Having a cup of tea with a meal decreases the body's absorption of non-heme iron (the only type of iron provided by plant-based foods, dairy products, and eggs) contained in that meal by roughly 60 percent.
This is particularly important for vegetarians as well as anyone diagnosed with anemia, as a meal moderately high in iron can lose quite a bit of that punch if you're accompanying it with tea (thanks to substances known as phytates).
If you're a tea lover, the safest bet is to drink it an hour before or after a vegetarian meal, so as to not inhibit absorption.
If you're interested in increasing non-heme iron absorption during a meal, be sure to include vitamin C (meat also aids in the absorption of non-heme iron, but this is a moot point for vegetarians and vegans.)
A cup of orange juice, for instance, increases absorption by approximately 80 percent!
Tea isn't the only inhibitor, by the way.
All foods containing phytates (mainly whole grains, coffee, and berries) decrease non-heme iron absorption rates.