Many people consume soy products because of its alleged anti-carcinogenic properties, especially with respect to cancer of the prostate and the breast. Well, sorry to disappoint you, but two large scale studies found no protective effect of soy on prostate cancer.
With respect to breast cancer, there is some weak evidence in the form of correlation studies. One study is especially weak. It showed that women whose husbands ate less beef and wieners and more miso (but not tofu) had less incidence of breast cancer. This assumes that the women ate the same things as the men, which is a huge assumption to make. (All of you reading this ask yourselves if you and your spouses or significant others eat the same foods all of the time).
Another study showed an inverse correlation between consumption of soybean paste soup and breast cancer. Again, this is weak evidence in it of itself because women who eat soy bean paste soup might also be more likely to eat vegetables or fruit, eat less processed foods or Omega 6 fats or smoke less, drink less milk, or a million other factors that one cannot possibly take into account [2]. Simply copying part of another diet based on speculation and weak correlation studies is quite frankly irresponsible. In fact, a few studies have shown increased soy consumption provides no protective effect or an unclear effect on breast and colon cancer or cancer in general [3], [4], [5], and one study showed an increase in breast tissue proliferation after supplementation with soy protein [6]. In fact, because soy acts as a weak estrogen, it might actually stimulate breast cancer growth, especially in postmenopausal women, where unopposed estrogen in the absence of menstrual cycles and progesterone can poses a potential health risk.
Thus, the bottom line is that soy seems to have no protective effect on prostate cancer, and probably cancer in general. Although it might protect against breast cancer, it might just as well promote tumor growth, and plus, there are many other things women can do to lower their risk, such as exercise, not smoke, and follow a low carb diet. That's right. Sugar has the highest correlation to breast cancer from any food group, and that is attributed to insulin[7].
Another interesting finding is that cerebrovascular disease is directly correlated with consumption of plant proteins, excluding those from cereals. In fact, Japanese men, who consume some of the highest amount of plant proteins in the world, often in the form of soy, also are world leaders in stroke mortality. Again, correlation obviously does not mean causation, but a possible explanation is proposed. Soy contains large amounts of phytoestrogens, compounds in plants which mimic estrogen. In men, this exogenous estrogen might lead to damaged cerebral arteries [8]. To be fair though, at this point it is conjecture, and further research is of course needed to elucidate the mechanism behind this.
Soy is also a potent food allergen and as such can weaken the immune system, which is busy getting rid of the soy proteins in the blood and thus is more susceptible to bacterial or viral infection.
Soy can also have a mild hypothyroid effect. Any person with hypothyroidism (an underactive thyroid), will tell you about how one gains weight, has no energy, and feels perpetually cold. Tofu anyone? Seriously folks, this is your health at stake. It is very interesting to note that pigs are often fattened with soybeans. Perhaps it is through the same thyrotoxic mechanism.
Furthermore, a lot of the soy products sold today, particularly the ones in the freezer section of your supermarket, are, pardon my language, complete crap. Don't take my word for it. Head on down to your local supermarket. Open the freezer door, and pluck out a package of soy hot dogs, patties, or whatever. First off, look at how much sodium you’re ingesting per serving. You might as well be drinking seawater. Second, look at all of the unpronounceable ingredients entering your body. If you still think that's healthy - well, enjoy. I'll stick to my natural food.
Finally (I bet you're going to love this one), soy consumption in pregnant women has been linked to a five-fold increase in hypospadias, a penile defect, possibly because of the high phytoestrogen content in soybeans[9].