Soy Isoflavones

Soy Isoflavones: Overview

Isoflavones are a class of phytochemicals, which are compounds found only in plants.  They are also a type of phytoestrogen, or plant hormone, that resembles human estrogen in chemical structure yet are weaker.  By mimicking human estrogen at certain sites in the body, isoflavones provide many health benefits that help us to avoid disease.

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Isoflavones are found in soybeans, chick peas and other legumes.  However, soybeans are unique because they have the highest concentration of these powerful compounds, especially genistein and daidzein.

Current findings suggest that eating soy foods – natural sources of isoflavones – can protect and enhance our overall health.  Isoflavones work together with soy protein in fighting disease.


The highest amounts of isoflavones and soy protein are found in tempeh, whole soybeans (such as edamame), textured soy protein, soy nuts, tofu and soy milk.

Function; Why it is Recommended

Isoflavones show tremendous potential to fight disease on several fronts.  They have been shown to help prevent the buildup of arterial plaque, which reduces the risk of coronary heart disease and stroke.  Isoflavones may help reduce breast cancer by blocking the cancer-causing effects of human estrogen.  They may also prevent prostate cancer by hindering cell growth.  Isoflavones can fight osteoporosis by stimulating bone formation and inhibiting bone resorption.  They may even relieve some menopausal symptoms as well.

Soy isoflavones have antioxidant properties which protect the cardiovascular system from oxidation of LDL (the bad) cholesterol.  Oxidized LDL cholesterol accumulates in the arteries as patches of fatty buildup which blocks the flow of blood, resulting in atherosclerosis.  Genistein inhibits the growth of cells that form this artery clogging plaque.  Arteries damaged by atherosclerosis usually form blood clots.  This can lead to a heart attack if the clot goes to the heart, or a stroke if it goes to the brain.

Being a weak form of estrogen, isoflavones can compete at estrogen receptor sites, blocking the stronger version naturally produced by the body from exerting its full effect.  Since high blood levels of estrogen are an established risk factor for breast cancer, weaker forms of estrogen may provide protection against this disease.

Genistein has been found to hinder breast cancer as well as prostate cancer.  Results from a new University of California study show that genistein slowed prostate cancer growth and caused prostate cancer cells to die.  It acts against cancer cells in a way similar to many common cancer-treating drugs.


Studies show that isoflavones account for approximately three-fourths of soy's protection, while its protein is responsible for about one-fourth.  The best way to consume isoflavones is in food form, so that you can benefit from all of soy's nutrients and beneficial compounds.

Researchers recommend consuming at least one to two servings of soy-based foods per day.  A serving is equal to 1 ounce of soy nuts; 4 ounces of tempeh, textured soy protein (cooked), or edamame; or 8 ounces of soymilk.

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Soy Isoflavones:

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Amongst 65 children aged 11 to 72 months with chronic constipation, 68% had a positive response with regard to bowel movements while receiving soy milk.


Menopausal Issues

The North American Menopause Society suggests that soy isoflavones can also be a natural alternative to estrogen replacement therapy for relief of mild menopausal symptoms.  It may help offset the drop in estrogen and regulate its fluctuations that occur at menopause.  Many women have reported a reduction in their hot flashes and night sweats when they regularly consume soy foods, such as tempeh or tofu.

Although study results are sometimes confusing on the issue of soy extracts and menopause, some studies have shown clear benefit.  One such study concluded that a particular soy product "...may be a safe and efficacious therapy for relief of hot flushes in women who refuse or have contraindications for hormone replacement therapy." [Menopause 2000;7: pp.105-111]

Another study found that consuming one capsule tid of a soy extract totaling 100mg of soy isoflavones per day effectively alleviated vasomotor symptoms, such as hot flashes associated with menopause, over a 4-month period compared with a placebo.  Total cholesterol and low-density lipoproteins were reduced also in the soy isoflavone group compared to placebo.  No difference in FSH and LH levels were observed between the placebo and the soy isoflavone group.  An increase in estrogen levels was observed only in the soy isoflavone group but this did not result in an increased endometrial thickness. [Obstet Gynecol 2002;99: pp.389-94]

