Can Your Gut Microbiome Protect Against Breast Cancer?

The gut is full of bugs. All these bugs make up the gut microbiome, which has been linked to chronic diseases and overall health. This gut-health connection has led to countless products promising to optimize your gut. There are probiotics in soda, at-home kits to help you “diagnose” your gut, and supplements that promise to “heal” it.  Some people even say that having a healthy gut may protect against — or “detox” — breast cancer.  While most of these claims are overblown, early research in animals and cells suggests the gut does play an important role in human health.  

The gut uses trillions of bacteria, fungi, and viruses to digest food and protect against infection. These microbes make up the gut microbiome. In the early 2000s, these microbes were still a mystery to scientists. Then, the U.S. National Institutes of Health (NIH) launched a project in 2007 called the Human Microbiome Project. The project funded research, totaling $170 million, into understanding the human microbiome. 

 Within a decade, scientists knew what microbes were in the gut, skin, and vagina and how the microbiome is linked to irritable bowel syndrome, prediabetes, and pre-term birth. In the process, they discovered that the gut microbiome is made up of a diverse community of bugs that can perform important gut tasks. 

If this community is disturbed — by diet, environment, lifestyle, disease, or medicine — the balance of good and bad bacteria is thrown off. Researchers have linked this disturbance, known as dysbiosis, with chronic diseases like cancer. 

Since the Human Microbiome Project began, research into the microbiome and human health has taken off, says Katherine Cook, PhD, a cancer biology professor at Wake Forest University. Cook is interested in how the gut microbiome influences breast cancer development and spread. This is a relatively new scientific area, so researchers have mostly done studies in cells and animals so far. It’s still too early to know how exactly the gut microbiome relates to human diseases like breast cancer, but scientists are starting to develop some theories. 

Early research suggests that gut microbes may play a role in many key functions in the body that may influence breast cancer risk. 

Chronic inflammation 

The immune system is how our body fights diseases. Most immune system cells live in the gut and are constantly communicating with the gut microbiome. Certain microbes control inflammation, the body’s natural response to infection or injury. Some microbes trigger inflammation while others are anti-inflammatory. 

If the gut microbiome and the immune system are always under stress, it can lead to chronic inflammation, a known risk factor for several types of cancer. Chronic inflammation speeds up cell growth, which may accelerate the development of cancer or help tumors grow and metastasize. Researchers have found signs of inflammation in the breast tissues of women who have breast cancer and are obese. 

Obesity, a well-known risk factor for breast cancer, is also a key driver of chronic inflammation and has been linked to changes in the gut microbiome. Scientists aren’t sure yet if the gut microbiome plays an independent role in breast cancer development, or if it’s linked only by the effects of obesity.  

“Just about everything can influence your gut, and everybody’s gut microbiome is different.”

Circulating estrogen

Another theory of how the microbiome may influence breast cancer risk is based on how the gut processes estrogen. Researchers have linked excess estrogen in the body to the beginning of estrogen receptor-positive breast cancer.

Some gut bacteria get rid of estrogen, while others send it back into the body. Early research suggests that when the gut microbiome is thrown off, there’s an increase in this second batch of bacteria, leading to an increase in estrogen. 

“The gut could control these levels of hormones or potentially the drugs used to treat [estrogen receptor]-positive breast cancer,” Cook says, since gut bacteria also control how drugs are excreted. 

Short-chain fatty acids 

Gut microbes use food we eat to make compounds that are beneficial for our health, including a class of compounds called short-chain fatty acids. These acids are anti-inflammatory and strengthen the gut barrier, one line of defense against disease. Gut microbes convert dietary fiber, rich in foods like beans, broccoli, and oatmeal, into these useful acids. 

Early research in cells reveals that some short-chain fatty acids may protect against breast cancer by halting cell growth or inducing cell death. Many studies use poop samples as a good indicator of gut microbiome health. Poop samples from people with breast cancer reveal they have less bacteria that make short-chain fatty acids and more estrogen-increasing bacteria. 

The breast microbiome and the gut

In 2014, researchers made a surprising discovery: breasts have their own  unique microbiome. Later research revealed breast cancer tumors may be linked to changes in the breast microbiome. 

Cook has investigated what changes the breast microbiome. In animal studies, she found an association between a high-fat diet and negatively altered gut and breast microbiomes, influencing breast tumor risk. Gut leakiness, which releases harmful bacteria into the rest of the body due to a weakened gut barrier, may  also change the breast microbiome.  A weakened gut barrier is common when the gut is disturbed. While research is still early, Cook says the breast microbiome may one day be shown to be a novel risk factor for breast cancer. 

Diet is one of the best ways to keep a healthy gut and ensure your gut microbiome is making good, anti-inflammatory products like short-chain fatty acids. Cook says the easiest place to start is by eating more fiber and incorporating more fermented foods into your diet — like kimchi, sauerkraut, and kefir — which help keep a healthy balance of microbes in your gut. 

Americans generally don’t eat enough fiber, which Cook says offers an opportunity for intervention: “We can use diet to affect the gut microbiome and the compounds they produce to improve health.” A healthy diet is known to reduce the risk for many diseases in ways independent of the gut microbiome.

It’s tempting to point to one food, supplement, or intervention you can try to reap the good, anti-cancer benefits of the gut while avoiding the bad. But Cook says it isn’t that simple. “Our diet is one of the most impactful and well-studied factors that influence our microbiome,” she says, but it’s not the only one.

Your gut microbiome is influenced by the world around you. It’s shaped by where you live, how much you exercise, and what medications you take. It can also change as you age. Extensive research has shown that early life factors — like vaginal versus C-section birth and breast- versus bottle-feeding — can affect an infant's microbiome. Gut imbalances in early life may be linked to diseases in adulthood. “Just about everything can influence your gut,” Cook says, and “everybody’s gut microbiome is different.” 

The bottom line is that while the gut microbiome is important for overall health, scientists don’t yet know how or if it’s linked to breast cancer, since there haven’t been a lot of human studies yet. It’s too early — and the research is too inconsistent — to scientifically back any clinical test or personalized “gut-optimizing” products that promise to prevent cancer or other diseases. The best way to protect yourself against breast cancer is through a healthy diet and exercise.