Tannins

Tannins are bitter astringent polyphenol chemicals of plant origin that are capable of binding and shrinking proteins. The astringency is what causes the puckery and dry feeling in the mouth after eating fruits rich in tannins or drinking teas and red wine.

Tannins are broadly divided into two categories: hydrolysable tannins and condensed tannins (proanthocyanidins) discussed in more detail here.

Hydrolysable tannins possess so called anti-nutrient qualities: they act as metal ion chelators considerably inhibiting non-heme iron absorption. Non-heme iron is found in plant foods and iron supplements. Animal sources of iron are not affected by tannins.

Thus, consuming too much tea or coffee may lead to anemia. The consumption of one cup of tea with a meal has been found to decrease the absorption of non-heme iron in that meal by about 70%.

In order to counter these problems, it is advised that one should take tea or coffee between meals and not with meals, and not consume iron supplements with these drinks either.

Also, adding lemon to tea helps reduce or neutralize tannins’ adverse effects on iron intake. However, adding milk to coffee or tea has very little to no influence on the inhibitory effect of tannins according to the study “Inhibition of non-heme iron absorption in man by polyphenolic-containing beverages” published in the British Journal of Nutrition in 1999. Consuming foods rich in vitamin C helps in neutralizing tannins' effects on iron absorption due to this vitamin's ability to increase iron absorption in the body.

Condensed tannins (proanthocyanidins) do not interfere with iron absorption.

Food sources of tannins: teas, coffee, pomegranates, persimmons, most berries (cranberries, strawberries, blueberries), grapes, red wine, chocolate (with cocoa content 70% and higher), spices (cinnamon, vanilla, cloves, thyme).