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Breast Cancer and the Microbiome: Part 2


  • Fruits and vegetables are full of powerful, anti-cancer phytochemicals.
  • Compounds that show positive effects against breast cancer include glucosinolates, carotenoids, and polyphenols.
  • Although fiber can’t be digested by the body, it directly nourishes the microbiome.
  • The best source of probiotics includes kefir, yogurt, kimchi, and sauerkraut.

In part 1 of this blog series, we discussed the surprising connections between breast cancer and the microbiome. Many factors can cause a dysfunctional microbiome (dysbiosis), such as processed foods, antibiotics, and stress. Fortunately for us, dysbiosis is not only preventable but reversible! The new research emerging about the breast cancer-microbiome relationship gives us a roadmap to move forward and harness the microbiome as a cancer-fighting tool. In this article, we’ll provide some research-backed strategies for maximizing the potential of a tool you may not have even realized you had—your gut!

Gut-Healing and Gut-Protecting Compounds

There are a plethora of food compounds that can heal the gut and make a big impact on cancer development and progression. From plant compounds found in fruits and vegetables, to fiber and probiotics, we’ll discuss the role of these substances on breast cancer. In general, these compounds heal the gut in a few key ways: (1)

  • By serving as a food source for healthy bacteria in the gut
  • Eliciting antimicrobial properties and preventing the growth of “bad” bacteria
  • Decreasing gut inflammation
  • Keeping the gut lining healthy and intact
  • Modulating the innate and adaptive immune systems

As you’ll see in the following sections, these effects end up making a big impact on restoring the microbiome to help prevent and fight breast cancer. But, while this article is focused on breast cancer, many of these compounds have benefits for all types of cancer!

Fruits & Vegetables 

It almost becomes tiresome to constantly hear about the health benefits of fruits and vegetables. How big of an impact can they really have on your health? Well, it turns out—quite a big one! A lot of research has been done over the years in regard to fruits, veggies, and breast cancer, and the results are striking. Not only does your consumption of these natural foods correlate to breast cancer prevention, but there is sufficient evidence that shows how these substances can inhibit the proliferation and promote apoptosis (cell death) of breast cancer cells (2).

One study showed that women who ate a plethora of raw vegetables and salads were 34% less likely to develop breast cancer (3). That’s a significant risk reduction in itself! But in digging deeper, we’ve been able to isolate three phytochemical families that really pack a punch against breast cancer: glucosinolates, carotenoids, and polyphenols.

Diagram showing the relationship between the gut and phytochemicals

Figure 1. Complex relationships between the intake of phytochemicals, fibers (soluble and insoluble), and probiotics that help prevent breast cancer growth and metastasis through a variety of mechanisms (Licensed image created with

n the previous blog article, we briefly mentioned glucosinolates, plant-based cancer-fighting chemicals that need a healthy gut in order to work. These compounds are specifically found in cruciferous vegetables, such as broccoli, cauliflower, kale, Brussels sprouts, and cabbage. They’re also found in more pungent foods such as mustard, horseradish, and rapeseed. Not only do these compounds induce cell death in cancer cells, but they also appear to aid and improve the effectiveness of popular breast cancer drugs (4).

The next phytochemical family is carotenoids, which include lutein, lycopene, and beta-carotene. First, lutein is a type of carotenoid that comes from the Latin word “lutea,” which means “yellow.” Perhaps not surprisingly then, yellow corn has the highest amount of lutein (over 60%!). But this compound is also found in sweet peppers, peas, egg yolk, spinach, zucchini, and squash. There have been numerous studies regarding the positive effects of lutein on cancer, and breast cancer specifically. One study showed that lutein decreased premenopausal breast cancer risk by a whopping 53%! (5) In another study, it was even shown to induce apoptosis (cell death) in breast cancer cells while keeping healthy cells safe and intact (6). These positive effects seem to be due to lutein’s antioxidant properties and ability to eliminate free radicals.

The second carotenoid is lycopene. Responsible for the deep-red color of tomatoes, this compound is also found in vegetables like cabbage, carrots, and asparagus and fruits like apricots, guava, papaya, and watermelon. Lycopene has a distinct chemical structure that allows it to function as a potent antioxidant and eliminator of free radicals. In the lab, lycopene has been shown to prevent breast cancer cell growth. It also decreases insulin-growth-factor, another contributor to breast cancer (7). In addition, lycopene also has anti-inflammatory properties and can improve communication between cells!

You may have heard of beta-carotene, the third type of carotenoid. The highest sources are found in green leafy vegetables and yellow and orange vegetables—pumpkin, squash, cantaloupe, sweet potato, and carrots, to name a few. Beta-carotene, especially in combination with lutein, eliminates free radicals floating around the body. In one study, individuals with the lowest levels of beta-carotene were twice as likely to develop breast cancer compared to those with the highest levels! (8) In other studies, beta-carotene inhibited cancer cells from proliferating and increased cancer cell death (9). Interestingly, the anti-cancer effects are only seen when eating a diet rich in beta-carotene—taking a supplement with beta-carotene doesn’t appear to be beneficial and may even be harmful to certain people (particularly smokers) (7).

