Taken from supplementwatch.com (an unaffiliated supplement review forum)

The term "soy" is used to refer to many products derived from the soybean. In terms of health and wellness, the two most important dietary supplements derived from soybeans are isolated/concentrated soy proteins and soy extracts which contain a high amount of compounds called isoflavones. The isoflavones have been associated with a wide variety of beneficial health effects including protection from cancer and osteoporosis to reduction in hot flashes and other symptoms of menopause. Soy protein, which may or may not contain a high level of isoflavones (depending on how it is processed) has been associated with a reduction in serum cholesterol and triglyceride levels and may protect against the development of coronary heart disease.

Depending on the method of processing, many soy foods contain a relatively high content of chemical compounds called isoflavones, which possess weak estrogen-like effects. Under conditions of high estrogen exposure, which may promote certain cancers, the isoflavone compounds tend to block the adverse effects of estrogen and may prevent growth of cancer cells. Under conditions of low estrogen exposure, such as during menopause, the isoflavones tend to act as weak estrogens, which may be just enough to help alleviate some of the symptoms associated with menopause, such as hot flashes, headaches and mood swings.

Consumption of soy foods and soy proteins has been associated with beneficial health effects for heart disease, osteoporosis, cancer and menopausal symptoms

Isoflavones
Soy is the richest dietary source of isoflavones. Typical soyfoods like tofu might provide 1-4 mg/g or about 40-100 mg of isoflavones per ounce. Soymilk provides about 100-150 mg of isoflavones per 8-ounce glass. The isoflavones function as phytoestrogens in the body, where they possess weak estrogen-like effects. The two primary isoflavones found in soy are daidzein and genistein, both of which have been associated with the health benefits mentioned above. The chemical structure of isoflavones is similar enough to that of estrogen so that they can bind to the estrogen receptor on cells, yet different enough so that they only perform very weak estrogen effects. For the different soy-based protein powders on the market, the isoflavone content can vary significantly, from almost zero for those products extracted using alcohol, to certified levels of 2-5 mg per gram of protein. In many Asian countries, where the incidence of heart disease, cancer and menopausal symptoms is low, the daily isoflavone intake is estimated at 25-50 mg per day- in contrast, the average Western intake is less than 5 mg per day.

Heart Protection
Results from a number of studies show the cholesterol-lowering benefits of including soy protein in the diet. Drops in total cholesterol, LDL and triglycerides, with no lowering of HDL (good) cholesterol levels, has been shown with soy protein intakes of 25-50 grams per day, typically taken in 2-4 divided doses throughout the day. Such intakes have also been shown to reduce the susceptibility of the LDL particles to become oxidized, which is thought to be a crucial step that promotes cholesterol buildup on artery walls.

Cancer Risk
Epidemiological studies have suggested that Asian diets may provide protection from several cancers, including those of the breast, prostate gland and colon. As mentioned above, the action of isoflavones as weak estrogens allows them to bind to estrogen receptors and block some of the detrimental effects of estrogen – such as promotion of cancer cell growth. Tamoxifen, a prescription drug for treating breast cancer, is though to act as an anti-estrogen by binding to the estrogen receptor and "blocking" the growth-promoting effects of estrogen in cancer cells. Women using tamoxifen have a lower incidence of breast cancer and a 30-40% reduction in breast cancer cell growth rate. The isoflavones in soy are chemically similar to tamoxiphen and, therefore, may also reduce the risk of hormone-dependent cancers via the same "estrogen-blocking" mechanism

Bone Health
Soy protein consumption has been shown to reduce bone loss and slow calcium loss in an animal model of osteoporosis, suggesting a possible beneficial role in preventing osteoporosis in humans. A diet high in soy protein has been shown to improve bone density after six months. In addition, soybeans contain a relatively high calcium content, a large portion of which may be retained in soy protein powders. It is also interesting to note that soy protein seems to cause less loss of calcium from the body compared to other dietary sources of protein, which may promote calcium loss and bone breakdown at high levels. Ipriflavone, a synthetic isoflavone drug prescribed in Europe, is metabolized in the body into daidzein, and has potent effects on reducing bone resorption in postmenopausal women.

