Tannins are said to decrease feeding, nutrient absorption and protein digestibility in animals, according to PubMed. They are also deemed to be carcinogenic because of their relation to the incidence of certain esophageal cancer.
According to the PubMed article, tannins are polyphenols found in tea and other plant foods. Tannin in tea is not necessarily bad. An article in PubMed mentions various reports about the negative effects of tannin and some possible health benefits depending on the type of tannin and dosage. Some findings support the beneficial effects of tannins. Well-documented studies show the anticarcinogenic, antimutagenic and antimicrobial properties of tannins. However, a study in another PubMed article even investigates how tannins can inhibit HIV. Reduced blood pressure, accelerated blood clotting, decreased serum lipid levels and controlled immunoresponses are some of the other positive physiological effects of tannins.