The Cancer Research
Research results differ on whether soy foods are likely to reduce cancer risk. The variety of study outcomes may reflect influences that differ by the type of soy food, time of life they are consumed, the type of cancer, and individual differences in genetics, gut microbiota or overall diet.
Consistent findings from population studies indicate no increased risk for breast cancer survivors who consume soyfoods. In fact, limited evidence shows the potential for greater overall survival and perhaps decreased recurrence, among women a year or more after diagnosis who include moderate amounts of soy.
Interpreting the data
After a systematic review of the global scientific literature, AICR/WCRF analyzed how foods and their nutrients affect the risk of developing cancer.
“Convincing” or “probable” evidence means there is strong research showing a causal relationship to cancer—either decreasing or increasing the risk. The research must include quality human studies that meet specific criteria and biological explanations for the findings.
A convincing or probable judgement is strong enough to justify recommendations.
- There is probable evidence that foods with dietary fiber DECREASE the risk of:
- Colorectal cancer
- Weight gain, overweight and obesity*
*This is important, because there is strong evidence that excess body fat increases the risk of at least 12 different cancers.
“Limited suggestive” evidence means results are generally consistent in overall conclusions, but it’s rarely strong enough to justify recommendations to reduce risk of cancer.
- Limited evidence suggests that foods containing isoflavones may DECREASE the risk of:
- Lung cancer
- Limited evidence suggests that foods containing soy may DECREASE the risk of:
- Deaths from all causes in breast cancer survivors 12 months or more after diagnosis
Ongoing Areas of Investigation
- Laboratory Research
Most of the laboratory research involving soy and cancer risk has involved soy’s isoflavones, especially genistein and daidzein.
Isoflavones, especially genistein and daidzein, are the focus of most of the laboratory research involving soy and cancer risk.
- In cell and animal studies, isoflavones influence gene expression and cell signaling in ways that increase antioxidant, anti-inflammatory and carcinogen-deactivating enzymes. They inhibit cancer cells’ growth and ability to spread, and activate signaling that leads to self-destruction of abnormal cells. They dial down expression of oncogenes (genes that have potential to cause increased cell growth that can lead to cancer) and increase expression of tumor suppressor genes.
- Isoflavones have a chemical structure similar to human estrogen, so you may hear them called “phytoestrogens”. But there are important differences.
- Scientists have now found that rodents metabolize isoflavones differently than humans. Rodent metabolism leads to much higher levels of the active form of isoflavones compared to humans. In some early studies in mice, soy isoflavones promoted growth of estrogen receptor-positive (ER+) breast cancer. But this involved much higher blood levels of the active form of isoflavones than would result from humans consuming soyfoods.
- Different kinds of estrogen receptors are present in different parts of the body. Activation of some receptors seems to promote cell growth. But studies suggest that isoflavones more often bind to estrogen receptors with other effects, potentially acting as a tumor suppressor at times.
Resistant starch and fermentable types of dietary fiber support growth of healthful bacteria in the colon in animal studies.
Phenolic acids increase cells’ antioxidant and anti-inflammatory defenses against damage that could lead to cancer in cell and animal studies. Emerging evidence in animal studies suggests they may also improve glucose metabolism and decrease insulin resistance, and alter the gut microbiota (microbes living in the colon), creating an environment in the body less likely to support cancer.
Folate helps maintain healthy DNA and keep cancer-promoting genes “turned off”. Animal studies, however, suggest that exceptionally high amounts or intervention after cancer cells have formed might promote development of cancer.
Phytic acid may reduce damage to colon cells from free radicals produced there. However, the effects of whole grains as a source of this protection are not yet known.
Lignans increase antioxidant, anti-inflammatory and carcinogen-deactivating enzymes in cell studies. They also decrease growth and increase the self-destruction of cancer cells. In studies with mice, lignans decrease cancer development and growth.
Protease inhibitors block certain enzymes. In cell studies, they inhibit the development and growth of cancer cells at several stages and increase their self-destruction. Protease inhibitors resist digestion and reach the colon, and in animal studies, they suppress inflammation and development of colorectal cancer. Research is actively looking at their cancer-fighting potential, but their effects as consumed in foods like legumes is not known.
Saponins regulate certain enzymes that seem to inhibit oxidative stress, inflammation, and growth of cancer cells, and promote their self-destruction (apoptosis), based on cell studies. Saponins seem to be poorly absorbed out of the digestive tract, however, and more research is needed about their effects when consumed.
