Black tea improves the body's ability to relax its arteries, but not when milk is added! They tested both humans and rats.
"Their study showed that the culprit in milk is a group of proteins called caseins, which they found interacted with the tea to decrease the concentration of catechins in the beverage."
There are a couple of suspicions I have about the study. First of all is their insistence on the word 'catechins' as in the above quote. While green tea has a large concentration of catechin type polyphenols, black tea only has about 1/4 the amount. Catechins are converted during the tea oxidation/fermentation process to theaflavins--an entirely different class of flavonoids which are practically non-existent in green tea (unoxidized) but abundant in black tea (fully oxidized). Theaflavins are responsible for the antioxidant activity that give black tea its benefits, and would presumably cause arterial dilation.
Whether their chemical nomenclature is actually faulted or not does not however call into question the empirical results of the study. But I wonder about these results as well. Sure, black tea on an empty stomach is going to be absorbed by the body quickly and all at once compared to black tea and milk. But those newly formed "catechin-casein" compounds will ultimately be broken down by digestion. Won't they then be absorbed by the body just the same? It doesn't seem the caseins would be able to somehow block the effects of the tea, but rather simply slow their onset until all the compounds in the beverage are broken back down by the body. The study only measured the test groups' arterial pressure twice, once before beverage ingestion, and once two hours after ingestion. It could easily be imagined that an hour later the effects of pure black tea have worn off and those of the milky tea are now more noticeable. In any case, the study is not too convincing.
Now what about the health of tea-drinkers that routinely put milk in their morning cup? It would be interesting to compare the health of nations with this reputation, such as England, compared to that of non-dairy purists such as China. I tried looking online for hypertension rates of these nations, but there was no one study that compared both in the same way. Depending on the study that analyzes hypertension rates in adults for either country, they are reported as anywhere from 10-40%, and aren't consistent enough to trust. A deeper look into this might be coming soon...