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.
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.
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