A number of recent claims about soyfoods suggest that they may raise risk for all types of diseases–cancer, nutritional deficiency diseases, and thyroid problems among them. All of these allegations are, in fact, based on scientific studies to some extent which makes it easy to believe them.

But citing a research study or two doesn't make something true. It's rare for every single study on any particular topic to show the same results. For one reason or another, there are always a few that stand in direct contrast to the bulk of the research. For the most part, the allegations against soy are based on a few non-conforming studies rather than on the majority of the research.

This selective picking and choosing of individual studies allows people to "prove" just about anything. To get at the truth, health experts examine all the research and base conclusions on the totality of the evidence, not just a few studies.

Furthermore, almost without exception, studies suggesting that soy is harmful have been conducted in rodents or test tubes. These types of studies rarely provide meaningful insight into human nutrition. In fact, more often than not, animal studies lead to erroneous conclusions about effects in humans.

So what do the data say about soy when we look at all of the research with an emphasis on human studies? You'll see below that there is considerable support for the safety of soyfoods at all stages of life.

Soy and Breast Cancer

Soybeans and foods made from them contain isoflavones, compounds that exert weak estrogen-like effects under some experimental conditions. 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.

Over the years, there has been much speculation that isoflavones reduce breast cancer risk by inhibiting the effects of estrogen. However, two studies suggested that soy could have effects on the breast tissue which might actually raise risk for breast cancer. Also, in studies using animals whose ovaries have been removed, soy isoflavones can stimulate the growth of certain types of breast cancer cells.

These animal studies have many methodological limitations that impact the value of their findings. More importantly, recent human studies indicate that soy does not increase breast cancer risk in healthy women or negatively impact breast cancer patients. For example, long-term studies on risk factors for breast cancer (breast cell proliferation and breast tissue density) showed no effects of soyfoods or isoflavones. And, soyfood intake among Chinese breast cancer patients was unrelated to survival.

Whether or not soy protects against breast cancer remains controversial. But the bulk of the evidence in humans shows that soy is safe for women to consume. And, research suggests that consuming soy during puberty 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.

Soy Intake and Reproduction

The idea that soy isoflavones could cause reproduction problems arose 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. Sheep also metabolize isoflavones differently than humans, making them a poor predictor of effects in humans.

Research shows that isoflavones have no effect on sperm quality or testosterone levels in men, nor do they prevent ovulation in women. There is no evidence of reproductive problems in populations with regular soyfoods consumption.

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.

Soy has been studied in relation to thyroid function for more than 70 years. In the 1950s approximately 10 cases of goiter–a symptom of thyroid problems–were identified 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. Today's formula is fortified with iodine and is processed differently than older formulas. As a result, there has not been one case of goiter reported in the medical literature in the more than 20 million babies fed soy infant formula over the past 40 years.

Soy does not 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 answer will be to consume adequate iodine, not give up soy.

This can be a relevant issue for vegetarians since those who don't consume milk can sometimes have low iodine intakes. It really depends on where you live since the amount in fruits and vegetables varies according to where they are grown. (Vegans living in northern Europe are at highest risk.) Sea vegetables can supply iodine, although the amount varies. The most reliable source of this nutrient is iodized salt which should be on the menu–in moderate amounts–for all vegans.

Soyfoods and Cognitive Function

An epidemiologic study published in 2000 involving Japanese men living in Hawaii came up with an unexpected finding: Those men who ate the most tofu in middle age showed the most signs of mental decline in old age. The researchers recorded intake of 26 foods–including tofu–in 3,000 men in the mid-1960s and again in the early 1970s. The study was actually looking for links between diet and heart disease–but they also gave the men cognitive function tests in the 1990s and looked at brain size in men who had died. Tofu consumption was associated with poorer test performance and more brain loss.

It's hard to understand how this could be. After all, people in Japan–who eat tofu all their lives–have much lower rates of dementia in old age than people in the U.S. But it's tricky to compare rates of dementia across cultures. For one thing, dementia is diagnosed differently–using different criteria–in other countries. And second, there are so many profound differences between western and Asian culture that it's nearly impossible to single out any one factor. What we really need is a study comparing two groups of people in Japan–those who eat tofu and those who don't. But we don't yet have such a study.

The Hawaiian research was an epidemiologic study–an observation of factors that occur together. It doesn't show cause and effect. Tofu could simply occur along with some other factor that raises risk for dementia–without impacting that risk at all. That is, tofu could be an innocent bystander. And there is reason to believe this may be the case.

In contrast to this one epidemiologic study, intervention trials in which the effects of soyfoods or isoflavones on cognitive function have been directly assessed suggest that soy may actually be beneficial. Three of these trials found statistically significant benefits, one found an improvement in response to soy (although the effects were not statistically significant), and the fifth found no effects–neither positive nor harmful. Thus, most evidence suggests that soy may favorably affect cognitive function although this remains very speculative.

Soyfoods and Mineral Absorption

Soyfoods are high in phytate–a compound known to inhibit the absorption of minerals such as iron, zinc, and calcium. Despite this, calcium absorption from soy is actually very good. (Also, the isoflavones in soyfoods may promote bone health. Therefore getting calcium from soyfoods that are naturally rich in this mineral or fortified with it is a good idea.)

Iron absorption from soyfoods is more controversial. Soyfoods are high in iron, but there are conflicting findings on how well it is absorbed. Zinc is absorbed more efficiently from soyfoods but these foods are not particularly rich in this nutrient. The bottom line: don't depend on soyfoods to meet your iron and zinc requirements. This is certainly not a reason to avoid these foods, though. They can make a contribution to iron and zinc intake (especially if you consume a good source of vitamin C at the same time you consume the soyfood) but you should be certain to include other sources of iron and zinc in your diet as well. The idea that consuming soyfoods will contribute to poor mineral status is unfounded.

Should You Eat Soy?

Isolated studies have been used to raise questions about the safety of soy, but the bulk of the evidence shows that it is safe. Average intake among adults in Japan is one to two servings of soyfoods per day and there is no reason to think that more than this–up to four servings per day–is unsafe. Diets should always be varied, of course, but there is no reason to avoid soyfoods as a part of that varied diet.

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