That said, many people, and especially women, do in fact have low iron levels in their blood. There is also reason to believe that low body iron stores may make for a feeling of lack of energy, and for difficulties in pregnancy. It is important to realize that certain foods inhibit iron absorption, whether from foods or from supplements. Among the foods that inhibit iron absorption, black tea is one of the most potent. Black tea contains tannins, which are chemical compounds which bind with both the iron in the food that you eat and the iron you take as a supplement, thereby preventing your intestine from absorbing the iron.
Tannins can bind proteins tightly together.
They are found in plants of all kinds, but certain plants and plant parts have a very high content of tannins. It appears that plants, and especially their outer surfaces such as the bark or the skin of a fruit, contain tannins to help fend off insects. For example, a major source of industrial tannins for tanning leather is the bark of chestnut trees. A number of foods contain tannins. The amount of tannins in these foods is probably too low to cause difficulties with iron absorption.