London - The idea of "gut health" barely existed a few years ago, but now, people spend around £750-million every year on probiotic products - "beneficial" bacteria which is supposed to boost our health.
People take probiotics believing they increase levels of beneficial bacteria and will boost their immune system, digestion, mental health and more. But last week, an expert warned that fermented foods such as sauerkraut - often consumed because of their probiotic effect - may trigger bloating, headaches and allergies.
Meanwhile, increasing evidence suggests that, in some circumstances, probiotics might harm the delicate eco-system of the gut.
A study published this year, from the Parker Institute for Cancer Immunotherapy in San Francisco and The University of Texas MD Anderson Cancer Center, found patients with melanoma, the most serious form of skin cancer, who took a probiotic supplement had a 70 percent lower chance of responding to immunotherapy, a treatment that uses a patient’s own immune system to fight off the disease.
Researchers also found taking probiotic supplements was linked to lower diversity in the microbiome, which is composed of communities of bacteria in the gut. A lower diversity was previously found to be associated with poorer immunotherapy response.
This is particularly alarming as the type of immunotherapy used - anti-PD-1 checkpoint inhibitors - only works for 20 to 30 percent of cancer patients, and yet 42 percent of those in the study were taking a probiotic supplement.
Dr Christine Spencer, a research scientist and one of the study authors, advised cancer patients and doctors to "carefully consider the use of probiotic supplements, especially before beginning immunotherapy treatment".
So how might probiotics interfere with treatment? "There are usually around 200 to 300 different species of bacteria in the average human gut," says Dr Caroline Le Roy, a researcher and expert on the microbiome at King’s College London.
"Usually, the more diverse the microbiome, the better for your health."
But she says a type of bacteria that’s "healthy" for one condition may be harmful for another. "A strain that might help your metabolism, may have little or no effect on your immune response."
A 2018 study at MD Anderson found patients on anti-PD-1 checkpoint inhibitors with more of a specific bacteria, Bacteroidales, had faster advancing disease and lower survival rates.
Bacteroidales appear to be linked to metabolism and a lower risk of obesity, so in some people could be considered "healthy".
"Introducing a single strain via a supplement could be an issue, as what most people should be doing is feeding the hundreds of diverse bacteria in your gut, as this means there is a greater likelihood of the right strain being there when you need it," says Dr Le Roy.
And the potentially harmful effects of probiotics may not be limited to people with cancer. Dr Le Roy says: "We know the effectiveness of common drugs, such as metformin for diabetes, depends partly on its effects on the gut microbiome.
"Some probiotics that contain the bugs affected by the treatment may be good and improve responses to treatment. But if you have too much of one single strain, you are reducing space for others to thrive."
Many people take probiotics after antibiotics in the belief it will rebuild bacteria killed off by the treatment. However, a study at the Weizmann Institute of Science in Israel, published this year in the journal Cell, found that taking the supplements, in fact, "markedly delayed" the recovery process.
Researchers gave 21 people broad spectrum antibiotics for a week. Participants then either had no intervention, a common probiotic for a month, or a faecal transplant of their own stool taken before antibiotic treatment.
The probiotic group was the slowest to recover and, even five months after taking antibiotics, their gut was still not normal.
However it’s not all bad news. The National Institute for Health and Care Excellence has approved use of probiotics in conjunction with antibiotics to treat C. difficile infection, which can cause severe diarrhoea. There is also promising research into probiotics for conditions including irritable bowel syndrome, depression and anxiety, obesity and eczema.
And the Parker Institute is now testing a microbiome pill containing bacteria that mimics the gut composition of those who best responded to immunotherapy.
So, what does this all mean for probiotic devotees? "Take several in combination, or ones that contain multiple strains," says Dr Le Roy. And, if in doubt, she says, eat a varied diet.