Mushrooms, nuts and collard greens: Nutrition and how to lower the odds of breast cancer

The origins of breast cancer are varied. Though often the source of much attention, single gene mutations (like the BRCA mutation) are responsible for only 5 to 10 per cent of breast cancer diagnoses. Similarly, family history, though frequently feared, only accounts for 15 per cent. Thus, the remaining 75 per cent of cases are to some degree random.

The randomness is the product of a constellation of lifestyle and environmental factors. Many of those factors are uncontrollable, like the timing of menarche and menopause. Others are minimally controllable, such as the age at which you deliver(ed) your first child and whether you are able to successfully breastfeed that child. That being said, there are some factors that we can control daily, including exercise, alcohol consumption and diet.

The connection between nutrition and breast cancer is still an active area of investigation. One of the most notable studies was published in JAMA Internal Medicine in 2015, proving causation and demonstrating a definitive link between diet and breast cancer. What made this study remarkable is that it was a randomized controlled trial, a first in nutrition-related breast cancer research. Cancer is the manifestation of a long-term disease process that is typically decades in the making. Therefore, most of the research investigating diet and breast cancer is typically based on long-term cohort studies that follow women for decades and then retrospectively compare the dietary intakes of those who developed breast cancer in comparison to those who did not. Such studies inform us of associations between diet and breast cancer, but they do not definitively prove a connection.

In that 2015 study, more than 4,000 women between the ages of 60 to 80 at high risk for cardiovascular disease were randomized to eat one of three diets. Some ate a low-fat diet; others ate a Mediterranean diet supplemented with extra-virgin olive oil; others ate a Mediterranean diet supplemented with nuts. Researchers found that after nearly five years, participants eating the Mediterranean diet with olive oil were significantly less likely to develop breast cancer. Among this group, only 1.1 out of 1,000 women were diagnosed with breast cancer. Meanwhile, for women eating the Mediterranean diet with nuts, 1.8 out of 1,000 were diagnosed. And among those eating a low-fat diet, it was 2.9 out of 1,000.

These numbers sound small, and frankly, they are. You would need to put 562 women on the Mediterranean diet with olive oil to prevent one case of breast cancer. To put things into perspective, mammograms – our current mainstream approach to breast cancer prevention – has similarly modest effect sizes. For example, you need to perform a mammogram on 233 women between the ages of 60-69 to save one life. In younger age groups, this number is even higher. To save one life, you would need to screen 351 women between 50 to 59, or 746 women between 40 to 49.

This illustrates that it takes a lot of effort to influence breast cancer outcomes. The small effect sizes seen with the Mediterranean diet are no surprise and not a reason to ignore or downplay the findings. Diet is only one risk factor; therefore, diet alone does not determine who gets breast cancer and who doesn’t. Nevertheless, the fact that dietary changes initiated so late in life can have any benefit whatsoever is remarkably impressive.

Advice to adopt a Mediterranean diet is everywhere these days. Though this might be convenient for some (full disclosure, me included since I am of Southern European descent), it may be outright culturally inappropriate for others. So, if you don’t want to adopt a Mediterranean diet, the truth is, you don’t have to. There are many different foods and nutrients that have been associated with decreased risk for breast cancer. Integrating these individual foods and nutrients into your existing diet can make a significant difference.


According to the literature, mushrooms are potentially the most powerful food for preventing breast cancer. A systematic review and meta-analysis combining data from 10 studies showed that every gram of mushrooms that’s eaten each day decreases risk for breast cancer by 1 per cent. If you consider that a single mushroom is typically five grams, this suggests that even just taking a bite of a mushroom has an impact. If you look closely at the data, eating more and more mushrooms results in lower and lower risk. In fact, the study showed that with intakes of 20 grams (about four mushrooms per day), relative risk for breast cancer decreases by 45 per cent, an impressive feat.

With about four mushrooms per day, relative risk for breast cancer decreases by 45 per cent.


Fiber is a frequently overlooked nutrient for breast cancer prevention because it’s a little-known fact that fiber aids in the excretion of estrogen, the fertilizer for breast cancer cells. A meta-analysis of 16 studies showed that every 10 grams of fiber that’s eaten per day lowers relative risk for breast cancer by an additional 5 per cent. The problem is that 97 per cent of people do not eat enough fiber.  Nevertheless, this can be easily rectified by incorporating one fiber-rich food into each meal. The richest sources include beans (just one cup has approximately 15 grams), nuts (one handful has five grams), seeds (two tablespoons of flaxseeds contain six grams), whole grains (a cup of oats contains eight grams), certain fruits (berries, apples, pears, avocados, mangos) and certain vegetables (broccoli, cabbage, cauliflower, brussels sprouts, green peas, artichokes, pumpkin).


