• Dr. Thomas Griffith

No soy? No Way!

Updated: Oct 18, 2019

Many patients with breast cancer have asked me if eating soy increases their risk. Soy contains compounds called isoflavones that effect estrogen receptors. This has led to concern that soy may drive growth in certain types of breast cancer.

Delicious Soy Beans

What is Soy's Role in Cancer Prevention?

Soy foods are a major protein source throughout the world which has been credited for reducing risk for cardiovascular, neurological, and metabolic (diabetes) diseases. Even with demonstrated health advantages like these, the question about harm or benefit in breast cancer has remained.

To date, the only studies that have raised questions about safety have been based on lab findings in petri dishes. There has not been one human study demonstrating increased risk, in fact these studies show quite the opposite. There is actually very strong evidence for soy’s role in breast cancer protection.

Soy Found to Beneficial for Breast Cancer Patients

Most recently, a March of 2017 study by Dr. Fang Fang Zhang et al, published in the journal Cancer examined the levels of soy isoflavone intake in 6235 US and Canadian women with breast cancer over the course of 9.4 years.

They found a decrease of 21% in all causes of death in highest soy isoflavone intake vs lowest. Lower death associated with greatest soy isoflavone intake was found in women with estrogen and progesterone receptor negative cancer. In these women the risk of death was 51% less!

There was a 34% reduction in deaths among women who were not treated with hormone therapies for breast cancer. They found no negative interactions with cancer treatments. They concluded that soy was not problematic or risky but was actually beneficial for breast cancer patients.

This study’s findings confirm past published studies. It’s of note that benefit with soy consumption was seen in women who were not Asian and consumed an average of merely 2 mg of isoflavones daily. Asian women eat 45 mg of soy isoflavones on average daily, and have been the primary focus of many previous studies showing that soy intake reduces risk of breast cancer. This enormous disparity in dietary intake helps to explain why it was previously thought western women would not benefit from increased soy intake.

It was theorized that Asian women metabolize soy differently and get more protection from soy than their Western counterparts. Studies in ethnically diverse populations suggest otherwise. As it turns out, the difference in soy isoflavone metabolism lies in the way gut bacteria metabolize soy isoflavones.

The Key to Benefiting from soy consumption is Feeding the Friendly Gut Bugs

The key to optimizing the health benefit of soy is feeding the friendly gut bugs with a diet rich in high fiber plant based foods. Soy foods like miso, yogurt, and tempeh are commonly eaten in Asia and make the bacteria’s work easier through pre-fermentation of isoflavones. Research has shown that diet and not ethnicity are the key determinants of soy health gains. Dr. Zhang’s study means it is reasonable to extrapolate findings in Asian women to non-Asian women eating a health vegetarian based diet and eating lots of soy.

The Shangai Women's Health Study

The Shangai Women’s Health Study was a massive study including data from 70,578 Chinese women from age 40-70 observed for a median of 13.2 years. Greatest vs lowest soy intake, proffered a 22% reduction in breast cancer risk.

Premenopausal women often have the most aggressive breast cancers and they saw their breast cancer risk drop by 54%! Their risk of aggressive hormone receptor negative cancers dropped by the same amount! Post-menopausal women also fared considerably better, decreasing their risk of hormone receptor positive cancers by 28%.

A 2013 study, “Post-diagnosis Soy Food Intake and Breast Cancer Survival: A Meta-analysis of Cohort Studies”, summarized the findings of studies including 11,206 survivors. Soy intake at any level after diagnosis reduced death by 15%, and recurrence by 21%. Women who ate the highest amount of soy were 22% less likely to die and they had 26% less cancer recurrence than women eating the least.

Women with estrogen receptor negative cancer were 25% less likely to die, and women with estrogen receptor positive cancer were 28% less likely to die when eating the highest vs lowest soy in their diets. Recurrence of cancer was reduced by 36% for estrogen receptor negative patients, by 45% with estrogen / progesterone receptor positive patients, and 33% for postmenopausal patients (highest vs lowest soy).

These studies were based on studies with Western and Asian women driving home the argument that diet and not race are the drivers of soy effects.

Eat Your Veggies, and Your Soy!

It is now pretty clear that soy is not only not harmful in regards to breast cancer risks, but that is a potent tool for not only prevention of cancer occurrence but should be considered as an adjutant compliment in breast cancer treatment.

Future studies should emphasize a diverse patient population and breast cancer types. Clinical trials employing vegetarian diets with large amounts of soy and supplemental forms of soy isoflavones would also be valuable in further defining the parameters and extent of benefit for women fighting breast cancer.

The moral of this story, is eat your veggies, eat your soy (non-GMO, organic), and live a long happy, healthy life.

Now that your mind is changed about soy and breast cancer risk, the challenge is making soy taste great. We suggest a few cookbooks including The How Not to Die Cookbook and The China Study Cookbook. Of course, there are a plethora of recipes on-line as well.

Bon appetit!


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