A diagram showing the different types foods that provide phytochemicals

Figure 2. Major classes of phytochemicals with some examples of food sources (Licensed image created with

While these carotenoids show incredible benefits on their own, there’s also quite a bit of research that shows they work best together (10). Roasted carrots, for example, are a healthy side dish that may exert some anti-cancer effects—but a spinach salad with carrots, pumpkin, peppers, and zucchini is even better! So diversify your carotenoids and get creative—experiment with salads and stir-fry, smoothies, and juices. In terms of carotenoids, less is not more!

The next family of phytochemicals is polyphenols. Polyphenols include flavonoids, lignans, and stilbenes. The first, flavonoids, are anti-viral, anti-fungal, anti-bacterial, anti-inflammatory, and antioxidants! They are found in green and black tea, blackberries, blueberries, dark chocolate, red wine, kale, almonds, and apples (it turns out there might be something to the saying, “an apple a day keeps the doctor away”). These compounds have been shown to prevent all types of cancer through complex mechanisms such as slowing cancer cell growth, inducing apoptosis, and preventing cancer-contributing inflammation (7).

Flavonoid-rich berries have been extensively studied in regard to breast cancer. These include blackberries, strawberries, blueberries, cranberries, and even grapes. Hundreds of studies have shown the anti-cancer effects of berry extracts in cancer cell cultures and laboratory animals. Berry consumption significantly slowed tumor progression in mice with breast cancer, and in another study, berries prevented both the initial development of breast cancer and the recurrence of breast cancer (11, 12).

A type of flavonoid you may have already heard of is called EGCG (epigallocatechin-3-gallate). This compound is found in large amounts in green tea and smaller amounts in black and oolong tea. EGCG appears to elicit dramatic effects against all types of cancer, breast cancer included. Multiple studies have found that the greater the consumption of green tea, the less the risk of developing breast cancer. It also prevents the recurrence of breast cancer amongst those in remission(13). These effects may be due to EGCG’s ability to slow the growth of cancer cells, which subsequently makes the development and spread of cancer very difficult! (14)

We briefly mentioned lignans, the second type of polyphenol, in the previous blog article—anti-cancer plant compounds that rely on a healthy microbiome to elicit their effects. Lignans are found abundantly in seeds and specific vegetables. Foods like flaxseeds, sesame seeds, pumpkin seeds, peppers, carrots, spinach, and cruciferous vegetables have the highest amounts of lignans. Lignans are considered “phytoestrogens,” or plant compounds that regulate the levels of active estrogen in the body. For this reason, lignans are wonderful warriors against breast cancer—and your microbiome loves them! (15)

High lignan intake reduces the risk of developing breast cancer. This effect is seen in multiple studies that show consuming a lot of flaxseed or other lignan-rich foods equates to a lower risk of developing breast cancer (16). But lignans also elicit positive effects in those with active breast cancer. In one study, high lignan intake significantly reduced mortality in breast cancer patients (17). These are powerful findings for foods that can easily be found at the nearest grocery store!

The final type of polyphenol is stilbenes, but you may have heard of its most well-known member, resveratrol. Resveratrol is found in grapes, wine, grape juice, peanuts, pistachios, cocoa, blueberries, and cranberries, and it elicits positive effects at nearly every stage of cancer development. In studies of cancer cells in a lab, this compound was found to induce apoptosis in malignant cells (18). In mice, it significantly slowed the growth and reduced the size of tumors (19). Resveratrol also kills breast cancer cells known to be treatment-resistant, such as triple-positive MCF-7 breast cancer cells (20). Yet another positive effect is its ability to enhance the effect of popular breast-cancer chemotherapy drugs (21).


We would be remiss to discuss ways to boost your microbiome and fight cancer without mentioning fiber. First, fiber: the part of plant-based food that can’t be broken down completely by the gut. Based on this definition, you may be thinking fiber is useless. After all, if it can’t be digested, what good is it? Well, the microbiome lives and thrives on fiber. When you consume fiber, the beneficial bacteria in your gut feast on it. This fermentation process stimulates their growth and leads to the production of healthy byproducts.

Consider the fact that only 1 in 20 people in the US consumes the recommended amount of fiber every day (22). This statistic means that a whopping 19 out of 20 people are essentially starving their microbiome, thus increasing their risk of developing cancer and other chronic diseases!

General recommendations suggest consuming 25-30 g of fiber daily. Ideally, your fiber should come from food, not supplements. There’s good reason for these recommendations— studies show that women who eat more than 30 g of fiber, fruits, and seeds have a significantly reduced risk of breast cancer (23). Some of the highest-fiber foods are beans, lentils, broccoli, popcorn, avocado, apples, and dried fruits. But because all fruits and vegetables contain fiber, you don’t have to be too picky.

So how does fiber elicit such prominent anti-cancer effects? First, it has to do with the specific bacteria populations that thrive in response to fiber. Studies show that increasing plant fiber increases the population of certain healthy bacteria, specifically Bifidobacterium and Faecalibacterium prausnitzii (24). What’s more, these two specific bacteria have been shown to have anti-inflammatory and anti-tumor effects (25).

