The “soy story” and its possible connection to breast cancer is complicated by many factors. It all began when women in Asian countries were observed to have much lower breast cancer rates than women in the U.S. Most women living in Asia depend on soy as their main source of protein but soy is not the only dietary difference between the two regions. Asian women consume only small amounts of beef, chicken, and pork — which means less animal fat and other possibly unhealthy substances (such as growth hormones and antibiotics). Also, compared to the average woman in the United States, the average Asian woman:
- eats more fresh vegetables
- is closer to her ideal body weight
- is more physically active
- is less likely to consume significant amounts of alcohol
The most troublesome component of soy is isoflavones. The isoflavones are known to bind to estrogen receptors and have both estrogen-like and anti-estrogen effects. Tamoxifen is a drug used to treat certain types of breast cancer in women and men. The concern is that isoflavones may increase the risk of cancer recurrence because breast cancer patients have low estrogen levels when taking tamoxifen. Since both tamoxifen and isoflavones have the ability to bind to estrogen receptors, the isoflavones may lessen the effect of the tamoxifen.
A study in 2009 was conducted in more than 5,000 Chinese women diagnosed with breast cancer, suggests that a diet rich in soy doesn’t worsen prognosis in women diagnosed with breast cancer and may offer some protection against recurrence.
Now, a new study shows that eating soy foods might not increase risk of breast cancer recurrence or death. Soy isoflavones intake was assessed for 16,048 women on average of 13 months after breast cancer diagnosis. Breast cancer outcomes were then assessed nine years after diagnosis. Outcomes among the survivors who consumed the highest amount of isoflavones (more than 23 mg a day) were compared with the outcomes of those with the lowest intakes (0.48 mg/day or lower). Women in the highest intake category had a 9% reduced risk of mortality and a 15% reduced risk for recurrence, compared to those with the lowest intake. Unfortunately the difference was not statistically significant.
The authors concluded that it’s fine to eat moderate amounts of soy foods as part of a balanced diet. One to 3 servings of soy a day (a serving is about a half cup) is similar to an average Japanese woman’s daily soy intake. If you are taking hormonal therapy to fight off a hormone-receptor- positive breast cancer, you should consult with your doctor about how much soy you can eat until the issue becomes clearer. Many breast cancer survivors say if the point of the breast cancer drugs is to minimize estrogen in breast cancers, does it make sense to eat a food that contains estrogen-like compounds? That is a great point when making decisions about soy intake.
Many women think that soy supplements help with menopausal symptoms and promote bone health. The isoflavones found in soybeans, sesame seeds and legumes are about one hundredth as powerful as natural female estrogens. If you’re getting isoflavones from dietary sources, you’d have a hard time overdosing yourself, unless you went on an all-soy diet. Soy supplements often contain a higher concentration of isoflavones. Pills with isolated soy isoflavones may cause trouble – not enough studies have been done yet on people to determine whether or not high concentrations of those isoflavones in supplements may encourage breast cancer cell growth. Presently, avoidance of soy supplements appears to be prudent especially for those who take hormonal therapy or who have estrogen-receptor-positive breast cancer.