Dietary consumption of soy-based protein concentrate or soy isolate is not associated with any significant side effects aside from the mild gastrointestinal issues (bloating, flatulence) associated with any high protein diet. High dose intake of concentrated isoflavone extracts are probably safe at levels up to at least 200 mg per day (the estimated amount contained in the average Japanese diet). Since the long term effects of isolated isoflavone supplements is unknown and the potential for pro-estrogenic effects may exist for mega-dose isoflavone consumption, it is prudent to keep total isoflavone intake close to those levels found in dietary amounts.

As a high quality protein source, soy-based protein powders provide an excellent amino acid profile along with the added health benefits for heart health, cancer protection, bone maintenance and, in postmenopausal women, relief from menopausal symptoms

For those women who cannot or choose not to select hormone replacement therapy following menopause, isoflavone supplements may provide an effective alternative to treating some of the symptoms associated with menopause, including hot flashes, night sweats, headaches, vaginal dryness and mood swings.

Dosage:
As a protein supplement – as needed. (Typical protein recommendations are 1-2 grams of protein per kg of body weight).
For heart health, 25-50 grams of soy protein isolate per day is effective in reducing cholesterol and triglyceride levels.
For menopausal symptoms, 25-50 mg of isoflavones per day is effective in alleviating some of the symptoms associated with menopause (e.g. hot flashes). Do not exceed more than 200 mg per day.


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From wholehealthmd.com I obtained basically the same review with this warning:

Because of their estrogenlike effects, pregnant or nursing women should not take soy isoflavone supplements. (Soy-rich foods pose no such problems, however.)

Further research found this:

Breast Cancer
Concerning soy’s possible role in breast cancer prevention, the picture remains a little murky at present.

On the one hand, several studies have shown that populations with a high soy intake, such as those in Japan and China, have a relatively low incidence of breast cancer. And other research suggests that soy may have cancer-fighting effects, inhibiting the growth of breast cancer cells.

On the other hand, other studies suggest that soy sometimes causes breast tissue to proliferate, a possible harbinger of cancer.

Soy’s effects on the breast may also differ at different times in a woman’s life. Dr. W. Mark Cline, associate professor at Wake Forest University, has observed, “It’s quite likely that exposure to soy early in life will turn out to be more important than exposure later in life.” This may help explain why Asians, who begin eating soy at a very young age, have lower rates of certain cancers.

Women's health specialist Dr. Tori Hudson does not regard the conflicting soy findings as a cause for concern. “I don’t think soy is a health risk for anybody,” she says. “The key is that soy is a weak estrogen mimicker, with only 1/400 to 1/1000 the potency of natural estrogen. I still advise soy for women of all ages, including breast cancer patients to treat menopausal symptoms.”

Dr. Hudson believes adding soy to the diet offers women a way of reducing the amount of estrogen they need to relieve symptoms of menopause. In some cases, she says, “A woman can take a half dose of estrogen replacement and fill the gap with soy.”

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The message is a valid one, and it is coming from independant studies and research. Soy is an equal and sometimes better source of protein, depending on your needs and requirements. No one should ever blindly follow recommendations, especially when their health is involved. Just as we should be careful when making blanket statements.

I understand your concerns, Carol, and I know that when faced with information from someone in a position of authority, such as your doctor, it can be easy to accept statements as fact w/o having all the information. There are many members here, like Lesley, who will read this and, for lack of a better word "freak out" unecessarily. If soy were dangerous to everyone, as your doctor is saying, there would be facts / studies/ research and it would be all over the 6 o'clock news.

This isn't that far from dieticians telling us that "LC is dangerous for you, Stay away from it".

Nat