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A research survey, and updated analysis of how flavonoids work and
function in the human body, were recently published in Free Radical
Biology and Medicine, a professional journal.
"What we now know is that flavonoids are highly metabolized, which
alters their chemical structure and diminishes their ability to
function as an antioxidant," said Balz Frei, professor and director of
the Linus Pauling Institute. "The body sees them as foreign compounds
and modifies them for rapid excretion in the urine and bile."
Flavonoids are polyphenolic compounds with some common characteristics
that are widely found in fruits and vegetables and often give them
their color they make lemons yellow and certain apples red. They are
also found in some other foods, such as coffee, tea, wine, beer and
chocolate, and studies in recent years had indicated that they had
strong antioxidant activity and because of that, they might be
important to biological function and health.
"If you measure the activity of flavonoids in a test tube, they are
indeed strong antioxidants," Frei said. "Based on laboratory tests of
their ability to scavenge free radicals, it appears they have 3-5
times more antioxidant capacity than vitamins C or E. But with
flavonoids in particular, what goes on in a test tube is not whats
happening in the human body."
Research has now proven that flavonoids are poorly absorbed by the
body, usually less than five percent, and most of what does get
absorbed into the blood stream is rapidly metabolized in the
intestines and liver and excreted from the body. By contrast, vitamin
C is absorbed 100 percent by the body up to a certain level. And
vitamin C accumulates in cells where it is 1,000 to 3,000 times more
active as an antioxidant than flavonoids.
The large increase in total antioxidant capacity of blood observed
after the consumption of flavonoid-rich foods is not caused by the
flavonoids themselves, Frei said, but most likely is the result of
increased uric acid levels.
But just because flavonoids have been found to be ineffectual as
antioxidants in the human body does not mean they are without value,
Frei said. They appear to strongly influence cell signaling pathways
and gene expression, with relevance to both cancer and heart disease.
"We can now follow the activity of flavonoids in the body, and one
thing that is clear is that the body sees them as foreign compounds
and is trying to get rid of them," Frei said. "But this process of
gearing up to get rid of unwanted compounds is inducing so-called
Phase II enzymes that also help eliminate mutagens and carcinogens,
and therefore may be of value in cancer prevention.
"Flavonoids could also induce mechanisms that help kill cancer cells
and inhibit tumor invasion," Frei added.
It also appears that flavonoids increase the activation of existing
nitric oxide synthase, which has the effect of keeping blood vessels
healthy and relaxed, preventing inflammation, and lowering blood
pressure all key goals in prevention of heart disease.
Both of these protective mechanisms could be long-lasting compared to
antioxidants, which are more readily used up during their free radical
scavenging activity and require constant replenishment through diet,
scientists say.
However, Frei said, its also true that such mechanisms require only
relatively small amounts of flavonoids to trigger them conceptually,
its a little like a vaccine in which only a very small amount of an
offending substance is required to trigger a much larger metabolic
response. Because of this, there would be no benefit and possibly
some risk to taking dietary supplements that might inject large
amounts of substances the body essentially sees as undesirable foreign
compounds.
Numerous studies in the United States and Europe have documented a
relationship between adequate dietary intake of flavonoid-rich foods,
mostly fruits and vegetables, and protection against heart disease,
cancer and neurodegenerative disease, Frei said.
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