Hoe werken bioactieve stoffen?*
Onderzoek door wetenschappers van het Linus Pauling instituut van de Oregon State University doet een nieuw licht schijnen over de rol van flavonoïden, bioactieve stoffen die gevonden worden in groenten en fruit en hen vaak de de karakteristieke kleuren geven. Uit eerdere, laboratorium, testen is altijd gebleken dat deze flavonoïden een grote antioxidatieve werking hebben en daarom zo gezond zijn. Nu blijkt dat in het lichaam flavonoïden slecht opgenomen worden, vaak niet meer dan 5%, en snel omgezet worden waardoor de antioxidatieve kracht snel afneemt. Ook probeert het lichaam, alsof het vreemde stoffen zijn, deze zo snel mogelijk uit het lichaam te verwijderen via darmen en urine. Toch heeft het bloed na inname van flavonoïdenrijk voedsel een fors hogere antioxidatieve kracht waarschijnlijk te wijten aan een verhoogde urinezuur waarde. Ook al zijn in het menselijk lichaam de flavonoïden geen antioxidanten, ze doen wel andere belangrijke dingen. Terwijl het lichaam er zo vlug mogelijk vanaf wil worden ook andere ongewenste soffen zoals fase2 enzymen afgevoerd waardoor mutagene en kankerverwekkende stoffen ook afgevoerd worden. Flavonoïden kunnen ook bepaalde mechanisme in het lichaam op gang brengen die kankercellen doen afsterven en het verspreiden tegengaan. Verder bevorderen flavonoïden stikstofoxide reacties in de bloedvaten waardoor deze gezond en soepel blijven en hierdoor de bloeddruk verlagen en ontstekingen tegengaan. Al deze belangrijke zaken gebeuren in het lichaam al bij relatief kleine hoeveelheden flavonoïden en ook nog langdurig. Dit is duidelijk in tegenstelling tot antioxidanten die bij hun werk, het vangen van vrije radicalen, meteen verbruikt worden en dus continu met de voeding ingenomen moeten blijven worden. Flavonoïden zijn als een vaccin, kleine hoeveelheden kunnen grote stofwisselingsveranderingen teweeg brengen. Vandaar is het zeer de vraag of extra aanvulling van flavonoïden in de vorm van supplementen, naast een gevarieerde voeding, wel goed is. Bijna zeker doet dat meer kwaad dan goed.
New
View On Biology Of Flavonoids
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
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.
Contact: Balz Frei
(Maart 2007)
(Opm Meer over bioactieve stoffen)