The second reason fiber elicits anti-cancer effects is due to the production of those “healthy byproducts” mentioned above. As the microbiome snacks on the fiber, it ferments and produces something called short-chain fatty acids (SCFAs). These SCFAs seem to play an anti-cancer role, as in one study that showed the ability of these compounds to prevent metastasis in breast cancer (26).

One type of SCFA that continues to appear in research as a seriously potent anti-cancer compound is called butyrate. Not only does it play a role in the activation and production of immune cells and cytokines, but it also selectively targets and kills cancer cells while keeping healthy cells intact (27, 28).

Another positive function of butyrate and other SCFAs is their ability to decrease the incidence of leaky gut syndrome. Normally, the cells that line your gut sit shoulder to shoulder, preventing any food compounds from leaking into the bloodstream. Leaky gut syndrome, however, is when that gut barrier becomes permeable and those food compounds wreak havoc on the rest of the body. A leaky gut has been shown to contribute to both the development and recurrence of breast cancer, so this condition is extremely important to prevent! (29)


The last and perhaps most obvious step to harnessing the microbiome against breast cancer is to increase the amount and number of healthy bacteria you consume. Probiotics can be taken as a supplement or through the consumption of probiotic-rich foods. Although getting probiotics from food is the better option, it’s not always possible to get enough healthy bacteria from food, in which case a supplement can be very helpful.

Arguably the best probiotic-rich food that exists is kefir, a fermented milk drink with both healthy bacteria and yeast. Kefir comes from the Turkish word for “good feeling,” and it has tons of health benefits; it’s packed with protein, calcium, vitamins, and minerals, and it even appears to play a direct role in preventing breast cancer! One study showed that kefir decreased the number of breast cancer cells by 56%! (30)

You’ve certainly heard of the microbiome-nourishing probiotics present in yogurt, and it’s true—yogurt is great for the gut. But there are many yogurt options out there that don’t help the microbiome and may do more harm than good. For this reason, it’s important to read the labels to ensure that the good bacteria haven’t been killed in processing. To make sure the bacteria are alive, look for “active” or “live” cultures on the packaging. Additionally, try to consume yogurt that does not have any added sugar. Avoid yogurt where sugar is the first ingredient, and aim for less than 13 g of sugar per 5.3 oz container.

Fermented cabbage is also excellent for the microbiome. Different regions of the world have different variations of this food; Eastern Europe has sauerkraut, and Asian countries have kimchi. As with yogurt, it’s important to ensure the bacteria are alive. Two tips for doing this are to look for unpasteurized and refrigerated products. Since heat kills healthy bacteria, we want to avoid pasteurized, canned, and non-refrigerated products. Also, when reading the ingredients, the best kimchi or sauerkraut should NOT contain vinegar, sugar, sodium benzoate, or sodium bisulfate. If it contains a “starter culture,” that’s okay—that just means there are more helpful bacteria.

The research is still evolving in determining which probiotics taken as supplements are effective against breast cancer. The specific bacterial strains, the dosage, and the frequency of administration are still being determined, but the research that does exist is quite promising. One study showed that consuming probiotics that contained Enterococcus faecalis and Staphylococcus hominis prevented growth and induced cell death in breast cancer cells (31). Another study showed that the probiotic Lactobacillus reuteri increased white blood cells and made breast cancer cells more susceptible to apoptosis (32) Additionally, probiotics containing Bifidobacterium can lessen the side effects of traditional cancer treatments, such as chemotherapy-related cognitive impairment (33).

Before selecting a probiotic, it’s important to talk to your healthcare provider. Certain bacteria can increase the toxicity of medications, and others may not be appropriate for those who are immunocompromised or at high risk of infection. Some research shows probiotics are safe for those with depressed immune systems, but others show the possibility of causing infection. Your health provider should be able to recommend the best probiotic combination for your condition.

When shopping for a probiotic, there are a few important things to look for and consider:

  • Seek probiotics with more than 1 billion cultures or colony-forming units (CFUs).
  • Make sure the label says the cultures are “live” or “active.”
  • Ensure the expiration date is far in the future since the bacteria decline or die over time.

Tying it All Together

We weren’t kidding when we said there are tons of breast cancer-fighting foods and phytochemicals! Unfortunately, these research-backed tools aren’t as well-known as they should be because they’re buried under a mountain of medical journals and complex language. But by using this guide, you can take your nutritional journey into your own hands—explore foods rich in carotenoids and polyphenols, overhaul your diet, crank up the fiber, and get creative with recipes. Most importantly, take a deep breath and have hope that healing is possible. Food can be your enemy or your ally in your fight against cancer—it’s never too late to start nourishing your gut!


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2 thoughts on “Breast Cancer and the Microbiome: Part 2

  1. I enjoyed the article and dietary helps
    .I am 76 and have no family to care for me so can’t take anything that would deminish my ability to care for myself.

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