Scientists are uncovering links between the microbes in our intestines and breast cancer, from how hormones are recycled to how patients respond to treatment. But, experts caution, a link is not a cause.
The human gut is home to trillions of microorganisms, and their work extends far beyond digestion. They break down food, communicate to the immune system, regulate inflammation and even influence how hormones circulate through the body.
Now researchers are asking a more provocative question: Could the bacteria in our intestines have something to do with breast cancer?
The findings demand careful reading. Researchers have found differences in the gut bacteria of some people with breast cancer, along with mechanisms that could plausibly influence the disease. What they have not shown is that an unhealthy gut causes breast cancer, or that changing the microbiome can prevent it.
To separate the known from the speculative, we spoke to Dr Purna Kashyap, director of the Mayo Clinic Microbiome Program, and Dr Soha Abdelbaky, consultant in medical oncology at Medcare Hospital Sharjah.
The oestrogen loop
Oestrogen is essential to female reproduction. It is also the fuel for many breast cancers, particularly those classed as oestrogen receptor-positive.
Scientists call the collection of gut bacteria that can process oestrogen the “estrobolome”. A 2016 review in the Journal of the National Cancer Institute, led by researchers at New York University, set out the theory.
A gut rich in bacteria that make beta-glucuronidase, the enzyme that removes oestrogen’s disposal stamp, could raise a woman’s circulating oestrogen. Diet appears to matter: the enzyme seems more active on high-fat diets and less so on fibre-rich ones. Antibiotics, alcohol and body fat may also shape the estrobolome. Crucially, the authors presented this as a hypothesis, not a proven cause.
So, what does it all mean?
“Some gut bacteria can increase the amount of estrogen that is reabsorbed into the bloodstream. This may lead to higher estrogen levels, which could increase the risk of developing breast cancer,” says Dr Kashyap, who is also the Bernard and Edith Waterman Director of the Microbiomics Program at the Mayo Clinic Center for Individualized Medicine.
Other bacterial products seem to push the opposite way. Short-chain fatty acids and lithocholic acid, both made by gut microbes, are found at lower levels in breast cancer patients, he says. They may help restrain tumour growth and spread while bolstering the body’s anti-cancer immune response.
Dr Abdelbaky widens the lens. The microbiome also trains the immune system, governs inflammation and regulates metabolism, so any shift could ripple through processes that matter in cancer.
Yet, the link, is not the cause. “We cannot yet establish a direct cause-and-effect relationship between gut health and breast cancer, and much more evidence is needed before these findings can be translated into specific clinical recommendations,” she says.
A puzzle in younger women
One of the most intriguing threads concerns breast cancer diagnosed at a younger age. Dr Kashyap was senior author of a study, published in the journal Cell, that examined gut microbiome patterns across five groups of cancers.
"In our study, we found that patients with early-onset breast cancer had a distinct gut microbiome and lower levels of primary bile acids," he says.
Bile acids help the body digest fat, but they also act as signals that influence metabolism. Gut bacteria can chemically transform them, potentially changing their effects elsewhere in the body.
The team also singled out one microbe, Clostridium scindens. "One particularly interesting bacterium was C. scindens, which, in addition to its role in bile acid metabolism, can process hormones such as androgens that may have protective effects in some forms of breast cancer," Dr Kashyap explains.
Levels of C. scindens were lower in patients with early-onset disease. That does not mean its absence causes cancer, or that boosting it would offer protection.
Dr Abdelbaky notes that breast cancer in younger women is biologically complex: hormonal status, life stage and tumour characteristics can all differ from those of older patients. "However, we should be careful not to assume that microbiome differences explain why breast cancer develops at a younger age. This remains an area of investigation, rather than an established clinical finding."
Signals from the gut
How can bacteria in the intestine reach as far as the breast? Through chemistry.
"The bacteria in our gut are constantly processing what we eat and producing small molecules that can enter the body and act as chemical signals," says Dr Kashyap.
Those molecules can tune immunity, metabolism and inflammation. Some reinforce the gut wall; others inflame tissue or even damage DNA. Short-chain fatty acids, released when microbes ferment fibre, are among the most closely watched: Dr Abdelbaky explains that they support the gut lining and help keep immune responses in check.
