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What's
in a name?
Phytoestrogens (fy-toe-es'-truh-jenz)
are molecules with estrogenic activity that are found in plants: 'phyto' =
plant; 'estrogen' comes from 'estrus' (period of fertility for female
mammals; derived from the name of an ancient Anglo-Saxon fertility goddess, Oestre) + 'gen,' to initiate, generate. Several different classes of
molecules have been identified as phytoestrogens so far; these are not
identical to human estrogens, but have some structural similarities which
allow them to interact with estrogen receptors in human cells.
'Phytoestrogen,' then, is more of a functional classification than a
chemical classification.
Three major types of known
phytoestrogens are coumestans (koo-meh-stans),
isoflavones (eye-so-flay'-vonez), aka isoflavonoids, and lignans (lig'-nanz).*
These are all examples of phenolic
phytoestrogens. Other kinds of molecules (including plant steroids and
terpenoids) have
demonstrated varying estrogenic activity as well; however, this discussion
will focus mainly on the three types mentioned above.
*Not to be confused with lignin,
the polymer that gives wood its rigidity.
Phytoestrogens in Nature
Plants invented lignans around 420 million years ago
when they started to grow on land. Their algal ancestors were supported by
water, but land plants needed a stiffer superstructure to deal with wind and
gravity. During the course of evolving lignin, the rigid woody polymer,
lignan subunits were formed. In addition to their structural function, some of these compounds
could bind proteins (including
enzymes), polysaccharides, and nucleic acids, which allowed them to act as
protective antibiotic and insecticidal molecules for the plants.
Isoflavones came along as flowering plants developed about
120 million years ago. Acting as phenolic
phytoalexins, they were synthesized for defense against bacterial,
fungal, or viral infection. Some also served as insecticides because their
steroid-mimicking structures could disrupt growth and development in insects
(steroids may have been the distant chemical ancestors
of isoflavones). Their hormone-like structures allow isoflavones
to have estrogenic activity in animals.1
Other botanical functions of isoflavonoids include
the ability to act as communication molecules. By attracting symbiotic nitrogen-fixing bacteria
to form
root nodules, they help legumes to acquire nitrogen. In addition, they are especially abundant in seeds where they
help regulate growth and development.
Human
estrogens and phytoestrogens
There are not one but three different kinds of estrogen made
by the human body: estradiol, estrone, and estriol. Known as 'endogenous'
estrogens, they are produced in the
ovaries, the placenta, and, in small amounts, in the testes. There are also
various metabolites of estrogen that circulate in the blood. Chemically,
all of the above are known as steroids. Some plant seeds (i.e. pomegranate, date palm) actually
contain small amounts of estrone, but many of the phytoestrogens are
not steroidal. The main ones we will consider are chemically classified as isoflavones, lignans, and coumestans; collectively known as phenolic
phytoestrogens, they are not identical to steroids but have enough features
in common that they can affect steroid receptors and hormone metabolism in cells.
Sources of phytoestrogens
Although phytoestrogens of one kind or another occur in many different
plants, only certain species contain medicinally significant amounts. Among
the food plants, legume seeds (beans, peas) and especially soy products are the most
prominent sources of isoflavones. Flax seed and flax seed meal are the
richest dietary sources of lignans, which are also present in some berries,
nuts, vegetables, and whole grains. Various kinds of phytoestrogens are also
found in many medicinal herbs, including red clover, alfalfa (lucerne), hops (with
especially high levels of prenylflavonoids2),
licorice, turmeric, and kudzu. The chart below lists the most
common sources of phenolic phytoestrogens.
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Common phytoestrogens and their sources
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Lignans
|
Sources & concentrations (mg/100 g dry weight)3 |
Matairesinol
|
Flax seed, 1.087
Sesame seed, 0.608
Teas (brewed), 0.090 - 0.305
Rye bran, 0.167
Oat bran, 0.155
Rye meal, 0.065
Broccoli, 0.023
Poppy seed, 0.012
Other grains, legume seeds, and nuts contain trace amounts, up to ~ 0.01
Black currants, 0.01
Strawberries, 0.005 |
Secoisolariciresinol
|
Defatted flax meal, 600 -
700
Flax seed, 369.9
Pumpkin, 3.870
Teas (brewed), 1.050 - 2.887
Lingonberry, 1.510
Strawberry, 1.205
Cranberry, 1.054
Blueberry, 0.835
Zucchini, 0.817
Sunflower seed, 0.610
Kudzu leaf, 0.476
Broccoli, 0.414
