This section is from the book "Practical Dietetics With Special Reference To Diet In Disease", by William Gilman Thompson. Also available from Amazon: Practical Dietetics with Special Reference to Diet in Disease.
Tannin is an astringent of vegetable origin which exists in tea, coffee, and many wines, especially the red wines, and as such it is worthy of brief separate consideration. It possesses no nutrient power whatever, and is mainly of interest to the dietetist from the harm it may occasion if taken too freely. In strong solution it precipitates the ferment of the gastric juice and renders it inert, and also gives rise to constipation by its astringency, which affects the mucous membrane of the intestine. For these reasons strong tea or tea drunk to excess materially hinders gastric digestion.
According to Fraser, the tannin in tea interferes with the digestion of fresh meat, but to a less extent with that of dried or smoked meat, such as tongue or ham, the fibres of which are already shrunken by curing.
Coffee contains much less tannin than tea, thus it has not the same effect upon the alimentary canal, in fact, it may be laxative.
Tannin is contained in red wines in considerable quantity, hence clarets are mildly astringent and constipating.
Tannin is useful for a variety of local astringent applications.