Milk can do your body good, but maybe not when it's in your tea.

Women in a study who drank black tea had improved cardiovascular function -- but that protection vanished if they drank it with milk. Temper the taste of your black tea with lemons instead. Or sip it as the Chinese traditionally do: straight up.

Researchers are not sure why milk may blunt tea's heart-healthy effects, but milk proteins called caseins are possible culprits. Tea is bursting with health-boosting polyphenols, but proteins have been shown to counteract them. In one study, when researchers added a small amount (10 percent) of milk to black tea, it reduced the tea's concentration of catechins -- polyphenols credited with giving tea its antioxidant punch as well as fighting heart disease and boosting weight loss. The study results may help explain, in part, why tea's heart benefits appear to be missing in the United Kingdom, where milk is usually added to the brew.

It's not a done deal, however. Other studies have concluded that milk has no effect on tea's antioxidant powers. But the conflicting study results may simply be due to the way the scientists measured the good stuff in tea.

In this recent study, researchers measured the effects of tea -- with and without milk -- on blood vessels in the cardiovascular system. Straight black tea helped blood vessels relax and widen, allowing blood to flow more freely. Tea with milk did not produce the same effect.

Which is best -- black or green tea? Read this tip.