Are tannins a double-edged sword in biology and health?

Vegetable tannins are water-soluble polyphenols that are present in many plant foods. They can be divided into hydrolysable and condensed tannins. Tannins are considered nutritionally undesirable because they precipitate proteins, inhibit digestive enzymes and affect the utilization of vitamins and minerals. Tannin components have also been implicated in the high levels of cheek and oesophageal cancers in certain regions of the world. However, many tannin molecules have been reported to reduce the mutagenicity of a number of mutagens. Tannins were also reported to have anticarcinogenic activity. The growth of fungi, bacteria, and viruses has been inhibited by tannins. Tannins in food plants serve as a natural defence mechanism against microbial infections. Thus, tannins can theoretically serve as natural regulators of the microbial population in different habitats including the human gastrointestinal tract. Tannins have also been reported to produce other physiological effects such as immune response, hepatotoxicity and lipid metabolism. Ingestion of large quantities of tannins may result in adverse health effects. However, the intake of a small quantity of the right kind of tannins may be beneficial to human health.