Black tea appears to work about just as well as green tea, but then why is green tea associated with lower heart disease risk while black tea is not?

Tea and Artery Function
by NutritionFacts.org on YouTube

Researchers found the “addition of milk to black tea completely prevents the biological activity of tea in terms of improvement of endothelial function.” So, that could explain it. It appears the milk protein casein is the culprit, though soy protein was recently found to have the same nutrient binding effect. The European Society of Cardiology issued a press release about the study showing the protective effect of tea “is totally wiped out by adding milk” and suggested consumers should consider cutting down.