Soybeans, Cabbages and Breast Cancer
by Dr. Joe Schwarcz
an excerpt from the book Radar, Hula Hoops and Playful Pigs
Why all the excitement about soybeans? Because Japanese women have one quarter the breast cancer rate of North Americans. And Japanese women eat a lot of soy products. Of course this does not necessarily mean that soy consumption has anything to do with breast cancer; after all, there is a very strong association between the disease and the wearing of skirts. But of course nobody thinks that the wearing of skirts causes breast cancer.
When we start considering the scientific evidence it certainly begins to look like the soy connection is more than just a chance association. Our story begins in the 1940s when Australian farmers noticed that sheep which grazed on a certain type of clover failed to reproduce normally. The urine of these sheep was found to contain a high level of a compound called equol, which had previously been found in the urine of pregnant horses.
Bacteria in the sheep's intestine, as it turned out, had converted a naturally occurring compound in clover to equol. And equol was known to have biological activity similar to estrogen! It was no great surprise that an estrogen-like substance should interfere with fertility; after all, estrogen was known to play an important role in human reproduction. Scientists began to wonder if other naturally occurring foods had compounds which possessed estrogenic activity.
Enter the soybean. Researchers discovered that this Asian staple contained compounds collectively known as isoflavones which indeed did exhibit estrogen-like behavior. Genistein and daidzen, in particular, were of interest because they were partially excreted in the urine and could be correlated with the amount of soy in the diet.
This raised eyebrows because it had already become apparent to scientists that estrogen and breast cancer were somehow connected. Women who were exposed to more estrogen over a lifetime were known to have a higher risk of contracting the disease. This includes women who came into puberty early, reached menopause late or had few or no children. In other words, it seems that any factor that lowers the total number of menstrual cycles over a lifetime lowers the risk.
Now we get back to our Japanese women. They have longer menstrual cycles, averaging 32 days compared with the North American 29 days. This could mean thirty to forty fewer periods in a lifetime. They also have up to a thousand times more phytoestrogens in the urine than North American women. But the soybean plot really thickens when we note that Japanese consume 30 times more soy products than we do and that Japanese who migrate to North America and take up the North American diet and lifestyle show cancer rates comparable to other Americans.
Recent research has even revealed a possible mechanism for the isoflavone-breast cancer connection. Some cells in breast tissue are known as estrogen-responsive, meaning that they contain certain proteins (estrogen receptors) to which estrogen can bind, very much in the fashion of a key fitting into a lock. This binding unleashes a sequence of events in the nucleus of the cell eventually leading to the manufacture of certain proteins which cause cell proliferation. Such abnormal cell multiplication can lead to cancer.
Isoflavones, it seems, are actually "weak estrogens." They fit into estrogen receptors but do not stimulate any cellular activity. At the same time, they prevent estrogen from binding with the receptor. It is as if the wrong key had been inserted into the lock. The key cannot be turned, but it effectively prevents another key from being inserted.
So much for associations and theory. What practical evidence can we muster to show that soy consumption may actually prevent breast cancer? A number of animal studies have demonstrated that the consumption of soy or isolated isoflavones reduces tumor development. Human data is less direct. But researchers have compared groups of breast cancer victims with matched controls and noted a decreased risk of up to 50% in premenopausal women who consumed soy daily.
A classic study in Singapore showed that breast cancer rates correlated inversely with the amount of soy protein eaten on a regular basis. More than twenty studies of Asian women have shown that even 1 cup of soy milk or half a cup of tofu a day is associated with reduced cancer risk.
In addition, it is found that menopausal women who start eating 20 grams of soy protein powder daily, (roughly equivalent to a soy burger, a cup of soy milk or a serving of tofu) show a reduction in the severity of menopausal symptoms. An added benefit is increased bone density in the spine. As far as premenopausal women go, the same kind of diet increases their menstrual cycles by 2.5 days. The isoflavone content of the urine also rises dramatically. It is certainly apparent that soy has estrogen-like activity!
And now we find out that genistein, the main isoflavone, may have yet another effect. It decreases the growth rate of blood vessels that nourish tumors. This inhibition of "angiogenesis" may turn out to be the most important anti-cancer effect. It may even explain why men who have high levels of genistein in the urine seem to be protected from prostate cancer.
Although the isoflavones appear to be the most interesting anti-cancer compounds in soybeans, there are others. Folic acid, for one, has been shown to prevent mutations in DNA. There seems to be no end to soy's benefits. Soy protein can even lower cholesterol and when mixed in with ground beef it reduces the amount of carcinogens that form on broiling.
There are, however, some inconsistencies in the soy saga. A study of Japanese women showed that women with breast cancer had consumed no less soy than a control group unaffected by the disease. Chinese women, who eat only about a third of the soy based foods that the Japanese eat have the same low rate of breast cancer. Of course it is possible that a certain amount of soy is protective but eating more carries no further benefit.
Even though there are uncertainties about the role of soy in protection from breast cancer, there is certainly no harm in increasing intake of isoflavones. But not all soy products are equally good sources. Soya oil, hot dogs and tofu ice cream are poor sources but tofu, soy milk, tempeh, miso, soy flour and textured soy protein contain good doses of isoflavones.
Before we get too carried away with the isoflavones, it is important to realize that breast cancer is a complex disease with many possible contributing factors. The disease is age-related and is linked to excessive alcohol consumption. There may be a connection to high levels of certain fat-soluble pesticides. As far as the fat content of the diet goes, studies have been ambiguous. Some studies noted an increased risk with saturated fat while others showed high risk with increased carbohydrate consumption. Monounsaturated fats like canola oil or olive oil appear to be the best choices. Exercise, fruits and vegetables are protective.
Particularly effective are "cruciferous" vegetables like broccoli and cabbage! These contain indole-3-carbinol which protects against estrogen-stimulated breast cancer. Could this be the reason why prior to unification, the breast cancer rate in East Germany, where cabbage was a dietary fixture, was much lower than in the affluent West?
Cabbages are easy enough to cook, but what do you do with the soybeans? You can soak them in water overnight and then cook them like any other bean or just roast them in the oven or in the microwave and eat them as a snack. Then again, there's tofu. How about an anti-cancer combo of cabbages and soybeans? I'm working on it. The taste is fine, and the ill wind that is produced may well blow good.
© 2002 Dr. J. Schwarcz.. This information is published and may not be reprinted.