Sphingolipids seem to play a role in regulating cell growth, self-destruction of abnormal cells and progression of tumors based on cell and animal studies.
- Human Studies
Some studies link regular consumption of legumes (which includes pulses, as well as soy) with a reduced risk of colorectal, prostate and breast cancers, but more research is needed. For now, the AICR Third Expert Report categorizes evidence for all three of these cancers as “Limited – No conclusion”.
Most Americans do not eat large amounts of soy. That makes it challenging for U.S. observational studies to compare how the highest versus lowest intake links to cancer risk. The majority of population studies involving soy come from Asia, where soy is a dietary staple. Even so, across populations, studies now point to soy possibly protecting against some cancers, and presenting no increased risk of overall cancer.
- The AICR Third Expert Report rated evidence “Limited Suggestive” linking isoflavone consumption with lower risk of lung cancer for people who have never smoked. This does not apply to people who have ever smoked, and further research is needed before there is enough evidence to support any recommendation.
- Studies of prostate cancer paint a mixed picture, with some showing no association and some linking soyfoods with decreased risk. For example, an analysis combining observational studies shows lower risk for unfermented soy foods (such as tofu, soy milk and edamame), although no association was identified for fermented soy foods (such as tempeh, miso and natto)
- Observational studies link moderate soy consumption (one to two servings a day) with lower breast cancer risk in Asia, where soy foods are commonly consumed throughout life. However, broader research now suggests that hormone-related protective effects of soy against breast cancer may come mainly from soy consumption during childhood and adolescence. More research is needed; for now, whether considering soy as a whole or some specific forms of soy foods, the AICR Third Expert Report analysis found mixed associations and categorized evidence on soy and breast cancer risk as “Limited – No conclusion”.
- Soy may reduce cancer risk in some people more than others, reflecting differences in genetics and in gut bacteria that metabolize soy compounds into their active form. About 30% of Western populations and up to 65% of Asian populations have gut bacteria that are able to produce a compound called equol from the soy isoflavone daidzein. Some research suggests that those who produce equol may particularly benefit from eating soyfoods.
- Population studies don’t link soy consumption with an increased risk of any cancer.
- Emerging research links soy food consumption with a greater variety of health-promoting bacteria in the gut microbiome. That suggests potential for reducing cancer risk and promoting other aspects of overall health, so more research is needed.
- Observational population studies link high dietary fiber consumption with reduced risk of colorectal cancer. Analyses combining 16 to 20 prospective cohort studies also link dietary fiber with a lower risk of breast cancer. However, analysis for the AICR/WCRF Third Expert Report found the evidence for an association of dietary fiber and this and several other cancers too limited to support a conclusion.
Among women without cancer, several randomized controlled trials have studied consuming soy protein powder or soy isoflavone supplements for six months to two years. The amounts of isoflavones consumed in the studies are equal to or higher than what Asian women typically eat – one to two servings a day.
Overall, randomized controlled intervention trials in premenopausal and postmenopausal women without cancer testing have found no clear beneficial or harmful effects on markers of breast cancer risk such as hormone levels, markers of breast cell growth, or breast density. Likewise, randomized controlled trials show no effect on prostate-specific antigen (PSA) levels or hormones related to risk of prostate cancer.
Whole soy foods that are good sources of dietary fiber provide additional benefits.
- Viscous fibers form a gel that slows down the body’s absorption of carbohydrates. Ongoing research is looking at how this might reduce elevated levels of insulin and insulin resistance that seem to support cancer development.
- Fermentable fibers can be used by gut bacteria to produce butyrate and other short-chain fatty acids (SCFAs). These SCFAs reduce markers of inflammation and oxidative stress in human clinical trials and show effects on gene expression that could reduce cancer development.
- What about cancer survivors?
Breast Cancer Survivors: Consistent findings from population studies indicate no increased risk for breast cancer survivors who consume soyfoods. In fact, limited evidence shows potential for greater overall survival, and perhaps decreased recurrence, among women a year or more after diagnosis who include moderate amounts of soy.
- The amounts classified as moderate are comparable to what Asian women consume. The analysis that showed decreased recurrence with soy, pooled results of studies involving almost 10,000 breast cancer survivors. Consuming at least 10 milligrams isoflavones daily – lower than in the average Asian diet — was associated with less breast cancer recurrence. Some of the studies suggest that benefit may vary based on cancer treatment or tumor characteristics, but no group showed increased hazards related to soy consumption.
