Quercetin appears to provide protection against cardiovascular disease even though the body breaks it down before it enters the blood, according to a new study conducted by researchers at the Institute of Food Research and published in the journal "Atheroscleroisis."

While many studies have linked quercetin - a flavonoid that naturally occurs in onions, apples, tea and wine - to a reduced risk of cardiovascular disease, they have usually tested the chemical's direct effects on cells. But because the intestines and liver break down quercetin rapidly, the chemical never actually reaches the blood when consumed in food.

"We tested compounds that are actually found in the blood, rather than the flavonoid in food before it is eaten, as only these compounds will actually come into contact with human tissues and have an effect on arterial health," lead researcher Paul Kroon said.

Yet when tested on the cells lining human blood vessels, both quercetin and its metabolites (which are produced as the body breaks it down) were found to reduce the inflammation that can lead to cardiovascular disease.

"The effect is more subtle than laboratory experiments using [quercetin]," Kroon said. "But we can confirm that eating quercetin-rich foods may help prevent chronic inflammation leading to cardiovascular disease, because the metabolites still have an effect on the cells lining the blood vessels."

Lower doses of quercetin metabolites actually appeared to provide a greater protective benefit than higher doses. The most effective dose was achieved from the equivalent of eating 100 to 200 grams (3.5 to 7 ounces) of onions, the researchers said.

In addition to its cardiovascular and general anti-inflammatory benefits, quercetin has been linked to a decreased risk or increased recovery from cancer, allergies, cataracts, prostatitis and respiratory diseases including asthma and bronchitis.

The global market for flavonoids, the plant compounds of which quercetin is one type, is expected to grow 12 percent annually.

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