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Soy Isoflavones

Soy has shown great promise in reducing the uncomfortable hot flushes of menopause, and new research indicates that it may also help to protect against certain chronic diseases, including osteoporosis, heart disease and some cancers.

What they are?

Found in soya bean products such as tofu and soy milk, and sold in supplement form, isoflavones are powerful compounds known as phyto-oestrogens.  These plant-based substances are chemically similar to the hormone oestrogen produced in the body, but are much weaker.  Phyto-oestrogens, however, can bind to oestrogen receptors in the cells and produce various important health benefits.  Most research on soy isoflavones has been done with people who regularly ate soy products, so even though most supplements contain the major isoflavones in soya beans, genistein and daidzein, it's not clear whether isoflavones are the only beneficial compounds in soy.

What they do?

As phyto-oestrogens, soy isoflavones have two important effects.  First, when oestrogen levels are high, phyto-oestrogens, can block the more potent forms of oestrogen produced by the body and may help to prevent hormone-driven diseases, such as breast cancer.  Second, when oestrogen levels are low, as they are after menopause, phyto-oestrogens can substitute for the body's own oestrogen, possibly reducing hot flushed and preserving bones.  Soy isoflavones may also have antioxidant and anticoagulant effects.


Research indicates that soya bean products help protect against heart disease by lowering LDL ('bad') cholesterol and significantly increasing HDL ('good') cholesterol.  Soy seems most effective in people with high cholesterol levels.  in those with near-normal cholesterol levels, its effects are less powerful, and larger amounts are needed to produce the same benefits.  Soya bean products may also inhibit the oxidation of LDL cholesterol, the first step in the accumulation of artery-clogging plaque.  In addition, laboratory studies show that the genistein in soy helps to prevent blood clots from forming.

In Asian countries where soy is a dietary staple, rates of certain cancers are much lower than they are in many Western countries.  Preliminary studies indicate that regular consumption of soy foods or supplements may protect against cancers of the breast, prostate and endometrium (the lining of the uterus).  And in animal studies, adding soy protein to the diet significantly reduces tumour formation and the likelihood that cancer, once developed, will spread.  The phyto-oestrogens in the soy are most likely responsible for this effect.  Researchers speculate that the isoflavone genistein may block a protein called tyrosine kinase, which promotes the growth and proliferation of tumour cells.  This effect may be why soy is also associated with a lower risk of prostate cancer.  Geninstein has potent antioxidant properties as well, and for these reasons it may one day prove useful against cancer – though more research is clearly needed.

Additional Benefits

Studies show that hot flushes and other symptoms of menopause are relatively rare in Asia, where women eat a lot of soya bean products.  In addition, in one Western Study, women who added 45 g of soy flour to their daily diet experienced a 40% reduction in hot flushes.

Soy isoflavones may also help women maintain bone density.  One US study of postmenopausal women found that consuming 40 g of soy protein a day resulted in a significant increase in bone mineral density in the spine, an area often weakened by osteoporosis.

Common Uses

  • Reduce the frequency and severity of hot flushes and other menopausal symptoms.
  • May protect against coronary heart disease.
  • May forestall certain cancers.
  • May help to prevent osteoporosis.


  • Capsule.
  • Tablet.
  • Soy protein powder.


  • Consult your doctor before taking soy supplements for the treatment of cancer.
  • Pregnant or breast-feeding women should not take soy isoflavone supplements.
  • Reminder:  If you have a medical condition, talk to your doctor before taking supplements.

How to take it?


Experts don't know the amount of soy isoflavones needed to produce a therapeutic effect.  In Asian countries, the isoflavone consumption ranges from 25 mg to 200 mg a day.  Some researchers believe that an intake of 50-120 mg a day might be minimum amount necessary.  The supplements on the market vary in the types of isoflavones they contain and the amount per pill.  Choose a product that supplies a mixture of isoflavones – including both genistein and daidzein – and take enough pills to obtain 50-100 mg of isoflavones a day.

Guidelines for use:

Most experts recommended that you try to get your soy isoflavones from soya bean products.  In addition to their isoflavone content, these foods are good sources of protein.

The amount of isoflavones in soya beans – and therefore any product made from them – varies.  In general, eating one or two servings a day is probably sufficient.  (A serving equals: 100 g of tofu or miso, 1 cup of soy milk, or ½ cup of soy flour, cooked soya beans or textured vegetable protein).  If eating this much soy is not to your taste, you can get your isoflavones from a combination of foods and supplements.  Another alternative is soy powder, which contains both soy protein and isoflavones; mix it into juice or milk.  Take soy supplements with a large glass of warm water right before eating breakfast and dinner.

Possible side effects

Soya bean products, even in large quantities, are not known to produce side effects.  However, the small percentage of people who are allergic to soya beans should avoid all soy supplements and soy-based foods.

Facts and Tips

  • A diet high in fibre may interfere with the absorption of isoflavones.  If you eat a high-fibre diet, be sure to increase your consumption of soy supplements or soy foods.
  • Even though they're made from soya beans, soy sauce and soya bean oil contain no isoflavones.

Latest Findings

  • In a recent study, people with moderately high cholesterol levels drank a daily 'milkshake' containing 25 g of soy protein, either with or without isoflavones.  After nine weeks, those who consumed the isoflavone-rich shakes experienced, on average, a 5% reduction in LDL ('bad') cholesterol levels.  People with the highest LDL levels had an 11% drop.  (For each 10-15% drop in LDL levels, the risk of a heart attack decreases 20-25%).
  • Women who ate the most soya bean products and other foods rich in phyto-oestrogens reduced their risk of endometrial cancer by 54% according to one study.  Soya bean products may be especially important for women who have never been pregnant.  Among this group, eating less than an average of 8 g of soya bean products a day was associated with a fourfold increase in the risk of endometrial cancer.