- Among postmenopausal Asian women, those who ate the most soy foods – about two to three servings a day – had the lowest risk of recurrence or death compared to women who ate the least, less than a few servings a week. These findings were also seen in studies among predominantly Caucasian women who consumed soy at levels comparable to average Asian consumption.
- Overall, these population studies do not show any harmful interactions between soy foods and anti-estrogen medications. A small number of studies even suggest soy foods may be most protective for women who take tamoxifen or an aromatase inhibitor, but more research is needed.
- Studies supporting benefits of a moderate level of soy foods should not be used to support use of isolated soy protein or isoflavones, especially in amounts beyond what can be reached with diet. One 2014 randomized controlled trial studied the effects of soy protein powder among women diagnosed with breast cancer, before their surgery. The soy protein supplement altered the expression of genes associated with promoting breast cancer. With consumption only lasting from 7 to 30 days and many unknowns about other gene changes, the clinical significance of the study is unclear. Amounts of isoflavones consumed were above those recognized as safe – which is 100 mg/day – underscoring the point that more is not necessarily better.
Prostate Cancer Survivors: Among men with prostate cancer before or after prostate surgery, or following a “watchful waiting” approach after diagnosis, supplements of soy protein or isoflavones for periods ranging from 6 weeks to 2 years have shown either no effect or a decrease in progression of prostate cancer. Further investigation in well-planned trials is needed.
- In some clinical trials of men with various stages of prostate cancer who were experiencing increases in PSA, the PSA tended to rise more slowly in those consuming soy milk, or isolated soy isoflavones. One intervention using a supplement combining soy isoflavones and some other potentially protective compounds reported improved PSA. However, most studies have not shown effects on PSA. It’s possible that effects are stronger in some men than others. It is not clear whether genetics and/or differences in metabolism might result in greater effects in some men than in others. Further, although PSA testing is widely available and frequently cited in prostate cancer studies, it is not necessarily an indicator of long-term outcomes after prostate cancer.
- What Is a moderate amount of Soy?
Moderate consumption is 1 to 2 standard servings daily of whole soy foods, such as tofu, soy milk, edamame and soy nuts. (One serving averages about 7 grams of protein and 25 mg isoflavones.) Examples of a standard serving are 1/3 cup (about 3 oz.) tofu, 1 cup soy milk, 1/2 cup edamame and ¼ cup (1 oz.) soy nuts. Studies have demonstrated up to 3 servings/day – up to 100 mg/day of isoflavones – consumed in Asian populations long-term does not link to increased breast cancer risk.
- Tips for Selection, Storage and Preparation
- Tofu comes in different consistencies: extra firm to soft. Firm is best for stir-frying or grilling, while soft tofu works best in dishes like lasagna. Some tofu is high in calcium, depending on how it’s made. Check the nutrition facts label.
- Tempeh, made from cooked and slightly fermented soybeans, is high in protein and has a nutty flavor. You can find it in your supermarket’s refrigerated or frozen sections.
- Soymilk is naturally a good source of protein; if you are substituting for milk, check the label to make sure the soymilk is fortified with calcium and vitamins D and B-12. Flavored varieties typically contain added sugar.
- Other soy foods you might enjoy include soy yogurt and cheese as substitutes for dairy, roasted soy nuts as a snack, soy nut butter (use like peanut butter) and soy flour for baking.
- Miso is a paste made from fermented soybeans and a grain like rice or barley. Its salty, savory flavor makes miso a popular base for soups.
- Edamame (green soybeans) are available fresh in their pods and in the freezer section. Frozen, you can get them in their pods or shelled, raw or pre-cooked.
- If you purchase the boxed shelf-stable tofu, once opened you need to refrigerate it. Keep refrigerated tofu in a container covered with water, and change the water every day or two to keep tofu fresh for up to a week.
- In the refrigerator, soymilk keeps five to seven days, tempeh up to ten days and edamame about two days. Frozen edamame, tofu and tempeh last for several months.
- Miso will keep in the refrigerator for several months.
- Tofu can be stir-fried, grilled, added to stews and soups, and used in mixed dishes like lasagna.
- Add tempeh to chili and pasta sauce or grill a slice for a sandwich.
- Silken tofu makes the smoothest salad dressing and smoothies.
- Use miso in soups, sauces and in marinades for meats and fish. Keep in mind it is high in sodium.
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