Teasing out tea's heart benefits.

Epidemiological studies have found that tea drinkers face a lower risk of heart attacks than people who eschew the brew (SN: 10/30/93, p. 278). Curious to know why, researchers have begun focusing on flavonoids--tea's natural antioxidants.

Cholesterol-carrying low-density lipoproteins (LDLs) in the blood can be oxidized by naturally occurring molecular fragments known as free radicals. Such oxidized LDLs may eventually become part of artery-clogging plaque. The antioxidant potential of flavonoids has fostered a suspicion that tea's heart benefits come from its quenching of free radicals.

Data presented 2 months ago by Lester A. Mitscher of the University of Kansas in Lawrence fueled these suspicions. At the American Chemical Society national meeting in Las Vegas, he reported finding that when it comes to inhibiting oxidation, some of green tea's flavonoids are up to 25 times as powerful as vitamin E and 100 times as powerful as vitamin C.

Now, a Dutch study questions whether flavonoids actually deliver this kind of antioxidant protection to LDLs.

Forty-five healthy men and women drank 6 cups of water, green tea, or black tea daily for 4 weeks. Then Karin H. van het Hof and her colleagues at Unilever Research Laboratory in Vlaardingen extracted LDLs; from samples of the volunteers' blood and subjected these fatty particles to oxidants. To their surprise, all of the LDLs proved equally vulnerable to oxidation.

When the LDLs were extracted from the blood, the water soluble flavonoids stayed behind. This suggests that flavonoids do not enter LDLs; to guard their cholesterol, the Unilever team reports in the November American Journal of Clinical Nutrition. However, van bet Hof notes, flavonoids might protect LDL cholesterol by quenching free radicals in the blood or vessel walls. It's even possible, she observes, that flavonoids protect the heart through a totally independent means, such as by acting on enzymes that play a role in the formation of blood clots.