Answer : In general, theres no need to be concerned about tea blocking the absorption of vitamins or other nutrients, according to Jeffrey B. Blumberg, PhD, director of the Antioxidants Research Laboratory at Tufts Jean Mayer USDA Human Nutrition Research Center on Aging, who has studied tea at length. The extensive history of daily, long-term intake of tea indicates a high level of safety, although that usage does not adequately address the issue scientifically, Blumberg says. Other than the issue of caffeine intake, I am aware of only two documented situations where tea may have an adverse impact on nutritional status. These involve the possibility that tea might affect levels of folate and iron in the body.
In the first case, while many observational studies and some clinical trials suggest tea reduces the risk of cancer, a few test-tube studies suggest a potential pro-carcinogenic mechanism of tea catechins (naturally occurring antioxidant compounds) by reducing folate levels. In the only human study available to date examining this effect, researchers tested a green-tea extract and a black-tea extract in a crossover randomized clinical trial. The study measured bioavailability-the ability of the body to use a nutrient-by giving a high dose of folic acid to seven healthy individuals and then measuring their blood levels of folate. Only the green-tea extract, which contained 51 milligrams of green-tea catechins, significantly reduced folate levels.
In the second instance, a systematic review addressed concerns about the potential of tea to inhibit the absorption of iron-specifically the nonheme iron found in plants as well as in animal tissue. The 35 studies on iron absorption and black tea that were reviewed didnt include any clinical trials. Based on the 12 highestquality studies, there was a measurable inhibition of iron bioavailability, but one that presents no threat to healthy adults with normal iron status. While this may present the potential for an adverse impact in people with iron-deficiency anemia, Blumberg notes, it may also present a benefit to people (especially older men) with a high iron intake, as some studies suggest increased iron status is a risk factor for cardiovascular disease.