TANNINS AND HUMAN HEALTH - A REVIEW

Tannins (commonly referred to as tannic acid) are water-soluble polyph enols that are present in many plant foods. They have been reported to be responsible for decreases in feed intake, growth rate, feed effici ency, net metabolizable energy, and protein digestibility in experimen tal animals. Therefore, foods rich in tannins are considered to be of low nutritional value. However, recent findings indicate that the majo r effect of tannins was not due to their inhibition on food consumptio n or digestion but rather the decreased efficiency in converting the a bsorbed nutrients to new body substances. Incidences of certain cancer s, such as esophageal cancer, have been reported to be related to cons umption of tannins-rich foods such as betel nuts and herbal teas, sugg esting that tannins might be carcinogenic. However, other reports indi cated that the carcinogenic acitivity of tannins might be related to c omponents associated with tannins rather than tannins themselves. Inte restingly, many reports indicated negative association between tea con sumption and incidences of cancers. Tea. polyphenols and many tannin c omponents were suggested to be anticarcinogenic. Many tannin molecules have also been shown to reduce the mutagenic activity of a number of mutagens. Many carcinogens and/or mutagens produce oxygen-free radical s for interaction with cellular macromolecules. The anticarcinogenic a nd antimutagenic potentials of tannins may be related to their antioxi dative property, which is important in protecting cellular oxidative d amage, including lipid peroxidaton. The generation of superoxide radic als was reported to be inhibited by tannins and related compounds. The antimicrobial activities of tannins are well documented. The growth o f many fungi, yeasts, bacteria, and viruses was inhibited by tannins. We have also found that tannic acid and propyl gallate, but not gallic acid, were inhibitory to foodborne bacteria, aquatic bacteria, and of f-flavor-producing microorganisms. Their antimicrobial properties seem ed to be associated with the hydrolysis of ester linkage between galli c acid and polyols hydrolyzed after ripening of many edible fruits. Ta nnins in these fruits thus serve as a natural defense mechanism agains t microbial infections. The antimicrobial property of tannic acid can also be used in food processing to increase the shelf-life of certain foods, such as catfish fillets. Tannins have also been reported to exe rt other physiological effects, such as to accelerate blood clotting, reduce blood pressure, decrease the serum lipid level, produce liver n ecrosis, and modulate immunoresponses. The dosage and kind of tannins are critical to these effects. The aim of this review is to summarize and analyze the vast and sometimes conflicting literature on tannins a nd to provide as accurately as possible the needed information for ass essment of the overall effects of tannins on human health.