Soy Isoflavones


Soy Isoflavones, from Glycine max, are phytoestrogens. These weak estrogens are chemically similar in structure to naturally produced estrogen hormones. Isoflavones are found in soy foods both with and without a sugar molecule attached. The two primary isoflavones in soybeans are daidzein and genistein and their respective glucosides, genistein and daidzein. Soy foods typically contain more genistein than daidzein, although this ratio varies among the different soy products. (1)

Isoflavones are studied for a wide spectrum of effects and have been the object of over 1,000 studies. Originally, interest in isoflavones was due to their estrogenic activity (isoflavones can reach relatively high blood levels relative to estradiol (2) in individuals consuming soy foods); however, they are actually very weak estrogens. The soy isoflavones have between 1/1,000 and 1/100,000 the activity of estradiol. (3)

Isoflavones reportedly exhibits effects that are not related to estrogen activity. In vitro studies have reported that genistein inhibits the growth of a wide range of cancer cells including those that are not hormone-dependent. (4) , (5) It has been hypothesized that the explanation for these anticancer effects is the ability of genistein to inhibit the activity of enzymes that control cell growth and regulation. (6)

Dosage Info

Dosage Range

10-300mg daily.

Most Common Dosage

60mg daily.

Dosage Forms

Powders, granules, tablets, capsules, and liquids.

Adult RDI

None estabished

Adult ODA

None established


  • : None established

Interactions and Depletions


Active Forms

Soy isoflavones consist of genistein, daidzein, daidzin, gylcitin, and glycetein.

A synthetic derivative of naturally occurring isoflavones is ipriflavone (7- isopropoxyisoflavone). In animal models, ipriflavone has been shown to prevent bone loss, (7) an effect believed to be related to its ability to enhance calcium absorption. (8)


Isoflavones are well absorbed orally.

Toxicities & Precautions


There is no known toxicity associated with isoflavones.

Side Effects

One study indicated that ipriflavone, a synthetically derived isoflavone, caused lymphocytopenia in a significant number of women. (9)

Functions in the Body

Mild Estrogenic Activity

Reduce menopausal effects of low estrogen.

Anti-estrogenic Activity

Inhibit cancer-inducing estrogens.


Studies suggest isoflavones reduce oxidation of LDL.

Enzyme Inhibitor

Enzymes promoting bone resorption are inhibited by isoflavones.

Clinical Applications


In women fed 45 grams of soy flour per day, menopause symptoms were reduced by approximately 40 percent. (10) Results of a 5 year randomized, double-blind, placebo-controlled study indicated that soy phytoestrogens administered to postmenopausal healthy women were associated with an increased occurrence of endometrial hyperplasia. (11)


Researchers found that after a two year randomized, placebo-controlled trial that daily consumption of two glasses of soymilk containing 76 mg isoflavones prevented lumbar spine bone loss in postmenopausal women suffering from osteoporosis or had at least 3 risk-factors for osteoporosis. (12) Soy isoflavone supplementation to postmenopausal women resulted in significant favorable effects on bone mineral content indicating a possible preventative for osteoporosis. (13)


Though the effect has not been confirmed in humans, soy based diets have reduced blood pressure in hypertensive rats. A randomized, double-blind study involving 40 men and women with mild to moderate hypertension compared the ability of soy milk to lower blood pressure to that of cow's milk. After 3 months of consumption, individuals in the soy milk group experienced modest, though significant decreases in their blood pressures compared to the those who consumed cow's milk. (14)

Atherosclerosis And Platelet Aggregation

Genistein reportedly inhibits platelet aggregation and smooth muscle cell proliferation. Smooth muscle cells are one of the primary cell types comprising plaques. (15) , (16) It has been proposed that isoflavones may prevent atherosclerosis including antioxidant effects, antiproliferative and antimigratory effects on smooth muscle cells, effects on thrombus formation, maintenance of normal vascular reactivity, and beneficial effects on plasma lipid concentrations. (17) , (18) , (19) , (20)

Reduce Cancer Risk

Phytoestrogens may have a role in reducing cancer risk. High blood levels of estrogen are a risk factor for breast cancer. By competing with endogenous, estrogen hormones for binding to the estrogen receptor, isoflavones may function as antiestrogens. (21) It has been suggested that the metabolites of genistein may exert anticancer effects. (22) Although only limited data is available, studies suggest that genistein inhibits tumor development (23) and provides anticancer activity. (24) But more studies are warranted before recommending isoflavones for estrogen positive breast cancer because it has also been reported that soy isoflavones stimulate the growth of estrogen-dependent breast cancer cells in vivo in a dose-dependent manner. (25) It has also been reported that high consumption of soy isoflavones may reduce the incidence of prostate cancer in men. (26)

Elevated Cholesterol

Soy protein has been reported to be hypocholesterolemic in individuals with elevated cholesterol. (27) , (28) , (29) Women consuming 60 mg/day of isoflavones for 12 weeks exhibited 3.7% increase in HDL, a 5.5% decrease in the total cholesterol/HDL ratio, and a significant reduction in the oxidation of LDL-cholesterol. (30)

Symptoms and Causes of Deficiency

None documented.

Dietary Sources

Soybeans and soy foods. Raw soybeans contain between 2-4mg of total isoflavones/gram by dry weight. (31) Most soy protein concentrates are low in isoflavones and neither soy sauce or soy oil contain isoflavones. Soy protein concentrates (65 percent soy protein), frequently used in soy burgers and many other soy products, may not contain nutritionally significant amounts of isoflavones. Soy products processed with alcohol to reduce the smell and taste of the soybean have much lower concentrations of isoflavones. Some research suggests that a single serving of soy foods (such as soymilk or tofu) contains enough isoflavones to exert clinical effect. (32)


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