First of all, I wish more people would act as you and your friends do and give blood, but currently only five out of every 100 Americans donate. You're eligible if you're over 17 years old (16 in some states), weigh at least 110 pounds, are in good health, and haven't donated blood in the last eight weeks.
As you know, when you go to donate blood, your hemoglobin (the iron- containing protein in red blood cells that carries oxygen) or hematocrit (the measure of red blood cells found in whole blood) is tested. And yes, it is true that drinking a lot of tea can cause a drop in hemoglobin. The tannins in tea are the culprits – these are naturally occurring polyphenols (antioxidant compounds) that can give tea an astringent taste if it is brewed too long.
Tannins can lower hemoglobin because they block iron absorption. However, the effect of tea tannins on iron absorption seems to be limited to non-heme iron found in vegetables and grains, not heme iron from meat, fish and poultry. You're unlikely to run into a problem with iron absorption or a drop in hemoglobin unless you're a vegetarian who drinks a lot of tea.
Incidentally, tea (green or black) isn't the only food that influences iron absorption. Other caffeinated beverages, eggs, milk, and bran also interfere. Foods that enhance iron absorption include orange juice and other foods high in vitamin C, as well as fermented foods such as miso, yogurt and sauerkraut. Foods that provide iron include red meat, beans, lentils, millet, chickpeas, dark leafy greens, molasses, dried apricots, dried peaches, pumpkin and sunflower seeds, pistachios, walnuts, almonds, scallops, clams, oysters and soybeans.
For information on becoming a blood donor go to the Red Cross website.
Andrew Weil, M.D.