DEFINITION: Soy foods or foods made from soybeans include tofu, tempeh, edamame, miso, and soy milk. Soy foods are an excellent source of protein and contain several substances that have been studied for their effect on cancer prevention and tumor growth.
Background: Since the late 1990s, researchers have conducted thousands of studies on the links between diet and cancer. Epidemiological studies (studies of population-wide patterns of disease) show that many cancers are less common in Asia than in Western countries. The Japanese breast cancer mortality rate, for instance, is far lower than that of the women with breast cancer in the United States. Unfortunately, these types of studies are hard to interpret because they cannot account for many potential confounding factors. For example, the average person in Asia may have a more active lifestyle than a typical Westerner or individuals of Asian descent may be less likely to have genetic factors contributing to cancer development.
Although it is not completely clear what factors are responsible for the lower rates of cancer in these countries, many researchers have focused on the potential influence of diet on cancer risks and outcomes. There are many differences between a traditional Asian diet and a modern Western diet, such as the amount of vegetables and fruits, processed foods, alcohol, meat, fish, and whole grains consumed. However, one thing that is very common in traditional Asian cuisine and historically absent from the typical Western diet is soy foods. However, this trend has begun to change in the twenty-first century as food production has become increasingly globalized. In addition, high-dose soy protein and isoflavone supplements have been developed and are available for sale.
How these compounds work: Soy products contain compounds called isoflavones. The two primary isoflavones in soybeans are genistein and daidzein. The US National Cancer Institute reports that genistein exhibits antioxidant, antiangiogenic, and immunosuppressive properties, which could indicate its potential as an anticarcinogenic substance. Isoflavones also act as weak estrogens, with less than 0.1 percent of the activity of estradiol (the main naturally occurring form of estrogen in humans). Isoflavones may also act as antiestrogens in the body because they can bind with the body’s estrogen receptors and block some of its more powerful estrogens. For cancers that have a hormonal basis, particularly estrogen-receptor-positive breast cancer, this estrogen blocking could help reduce a person’s cancer risk. However, while dietary soy is not considered an issue, it is prudent to avoid high doses of soy, such as those found in soy supplements, as their health effects are not well understood.
Research issues: Three basic kinds of studies have been conducted on the link between soy and cancer: population, animal, and laboratory. There are always concerns about the accuracy of the methods used in population studies. For instance, food frequency questionnaires depend on study participants remembering what they ate over days, weeks, or months. In addition, many of these studies have been conducted in small groups, so even when there is an apparent association, the link is often not statistically significant.
Animal studies have shown many potential benefits (and some risks) of soy consumption. Still, there is controversy over how applicable these results are to humans because animal biology, anatomy, and physiology are different from those of humans. Significantly, soy is metabolized differently by the human body than mice and rats, so research findings in rodents may not apply to people. Furthermore, animal studies often utilize isolated soy compounds or high doses of isoflavones, which may have significantly different effects than the average consumption of soy foods in a balanced diet. Laboratory studies, such as those that use cultured human cells to investigate the biochemical response of specific human tissues to elements found in soy, can be similarly controversial because they cannot consider the complexity of the whole human body and its systems.
Possible risks: Although several published studies suggest possible harm from eating soy, most are lab and animal studies. One of the more prominently reported studies dealt with the interaction between soy compounds and drugs to treat breast cancer. One soy compound (genistein) was shown to decrease the effects of tamoxifen, a standard breast cancer treatment, while another compound (daidzein) was shown to enhance tamoxifen’s effects. There is no consensus on whether high levels of soy foods are appropriate for current or former breast cancer patients. In addition, animal studies have found that soy (in supplement form or highly processed isolated proteins) can promote tumor growth under some circumstances.
Possible benefits: Some human and animal studies have shown that soy foods are associated with statistically significant reductions in prostate, colorectal, breast, endometrial, and stomach cancer risk. Soy seems to have a powerful protective effect against prostate cancer.
The best estimate is that those who eat the most soy have a 30 percent lower risk of developing cancer than those who eat the least. Regarding stomach cancer, intake of at least 10 grams per day of unfermented soy foods such as tofu may result in lower risk. Still, there is some question about whether these results reflect other factors, such as fruit and vegetable consumption.
As for breast cancer, modest reductions in risk have been shown for some groups of women, but most of the medical research suggests that soy intake protects against breast cancer mainly if consumed in childhood and adolescence. Postmenopausal women seem to receive little, if any, reduction in breast cancer risk. In the mid-2020s, the medical advice regarding soy foods and cancer suggests that eating soy foods in moderation is safe for people who have had breast cancer or are at a high risk of developing it.Soy-based foods do contain estrogen. However, the estrogen found in soy is different from the cancer-fueling estrogen produced by the body. Consuming soy products is believed to help reduce cancer risk and lower recurrence and mortality.More research is needed for the effects of soy foods on cancer risk to be fully understood.
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