Wood smoked meat and fish
Tannins have been associated with fatal liver damage from extensive use on burns or in enemas. However, the toxicity may be due to an impurity, digallic acid, rather than the tannins themselves.
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