Black currant, 0.388
Garlic, 0.380
Carrot, 0.370
Peanut, 0.298 - 0.333
Soybean, 0.013 - 0.273
Cashew, 0.257
Caraway seed, 0.221
Cow pea (Vigna unguiculata), 0.195 - 0.196
Walnut, 0.163
Kidney bean, 0.056 - 0.153
Red cabbage, 0.141
Raspberry, 0.139
Rye bran, 0.132
Hazelnut, 0.119
Wheat bran, 0.110
Sesame seed, 0.090 |
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Isoflavones |
Sources & concentration (mg/100 g) |
|
Biochanin A |
Red
clover (mature plant leaf) - rich
source; content varies by variety; used for commercial extraction. One
group reported 833 mg/100 g.4 A more recent analysis*
detected 417 mg/100 g.5
One commercial red clover extract contains 40 mg mixed isoflavones/tablet
(including 24.5 mg biochanin A).6
Chickpeas, 0.838 - 3.0801
Kudzu Root, 1.4001
Kudzu Leaf, 1.2401
Red clover sprouts, 0.441
Red clover seed, 0.3811
Peanuts, 0.0311
Alfalfa (mature), trace11
Red clover tea (flowering tops), trace7 |
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Common phytoestrogens and their sources
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Isoflavones, cont. |
Sources & concentration (mg/100 g) |
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Daidzein |
Soybeans and soy products:
Soy flour, 1.65 - 130.92
Soybeans, raw, 9.89 - 124.20
(A different assay measured 10.5 - 56.0)1
Soy protein isolate, 7.70 - 68.89
Miso, 7.10 - 36.64
Tempeh, 4.67 - 27.30
Tofu, 8.00 - 25.80
Soy cheese, 0.20 - 21.10
Soy milk, 1.14 - 9.84
Infant formulas, 0.75 - 9.65
Soy dog, 3.40
Shoyu, 0.60 - 1.40
Soy sausage, 0.75
Commercial soy sauce, 0.10
Soy oil, 0
Other sources:
Kudzu root, 1851
Split peas, raw, 0 - 7.26
(A different assay measured 0.007 - 0.036)1
Red clover leaf** 4.25
Mung bean sprouts, 0.701
Kudzu leaf, 0.3751
Red clover seed, 0.1781
Sesame seed, 0.1401
Chick peas, raw, 0 - 0.08
(A different assay measured 0.011 - 0.192)1
Alfalfa (mature) 0.0629
Peanuts, raw, 0.01 - 0.05
(A different assay measured 0.058)1
Black-eyed peas, raw, 0 - 0.03
Pinto beans, raw, 0 - 0.02
Fenugreek seed, 0.01
Lentils, raw, 0 - 0.01
Alfalfa sprouts, 0
Black beans, raw, 0***
Kidney beans, cooked, 0
(A different assay measured 0.007 - 0.040)1
Red clover sprouts, 0 |
Formononetin
|
Red
clover (mature plant) - rich source;
content varies by variety; used for commercial extraction. One group
reported 1322 mg/100 g.4 A more recent analysis* detected 647
mg/100 g.5
One commercial red clover extract contains 40 mg mixed isoflavones/tablet
(including 8 mg formononetin).6
Kudzu Root, 7.0901
Red clover sprouts, 2.281
Red clover seed, 1.2701
Alfalfa sprouts, 0.3410
Chickpeas, 0.094 - 0.2151
Soybeans, 0.018 - 0.1211
Kudzu Leaf, 0.0871
Alfalfa (mature), trace11
Red clover tea (flowering tops), trace7 |
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Genistein |
Soybeans and soy products:
Soy flour, 2.75 - 145.23
Soybeans, raw, 13.00 - 138.24
(A different assay measured 26.8 - 84.1)1
Soy protein isolate, 27.17 - 105.10
Miso, 11.70 - 52.39
Tofu, 11.10 - 42.15
Tempeh, 1.11 - 39.77
Soy cheese, 0.50 - 38.20
Infant formulas, 1.58 - 15.43
Soy milk, 1.12 - 11.28
Soy dog, 8.20
Soy sausage, 2.70
Shoyu, 0.30 - 1.54
Commercial soy sauce, 0
Soy oil, 0
Other sources:
Red clover leaf** 56.05
Kudzu root, 12.601
Kudzu leaf, 2.521
Mung bean sprouts, 2.001
Pinto beans, raw, 0.52
Peanuts, raw, 0.08 - 0.39
(A different assay measured 0.064)1
red clover Sprouts, 0.35
Split peas, raw, 0 - 0.10
(A different assay measured 0 - 0.023)1
Chick peas, raw, 0 - 0.12
(A different assay measured 0.069 - 0.214)1
Black-eyed peas, raw, 0 - 0.03
Fenugreek seed, 0.01
Lentils, raw, 0 - 0.01
Alfalfa (mature), trace9
Alfalfa sprouts, 0
Black beans, raw, 0***
Kidney beans, cooked, 0
(A different assay measured 0.018 - 0.518)1 |
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Glycitein |
Soybeans and soy products:
Soy protein isolate, 5.40 - 26.40
Soy flours, 3.95 - 28.8
Soy beans, raw, 6.72 - 20.40
Soy cheese, 2.70 - 4.10
Soy dogs, 3.40
Miso, 2.30 - 3.80
Infant formulas, 0.28 - 3.45
Tempeh, 0.90 - 3.20
Tofu, 1.70 - 2.90
Soymilk 0.36 - 0.86
Shoyu, 0.45
Soy sausage, 0.30
Soy oil, 0
Commercial soy sauce, 0 |
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Coumestans |
Sources & concentrations (mg/100g) |
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Coumestrol |
Red clover sprouts, 28.14
Mature alfalfa, 2.5 - 6.5 11
Alfalfa sprouts, 4.7 10
Kudzu leaf, 1.578
Soybean sprouts, 0.45 - 1.2112
Mung bean sprouts, 1.00
Soybean, 'Santa Rosa' variety, 0.19
Red
clover
leaf, 0.1058
Soybean (mature, other varieties) and soy products, ~ 0
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Because coumestrol acts as a
phytoalexin, values can be considerably
higher in plants defending themselves from various pathogens or insects.
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