Making tofu in Chinese village courtyard.
Many of the questions about soy safety have focused on their isoflavone content. These are compounds that exert weak estrogen-like effects under some experimental conditions.
But isoflavones are actually often classified as SERMS—selective estrogen receptor modulators. SERMs have estrogen-like effects in some tissues and under some experimental conditions, but have either no effects or anti-estrogenic effects in others.
Soy and breast cancer
While some animal studies have shown that soy isoflavones can stimulate growth of existing mammary tumors, the research in humans indicates that soy does not increase risk of developing breast cancer risk in healthy women. Nor does it adversely impact the prognosis of breast cancer patients.
In fact, studies suggest that consuming soy during early adolescence reduces risk for breast cancer later in life. This may help explain why Japanese women—who typically consume soy all their lives—have lower breast cancer rates than western vegetarians, who may not consume soy until adulthood. I wrote about this in more detail a few weeks ago.
Soyfoods and thyroid
Many foods—including soy, broccoli, and millet—contain goitrogens, which are compounds that interfere with the thyroid gland. Most people can eat these foods regularly without problems. Problems do occur in people who have low intakes of the mineral iodine, which is needed for thyroid function.
In the 1950s approximately 10 cases of goiter—a symptom of thyroid problems—were found in infants consuming soy infant formula. These 50-year-old studies have been used to fuel arguments that soy is dangerous for infants—but in fact, they have no relevance to infants who are currently fed soy infant formula. This is because soy formula is now fortified with iodine and is processed differently than the older formulas.
In the past 40 years more than 20 million babies have consumed soy infant formula that is fortified with iodine. In that time, there has not been a single case of goiter reported that has been linked specifically to the use of soy formula.
The clinical intervention studies show quite clearly that soy doesn’t adversely affect thyroid function in healthy adults either. As long as people meet the recommendations for iodine intake, consuming large amounts of soy has not been shown to have adverse effects. Whether soy is harmful to those who have deficient iodine intakes hasn’t been determined. But if this turns out to be the case, the response is to be sure to consume adequate iodine, not give up soy. Vegans can make sure they are getting adequate iodine by using small amounts of iodized salt.
For those who are hypothyroid, some adjustment in medication dose may be required if large amounts of soyfoods are consumed. The same is true for those who use fiber supplements and certain herbal supplements.
Soy intake and reproduction
The idea that soy isoflavones could cause reproduction problems arose in part because Australian sheep consuming clover that was very high in isoflavones developed breeding problems. Their isoflavone intake was far higher than what people in Asia typically consume and sheep also metabolize isoflavones differently than humans. This makes them a poor predictor of effects in people.
Clinical studies show that isoflavones do not affect sperm quantity or quality or testosterone levels in men, nor do they prevent ovulation in women. There is no evidence of reproductive problems in populations with regular soyfood consumption. You can read more about the effects of soy on sperm and testosterone on my blog.
How much soy should you eat?
Average soy intake among adults in Japan is one to two servings of soyfoods per day. Many epidemiologic studies show that those who consume two to three servings per day have lower disease rates than those who consume relatively little soy. So a reasonable soy intake is 2 to 3 servings per day, although there no reason to think that more than this is in anyway harmful. All types of soyfoods including tofu, soymilk, tempeh, and veggie burgers or other meat analogs can have a place in a healthful vegan diet.
For more information about soy, take a peek at my two previous articles this week:
Health effects of different types of soy can be found here.
Soy and cognitive function can be found here.
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