[PHNUTR-L] Studies force new view on biology of flavonoids

[Kathrynne Holden]

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Public release date: 5-Mar-2007
http://www.eurekalert.org/pub_releases/2007-03/osu-sfn030507.php

Contact: Balz Frei
balz.frei at oregonstate.edu
541-737-5078
Oregon State University

Studies force new view on biology of flavonoids

CORVALLIS, Ore. – Flavonoids, a group of compounds found in fruits and 
vegetables that had been thought to be nutritionally important for their 
antioxidant activity, actually have little or no value in that role, 
according to an analysis by scientists in the Linus Pauling Institute at 
Oregon State University.

However, these same compounds may indeed benefit human health, but for 
reasons that are quite different – the body sees them as foreign 
compounds, researchers say, and through different mechanisms, they could 
play a role in preventing cancer or heart disease.

Based on this new view of how flavonoids work, a relatively modest 
intake of them – the amount you might find in a healthy diet with five 
to nine servings of fruits and vegetables – is sufficient. Large doses 
taken via dietary supplements might do no additional good; an apple a 
day may still be the best bet.

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 what’s 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, it’s also true that such mechanisms require only 
relatively small amounts of flavonoids to trigger them – conceptually, 
it’s 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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Studies Force New View on Biology, Nutritional Action of Flavonoids

By David Stauth, 541-737-0787

The Linus Pauling Institute is a national leader in the study of such 
phytochemicals, or plant chemicals that may affect human health.

This research was supported by the American Heart Association and the 
National Center for Complementary and Alternative Medicine, which is 
part of the National Institutes of Health.
-- 
Kathrynne Holden, MS, RD < fivestar at nutritionucanlivewith.com >
"Ask the Parkinson Dietitian"  http://www.parkinson.org/
"Eat well, stay well with Parkinson's disease"
"Parkinson's disease: Guidelines for Medical Nutrition Therapy"
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