TANNINS

Two distinct types of tannins have been identified on the basis of structure. Condensed tannins are composed of many flavonoids or proanthocyanidins joined together. However, the properties of condensed tannins are distinct from their building blocks. Hydrolyzable tannins are composed of a glucose (or rarely, other monosaccharide or polyol) core with several catechin derivatives attached. These medium to large polymers are very widely distributed in the plant and fungus kingdoms. Condensed and larger hydrolyzable tannins have most properties in common, although hydrolyzable tannins are less stable (being susceptible to hydrolysis) and have greater potential to cause toxicity.

Smaller tannins are hot water soluble, but as the molecules get larger, they become less and less soluble in any solvent. Tannins are generally much less soluble in cold water. Thus, relatively tannin-free extracts can usually be made through cold infusion of crude herbs.

Tannins generally possess an astringent flavor and activity, which relates to their ability to indiscriminately bind proteins. Only triester and larger hydrolyzable tannins are astringent. Tannins draw tissues together as proteins congeal, causing a peculiar puckering sensation in the mouth. This also tends to inactivate bound proteins. Tannin-rich herbs are frequently used to slow proteinaceous discharges of all types, most notably, transudates such as those associated with atopic dermatitis skin lesions, diarrhea, and hemorrhages from the skin or in the gastrointestinal tract.

In vitro, condensed tannins inhibit a wide variety of enzymes, in keeping with their ability to bind and inactivate proteins. For example, various tannins have been shown to partially inhibit angiotensin-converting enzyme and aldose reductase, as well as human immunodeficiency virus (HIV) integrase, protease, and reverse transcriptase. However, their bioavailability is extremely low, so it is unknown to what extent they have relevant activity systemically, as opposed to on the skin or in the gastrointestinal tract. Nevertheless, tannin-rich herbs are commonly used in traditional herbalism to quell internal bleeding and for a variety of actions, such as modulation of systemic inflammation, that suggest that they are circulating and active.

Tannins tend to bind or chelate many other types of molecules besides proteins, most notably, divalent cations and alkaloids. This can render the bound agents inactive and insoluble. Thus, tannins generally should not be administered simultaneously with any medication or nutrient because of the potential for reduced absorption and activity. Tannin-rich herbs do not tend to mix well with other herbs in formulas, although sometimes, this may reduce the potential toxicity of some herbs.

Tannins can cause nausea fairly readily, perhaps as a result of protein binding within the stomach and duodenum (Major Tannin-Rich Herbs). Consumption of tannins with food reduces this problem. For herbs that contain tannins and other beneficial constituents, but with which the tannins are causing nausea or otherwise are interfering, cold infusions are recommended to produce low-tannin extracts. Tannins also tend to be mildly constipating. High levels of tannins absorbed into the bloodstream can cause serious constipation and hepatotoxicity, as well as damage to other organs. Ruminants are particularly susceptible to these effects.