Drinking tea could cause iron deficiency, if you drink a lot of tea with meals and don't consume adequate iron. Studies have shown that tea—notably black tea, but also green—decreases the absorption of iron, especially nonheme iron, the kind found in plant foods (heme iron comes from animal-derived foods).
Compounds in tea called tannins can combine with nonheme iron and make it less available for absorption. Drinking tea with a meal can decrease iron absorption by 50% or more, and that may increase the chances of iron deficiency. Putting lemon juice in the tea helps counter the negative effect by increasing iron absorption. Coffee may also decrease iron absorption, though not as much as tea.
Even though tea inhibits iron absorption, you still absorb some iron. But if you're a premenopausal woman, for instance, and drink tea with every meal, or drink many cups a day, while eating minimal amounts of iron-containing food, it's possible that the tea could promote iron deficiency. If you have an iron deficiency, or are at high risk for it, wait at least an hour after a meal before drinking tea.
Keep in mind: The body is able to use only a portion of the nutrients it takes in, an amount determined by many variables. Interactions among nutrients and other compounds are a major factor. Thus, tannins in tea may decrease iron absorption, but vitamin C—as in lemon juice—increases it. To get the nutrients you need, you have to balance the positives and negatives. Although drinking a lot of tea may cause iron deficiency, tea in moderation can be part of a healthy diet.
Originally published in The University of California, Berkeley Wellness Letter (January 2011)