The term
flavonoid refers to a class of
plant
secondary metabolites. According to the
IUPAC nomenclature,
[1]
they can be classified into:
- flavonoids, derived from 2-phenylchromen-4-one
(2-phenyl-1,4-benzopyrone)
structure
- isoflavonoids, derived from 3-phenylchromen-4-one
(3-phenyl-1,4-benzopyrone)
structure
- neoflavonoids, derived from 4-phenylcoumarine
(4-phenyl-1,2-benzopyrone)
structure.
Flavonoids are most commonly known for their
antioxidant activity. However, it is now known that the health benefits
they provide against cancer and heart disease are the result of other
mechanisms.[2][3]
Flavonoids are also commonly referred to as bioflavonoids in the
media � the terms are largely equivalent and interchangeable, for most
flavonoids are biological in origin.
Biosynthesis
Flavonoids are synthesized by the
phenylpropanoid
metabolic pathway in which the
amino
acid
phenylalanine is used to produce
4-coumaroyl-CoA[3].
This can be combined with
malonyl-CoA to yield the true backbone of flavonoids, a group of
compounds called
chalcones,
which contain two
phenyl rings (see
polyphenols). Conjugate ring-closure of chalcones results in the
familiar form of flavonoids, the three-ringed structure of a
flavone. The metabolic pathway continues through a series of enzymatic
modifications to yield
flavanones →
dihydroflavonols →
anthocyanins. Along this pathway, many products can be formed, including
the
flavonols,
flavan-3-ols,
proanthocyanidins (tannins) and a host of other polyphenolics.
Biological
effects
Flavonoids are widely distributed in plants fulfilling many functions
including producing yellow or red/blue
pigmentation in flowers and protection from attack by
microbes and
insects. The widespread distribution of flavonoids, their variety and
their relatively low
toxicity
compared to other active plant
compounds (for instance
alkaloids) mean that many animals, including humans, ingest significant
quantities in their diet. Flavonoids have been referred to as "nature's
biological response modifiers" because of strong experimental evidence of
their inherent ability to modify the body's reaction to
allergens,
viruses, and
carcinogens. They show anti-allergic,
anti-inflammatory[4]
, anti-microbial and anti-cancer
activity.
Consumers and food manufacturers have become interested in flavonoids for
their medicinal properties, especially their potential role in the
prevention of cancers and
cardiovascular disease. The beneficial effects of fruit, vegetables, and
tea or even red wine have been attributed to flavonoid compounds rather than
to known
nutrients and
vitamins[citation
needed].
Health
benefits aside from antioxidant values
In 2007, research conducted at the
Linus Pauling Institute and published in
Free Radical Biology and Medicine indicates that inside the human
body, flavonoids themselves are of little or no direct antioxidant value.[5]
Unlike in the controlled conditions of a test tube, flavonoids are poorly
absorbed by the human body (less than 5%), and most of what is absorbed is
quickly metabolized and excreted from the body.
The huge increase in antioxidant capacity of blood seen after the
consumption of flavonoid-rich foods is not caused directly by the flavonoids
themselves, but most likely is due to increased uric acid levels that result
from expelling flavonoids from the body.[2]
According to Frei, "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. 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."[2]
Their research also indicated that only small amounts of flavonoids are
necessary to see these medical benefits. Taking large dietary supplements
provides no extra benefit and may pose some risks.[2]
Diarrhea
A study done at Children's Hospital & Research Center Oakland, in
collaboration with scientists at
Heinrich Heine University in Germany, has shown that
epicatechin,
quercetin
and
luteolin can inhibit the development of fluids that result in diarrhea
by targeting the intestinal
cystic fibrosis transmembrane conductance regulator Cl� transport
inhibiting cAMP-stimulated Cl� secretion in the intestine.[6]
