A study at the Charité Hospital of the Berlin Universities showed that adding milk to tea will block the normal, healthful effects that tea has in protecting against cardiovascular disease. This occurs because casein from the milk binds to the molecules in tea that cause the arteries to relax, especially EGCG. Milk may also block tea's effect on other things, such as cancer. Other studies have found little to no effect from milk on the observed increase in total plasma antioxidant activity. Teas with high EGCG content, such as green tea, are not typically consumed with milk. Previous studies have observed a beneficial effect from black tea which was not attributable to the catechin content. Plant-based "milks", such as soy milk, do not contain casein and are not known to have similar effects on tea.
Milk binds catechins, most notably EGCG. Milk also binds tannin, rendering it harmless, which helps to exemplify the effect on tea's constituent parts (i.e. EGCG binding)
Catechins are a kind of polyphenol, a group of chemicals long thought to underlie the beneficial effects of a plant based diet. EGCG is the primary antioxidant in green tea.
Dr. Robert Vogel, a professor of medicine at the University of Maryland Medical School who agrees that tea is one of the greatest sources of antioxidants. In countries where they drink a lot of tea, heart disease is decreased, except for the British Isles, where it is typical to add milk to the tea. His advice is to add lemon to tea, but not milk or cream.
