Effects of Soy Foods on Cancer

• Clinical Evidence of Anticancer Effects
• Other Potential Health Benefits
• Safety Concerns

Soy beans are a rich source of high-quality protein and isoflavones. You may have heard that there was some concern about these plant compounds could increase the risk of the development or progression of breast cancer. That's because isoflavones have estrogen-like properties. ^(ii.1)

But there's good news, especially if you're a fan of tofu or miso. You'll be happy to know that research suggests the opposite. Foods made with soy beans and fermented soy milk beverages are actually linked with cancer-fighting effects: ^(ii.1, 2)

• Clinical trials show that soy isoflavone intake does not increase markers linked to risk of breast cancer.
• Population studies involving thousands of women suggest consuming foods and drinks made with soy reduces the recurrence of breast cancer and increases survival.

Clinical Evidence of Anticancer Effects

In fact, soy and its compounds may also help prevent or treat other types of cancer (besides breast cancer). ^(ii.1-6)

Prostate Cancer

In a randomized phase II clinical trial 32 men with prostate cancer were assigned to eat soy bread. One group was given soy bread enriched with more isoflavones. ^(ii.7)

Results of blood tests after 56 days, compared to baseline, showed significant benefit: ^(ii.7)

• Reduces inflammatory (cytokine) proteins that promote cancer.
• Inhibits cells that suppress the immune system.

Natural killer cells that help fight cancer were also increased. An earlier small clinical trial also showed the benefits of soy bread. In that study, the prostate cancer patients who ate 4 slices a day of bread made with soy grits showed significantly lower PSA levels. ^(ii.7, 8)

In preclinical studies, soy isoflavones and their metabolites reduce PSA levels. However, results from clinical trials measuring the ability of soy or its compounds to reduce high PSA levels are mixed. ^{(ii.5, 8-10)}

Endometrial Cancer

Increased thickness of the lining of the uterus is a risk factor for endometrial cancer in women. A review of randomized, controlled clinical trials suggests that the effects of soy may depend on different populations: ^(ii.11)

• In Asian women, soy isoflavone supplements were linked with increased endometrial thickness.
• In North American women, soy isoflavone supplements were lined with reduced endometrial thickness. The soy effect was only significant at doses of more than 54 mg/day.

The study authors suggested that dietary soy may explain the difference. Asian countries have high soy consumption compared to North America. So, in diets that are high in soy already, soy supplements may not confer any benefit. ^(ii.11)

Gastrointestinal Cancers

Analysis of numerous prospective studies suggests a link between soy consumption and lower risk of stomach cancer. However, the beneficial effect of soy on preventing gastric cancer only seems to apply to women, not men. ^(ii.12)

Research suggests soy consumption is linked to a slight reduction of risk for colon cancer. ^(ii.13)

Other Potential Health Benefits of Soy

Research, including clinical trials, suggests that soy isoflavones could also help prevent and treat various health conditions. ^(ii.6)

These include:

• Depression ^(ii.14)
• Diabetes ^(ii.15)
• Excess body weight or obesity (with exercise) ^(ii.16, 17)
• Heart disease ^(ii.18)
• High cholesterol ^(ii.19)
• Hot flushes in menopausal women ^(ii.20)
• Insulin resistance (in some menopausal women) ^(ii.21)
• Kidney disease ^(ii.22)
• Metabolic syndrome ^(ii.15)
• Osteopenia and osteoporosis (in the spine) ^(ii.23)
• Vaginal dryness and painful intercourse in postmenopausal women (topical soy isoflavone cream) ^(ii.24)

The most beneficial effects of soy depend on its metabolism. Bacteria in the gut break down the soy isoflavone daidzein into equol. However, not everyone has the bacteria necessary to produce equol. Some studies suggest that people who can break down daidzein to equol are likely to garner more health benefits (including against cancer) than those who cannot. However, more research needs to be done to clarify and confirm these results. ^{(ii.11, 25-27)}

Potential Drug Interaction

Animal studies indicate that soy interacts with valproic acid, a drug used to treat epilepsy. Groups pretreated with soy extract showed significantly lower levels of the medication in both the brain and the blood. ^(ii.28)