Another underrecognized nutrient for breast cancer prevention is alpha-carotene, namely found in orange vegetables like pumpkin, carrots and squash, as well as collard greens. A single serving of an alpha-carotene-rich vegetable can reduce risk for breast cancer by 9 per cent, an amount that supersedes a single serving of most other breast-cancer reducing nutrients.


No discussion of diet and breast cancer is complete without addressing the topic of phytoestrogens. Largely derived from soy-containing foods, these plant-based compounds look, and somewhat act, like estrogen. The fear of phytoestrogens because of their similarity to estrogen – the aforementioned fertilizer for breast cancer cells—is unmerited as the science of phytoestrogens is far more complex. Estrogen exerts its effects on the body by stimulating receptors, akin to pressing buttons. What was unknown for many years is the fact that there are different types of estrogen receptors that produce vastly different effects. While estrogen pushes “pro-cancer” buttons, phytoestrogens push “anti-cancer” buttons. The safety of soy-containing foods (like soybeans, edamame and tofu) was best demonstrated in a study of more than 3,000 breast cancer survivors who were followed for seven years post-diagnosis. The study found that soy consumption was not only safe (and did not increase risk for recurrence), but in fact, those with the highest intakes had half the risk for recurrence compared to those with the lowest intakes.

That being said, there are complicating factors that ultimately determine the potential healthfulness of soy foods. For starters, the benefits of phytoestrogens are likely greatest if they’re consumed earlier in life, particularly around the time of puberty. Furthermore, to exert their effects, phytoestrogens must be activated post-ingestion by the bacteria that live in our large intestine. The ability to activate these phytoestrogens depends on the health of the microbiome (the scientific term for the half pound of bacteria that we all carry around in our large intestines). Thus, simply adding soy foods to your diet is not enough, because without the help of healthy bacteria, you cannot process the phytoestrogens and reap maximal benefits. The bottom line is, it’s more complicated than simply the presence or absence of soy. Soy foods, when added to a high-fiber, vegetable-rich diet will likely decrease risk for breast cancer. However, soy foods in the context of a less healthful, processed/fast-food diet, likely will not.


While flaxseeds have been associated with decreased risk for breast cancer, their most remarkable benefit is their demonstrated ability to reduce tumor growth in patients already diagnosed with breast cancer. In one study, 32 women who were diagnosed with breast cancer and awaiting surgery were randomized to either receive a muffin containing 3.5 tablespoons of flaxseed or simply a plain muffin without flaxseeds. The researchers then compared the core tumor biopsies taken at the time of diagnosis (before patients started eating flaxseeds) in contrast to biopsies taken a month later at the time of surgery. They found that the group eating the muffins with flaxseeds had significantly decreased cancer cell growth and increased cancer cell death.


Folate, largely found in “beans and greens” is another nutrient associated with decreased risk for breast cancer. For breast cancer prevention, you don’t need a mega dose or supplements. Everyone needs 400 micrograms of folate per day and meeting this universal daily requirement from folate-rich foods – which can be easily obtained from a daily cup of beans alongside a half cup of cooked spinach – is the ideal way to decrease risk.

No conversation about diet and breast cancer is complete without also addressing the foods that increase risk. This most notably includes alcohol (even very light drinking at a dose of less than half of a standard drink per day can increase risk for breast cancer), processed meat, red meat and high-glycemic index carbohydrates (including white bread, white rice, desserts and baked goods). This is not to say that these foods should be avoided completely, but rather should be viewed as treats.

In summary, breast cancer is the product of many factors over a lifetime. While some of those factors are out of our control, others like diet offer opportunities to mitigate risk.

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Mary Sco.


Dr. Mary Sco. is a family physician with a PhD in nutrition. She completed her degrees at the University of Toronto and her residency at Women’s College Hospital. She is currently pursuing additional training at the Eastern Virginia Medical School, in Norfolk, Va. She is passionate about translating nutrition science into clinical settings and shares her evidence-based educational nutrition guides at:

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