When the system is disrupted, chronic inflammation and metabolic change may follow, though their exact role in breast cancer is unclear. And no molecule is simply hero or villain. As Dr Kashyap says, much depends on the compound and the setting in which it acts.
A hand in treatment
The gut microbiome can definitely affect both how well cancer treatments work and what side effects a person experiences, as Dr Kashyap notes.
Some bacterial enzymes can disarm cancer drugs, or reactivate them inside the gut, potentially contributing to diarrhoea and mucositis, an inflammation of the digestive tract's lining.
Immunotherapy is a particular focus. As the gut is in constant dialogue with immune cells, researchers are testing whether certain microbial communities help patients respond better to drugs that turn the immune system on cancer. Dr Abdelbaky adds that links between microbial diversity and chemotherapy response are also under study, as is its role in side effects such as nausea and diarrhoea.
None of this makes probiotics or diet a proven way to improve results. Microbiome-based therapies, Dr Kashyap stresses, are an active area of research, not yet routine cancer care. Dr Abdelbaky is equally firm: Gut health is no substitute for established oncology, and patients on chemotherapy or immunotherapy, who may have particular nutritional needs or weakened immunity, need individual medical advice.
The unglamorous truth about a healthy gut
The microbiome has become a wellness obsession sold in capsules and powders. The reality is plainer.
Some of the biggest everyday influences, says Dr Kashyap, are diet, medications and transit time, the speed at which food moves through the gut. The amount and variety of fibre-rich plant food you eat can reshape microbial communities. Dr Abdelbaky adds that diets low in fibre and heavy in ultra-processed food can narrow the range of beneficial bacteria. So can inactivity, poor sleep, smoking and needless antibiotics.
There is no perfect microbiome to chase. “There is no single ideal composition that applies to every individual,” she says. The aim is habits that support long-term digestive and metabolic health.
Thirty plants a week
If the science has one practical message, it is variety, because different plants feed different microbes. Dr Abdelbaky recommends vegetables, fruits, wholegrains, lentils, beans, chickpeas, nuts and seeds, which are rich in the prebiotic fibres gut bacteria thrive on. Where suitable, these can sit alongside fermented foods such as live plain yoghurt, kefir and traditionally fermented vegetables.
“Rather than focusing on one particular superfood or supplement, I would encourage women to aim for variety and consistency,” she says.
Dr Kashyap puts a number on it: “As a practical goal, you can aim for around 30 different plant foods each week, including fruits, vegetables, whole grains, legumes, nuts and seeds.”
For women in treatment, diet may need adjusting around appetite, symptoms, therapy and immune status.
The hype gap
The microbiome boom has filled shelves with products promising to restore bacterial balance. On breast cancer, the evidence does not back them.
“While probiotics and a healthy gut microbiome are receiving considerable research attention, there is currently no conclusive evidence that probiotic supplements can prevent breast cancer or replace established cancer treatments,” says Dr Abdelbaky.
Strains differ, effects vary between people, and not every fermented food contains probiotics. For patients with weakened immunity, some supplements may be unsuitable, so a doctor’s advice comes first.
Antibiotics raise a different question. They can strip out beneficial bacteria, and studies have linked antibiotic exposure to treatment outcomes. Dr Abdelbaky notes that this falls short of proving cause. Both experts are adamant that necessary antibiotics must never be skipped. “Antibiotics can affect the gut microbiome, but they are also life-saving medications and should be taken when clinically indicated,” says Dr Kashyap. For cancer patients, treating infection promptly can be especially important.
What it means for now
Microbiome science is opening new windows on cancer biology. But spotting a microbial pattern is a long way from a reliable clinical tool.
There is no established gut test that can predict an individual’s breast cancer risk, and no proven probiotic that prevents the disease. A varied, fibre-rich diet, regular exercise, good sleep and sensible use of medication are worthwhile for overall health. They do not replace screening, knowing your risk factors and getting any breast change checked promptly.
Meanwhile, deep in the gut, the bacteria are still at work on that discarded oestrogen. The real promise of research lies in what that hidden world may one day teach us about cancer, and how to treat it.