Tannins

Tannins are a class of plant polyphenols that bind to proteins and metals, giving them their characteristic astringency — that drying, puckering sensation in black tea, red wine, dark chocolate, walnut skins, and underripe persimmons. They're also present in coffee, pomegranate, and many beans and grains, especially in their seed coats.

For most people, tannins are part of the flavor and antioxidant profile of healthy foods and not a clinical concern. The main practical effect is on iron absorption — tannins bind non-heme (plant-source) iron in the gut and significantly reduce its bioavailability when consumed at the same time as iron-rich meals.

Iron absorption interaction

A cup of black tea with a meal can reduce non-heme iron absorption by 50–70%. This matters most for people at risk of iron deficiency: menstruating women, pregnant women, vegetarians, vegans, distance runners, and people with chronic blood loss. Heme iron (from meat) is much less affected by tannins. Practical workaround: drink tea and coffee between meals rather than with them, and separate iron-rich meals from tannin-rich beverages by an hour or two.

Sources

Highest: strong black tea, red wine, dark chocolate, walnut skin, unripe persimmon, sorghum. Moderate: green tea, coffee, pomegranate juice, beans, some grapes. Tannins also have positive sides — antioxidant activity, mild antimicrobial effects, possible cardiovascular benefits from polyphenols in tea and wine. The case for moderation is about pairing, not avoidance.