Four months of treatment with a soy isoflavone product reduced the number of hot flashes and other menopausal symptoms in a study of 190 postmenopausal women in Spain. [Phytomedicine 2002;9(2): pp.85-92]

Perimenopausal Issues

A randomized double-blind placebo controlled clinical trial (the "gold standard" study design) has shown that the isoflavones in soy appear to protect menopausal women from bone loss just as powerfully as hormone replacement therapy, but without the side-effects [Nutrition Reviews 61 (2003): pp.346].  In other words, drinking the equivalent of about 2 cups of soymilk each day, women can go through menopause with strong bones, and without the cancer, heart disease, a stroke or two and blood clots in the lung.  And, using soymilk instead of Premarin (now known to increase risk of these conditions), your smoothies won't taste like urine!

Lab Values

High Total Cholesterol

Soybean isoflavone fraction, which contains primarily genistein, daidzein and glycetein, has been shown to have a hypocholesterolemic effect.

A soy protein isolate reduced total and LDL-cholesterol concentrations in a study of 60 patients with high cholesterol levels. [Eur J Clin Nutr 2002;56(4): pp. 352-35]


Osteoporosis - Osteopenia

Higher intake of dietary phytoestrogens (isoflavones) was associated with higher lumbar spine and hip bone mineral densities in a study of 357 postmenopausal southern Chinese women. [J Clin Endocrinol Metab 2001;86(11): pp.5217-5221]

A one-month of treatment with a soy isoflavone extract reduced the excretion of bone resorption markers, in a placebo-controlled study of 23 healthy perimenopausal women. [J Am Coll Nutr 2002;21(2): pp.97-102]

It should be noted, however, that this benefit has not been seen in other studies.  It seems that the best chance of receiving any benefit by this means is if you are a Japanese premenopausal woman.  The closer you are to menopause, the less likely to be benefited. [Am J Epidemiol 2002;155(8): pp.746-754]

Poor Bone Health

Preliminary and sometimes conflicting research suggests that soy foods have a positive effect on bone health.  Two small 6-month studies showed that soy protein and isolated isoflavones (a component of soy), respectively, had beneficial effects on spinal bone density [Potter et al. 1998; Alekel et al. 2000].


Tumors, Benign

Colon Polyps

Men who eat one or more servings of soy per week reduce the risk of developing colon polyps by half. [ Am J Epidemiol 1996;44(11): pp.1015-25]

Tumors, Malignant

Prostate Cancer

Soy protein is one of the dietary elements that has been talked about for a long time as having a potentially beneficial effect for preventing prostate cancer.  Soybeans are full of chemicals called isoflavones which closely resemble the structure of estrogen-like hormones once widely used to treat prostate cancer.

Phytochemicals in soy protect against cancer via several different mechanisms, including interacting with intracellular enzymes, regulating protein synthesis, controlling growth factor actions, inhibiting malignant cell proliferation, inducing differentiation, deterring cancer cell adhesion, and inhibiting angiogenesis.

Soy extracts also provide doses of soy isoflavones such as genistein.  Cancer cells use the enzyme protein kinase as a growth factor and genistein is a potent inhibitor of its activity.  Genistein may reduce the metastatic capacity of hormone-dependent cancers.  Studies have shown that genistein inhibits proliferation of prostatic cancer cells.

Laboratory studies are backed up by observations in Asian countries, particularly Japan, where men may develop small prostate tumors but rarely the kind of large, aggressive tumors seen in American men.  However, the aggressive form of the cancer becomes more prevalent when Asian immigrants come to the US and are likely to substitute their soy-rich diet with one based mainly on animal protein.

Since prostate cancer cells usually multiply slowly, the development of prostate cancer can take many years before symptoms appear.  During this time period, the benefits of natural therapies like soy consumption are more effective at dealing with the problem while it is still small.

Note: Do not take any soy genistein products 10 days prior to, during, or 3 weeks after any form of radiation therapy.  Genistein may protect cancer cells against radiation-induced death.

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