Phytoestrogens are non-steroidal plant – derived phytochemicals
with estrogenic activity. The common biological roles of phytoestrogens are to
protect plants from stress or to act as part of a plant’s defense mechanisms. The
name ‘phyto’ means plant and ‘estrogen’ comes from estrus (period of fertility for female mammals). All phytoestrogens are diphenolic compounds
with chemical structures similar to natural estrogens and antiestrogens. Phytoestrogens
are made up of more than 20 compounds and can occur in more than 300 plants
such as fruits, herbs and grains. Three major subclasses of phytoestrogens have
been identified and chemically defined as Isoflavones, lignans and coumestans. The
best studied dietary phytoestrogens are the soy Isoflavones and the flaxseed
lignans. Phytoestrogens cannot be stored in the body and can be easily broken
down and eliminated. Dietary phytoestrogens have weaker estrogenic effects than
human estrogens. Human clinical trials suggest that phytoestrogens may potentially
confer health benefits related to cardiovascular diseases, cancer, osteoporosis
and menopausal symptoms.
The soy hypothesis
Historically, Asian countries have had low incidence rates
of breast cancer compared to Western countries. Soy, a dietary staple in many
Asian countries may protect women against breast cancer. Anticancer effects of
soy Isoflavones may be attributable to their structural and functional
similarities to estrogens. Epidemiologic
studies in Asian and Asian-American populations support the hypothesis that
early exposure to dietary soy may decrease breast cancer risk, while exposure
in adulthood may not be positive. A regular intake of soy foods appear to be
protective. This is evidenced by Japan, for example only 25% of menopausal
women suffer hormone withdrawal symptoms as compared with 85% of women in the
U.S (Notelovitz 1989). In menopausal women, dietary phytoestrogens can help
compensate for the hormone deficits and thereby moderate the hormonal
withdrawal symptoms.
Phytoestrogens – a definition
Phytoestrogens are defined as “any plant substance or
metabolite the induces biological responses in vertebrates and can mimic or
modulate the actions of endogenous estrogens usually binding to estrogen
receptors” (MAFF UK, 2003).
Estrogenic potential of phytoestrogens
Phytoestrogens have biochemical structures similar to those
of mammalian sex hormone estrogen. The estrogenic activity of phytoestrogens
has been related to their ability to bind to the mammalian estrogen receptor
(ER). The phytoestrogens such as genistein and coumestrol were shown to have
higher binding affinities to estrogen receptor(ER). Dietary phytoestrogens have
weaker binding affinities when compared with the female endogenous estrogen
β-estradiol. Isoflavones are best researched and most common form of
phytoestrogens.
Dietary sources
The main dietary source of Isoflavones (e.g. genistein and
daidzein) for humans is soybean, while
flavonoids (e.g. apigenin, naringenin and luteolin) are found in several
different vegetables, fruits, berries, herbs and green tea. The soy foods
contain approximately 0.2 – 1.6 mg of Isoflavones/ g dry weight. For Coumestans( e.g. coumestrol), the main
sources are sprouts of alfalfa and various beans. Lignans (e.g.entrolactone and
enterodiol ) are not present in our diets as such, but precursors are converted
to lignans by the gut microflora. Lignan precursors are present in fibre- rich
foods such as flaxseed, unrefined grain products particularly rye and some
berries. Flax seeds are one of the best sources for lignans. Dietary intake of
phytoestrogens is greater in vegetarians than in non-vegetarians. Asians,
Japanese, Koreans and Chinese consume more phytoestrogens than the people of
Western countries including Americans. It is estimated that dietary phytoestrogen
intake is up to 30 times higher in Eastern Asia than in Europe and North
America.
Dietary reference intakes
The total phytoestrogen consumption in Eastern population or
in adults taking phytoestrogen supplements may be approximately 60 to 75
mg/day. Therefore an average 60-75 kg adult would consume approximately 1 mg
phytoestrogens per kg body weight (bw). Many Asian populations that exhibit low
incidence of prostate and breast cancers consume 20 and 80 mg/day of the
isoflavone genistein mainly from soy foods. The mean daily isoflavone intake in
Asian populations has been estimated to approximately 30 mg/day. It has been
recommended that dietary phytoestrogens sources containing 30 to 120 mg of
Isoflavones can be given daily for relief of post menopausal symptoms. The
dietary intake of phytoestrogens can affect the menstrual cycle and the
concentration of reproductive hormones in the blood of premenopausal women. In
1999, the U.S. Food and Drug Administration (FDA) approved the health claim
that a diet low in saturated fat and cholesterol that include 25g of soyprotein
per day reduce the risk of heart disease. A meta-analysis study concluded that
an intake of an average of 47g soyprotein / day lowered total cholesterol and
LDL- cholesterol. This was supported by studies in the monkey indicating that
isoflavone increased HDL-cholesterol, enhanced vasodilation and decreased
atherosclerosis.
Therapeutic properties
Several health effects have been hypothesized for
phytoestrogens: they can be estrogenic or antiestrogenic, antioxidative, antiproliferative,
antiviral, antibacterial, insecticidal or fungistatic, cardioprotective,
antiatherogenic, hypocholesterolemic, bone maintaining, cancer protective and
anticarcinogenic (Branca F and Lorenzetti S, 2005). Phytoestrogen such as
genistein affect the blood vessel wall to inhibit atherosclerosis by binding to
hormone receptors. Phytoestrogens exert a cardiovascular protective effect by
regulating blood lipid levels. Isoflavones and other phytoestrogens have been
considered to exert anticarcinogenic actions mainly through antiestrogenic,
antiaromatase or antiprolifertive mechanisms. Isoflavones can have a beneficial
effect on balancing male hormones and the risk of prostate cancer. Dietary soy supplementation
has been shown to increase bone mineral density. Sufficient dietary intake of
soyprotein relieves hot flashes in post- menopausal women. The lignan phytoestrogens
provide the building blocks of cell walls in plants and in humans. Lignans have
the ability to inhibit fungus growth and kill various bacteria and viruses.
Soy estrogens (isoflavonoids) play beneficial role in menopausal health
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