This is i’s science newsletter with Clare Wilson, a subscriber-only newsletter from i. If you’d like to get this direct to your inbox, every single week, you can sign up here.
Hello and welcome back to i‘s science newsletter. I’m Clare Wilson, i‘s new Science Writer, and I’ll be taking over from this week.
I’m starting my first newsletter with a topic I’ve been waiting to get my teeth into: the current obsession with our gut bacteria.
Whether it’s TV ads, best-selling cookery books or TikTok influencers, it feels like these days we are constantly being encouraged to nurture and monitor the microbe ecosystem living deep in our bowels – even though, until recently, most of us might not have given this subject much thought.
To keep our microbial residents happy, growing numbers of us are eating special foods, buying pricy supplements or even holding our noses and collecting a sample of faeces to send off for analysis in a lab.
But speak to practising microbiologists and you might be surprised at just how sketchy the supporting scientific evidence is.
As there have been several recent studies questioning how much faith we should place in microbiome science, I thought it was time to take a deep dive into which bacteria-based health advice is worth following – and which is just a modern-day form of snake oil.
Bewildering array of consumer products
Doctors have of course known for decades that while certain infectious diseases are caused by harmful bacteria, our guts also contain trillions of so-called “good bacteria”, which help digest our food. This idea came to public attention in the 90s, mainly thanks to TV advertising campaigns by the manufacturers of yogurt drinks containing live cultures of these bugs.
Since then, advances in DNA sequencing made it easier and cheaper to identify the bacteria in a stool sample, and that led to an explosion of research into the influence of the microbiome on all aspects of our health.
There have been claims that gut bacteria disturbances are behind a range of diseases not previously connected with digestive health – including cancer, obesity and even mental health problems. They have raised hopes of treating such diseases in novel ways, for instance with faecal transplants – where stool is transferred from one person to another – or medicines that boost beneficial bacteria.
The field has also led to a bewildering array of consumer products claimed to adjust our microbiome, whether in the form of probiotics – the bacteria themselves – or prebiotics, a term for food supplements that supposedly nurture our good bacteria. For good measure, you can also now buy “postbiotics” – these are compounds thought to be produced by good bacteria after they have digested the prebiotics.
If personal health monitoring is your thing, some firms also sell microbiome testing kits, where you post off a sample of your faeces to find out if you have a good gut bacteria score. Many such firms then advise you to improve your results by buying their pre-, pro- and postbiotic supplements – which seems like something of a conflict of interest.
If you think you can detect a note of cynicism creeping into my voice, you’d be right.
That’s because the field of microbiome research is not nearly as settled as the companies and influencers would have you believe.
Research still in infancy
Most of the studies claiming that a certain health condition is linked with having the wrong gut bacteria have just shown correlations between the disease and the bacteria. They haven’t proven that the bad bacteria cause the condition.
That means it’s quite possible that the effect is happening the opposite way round – that having a certain medical condition changes someone’s gut bacteria. In which case, trying to improve your microbiome would be pointless.
More worryingly, even the claims of correlations seem to be on shaky ground, as studies by different groups regularly contradict each other, according to an article published last year in Nature Microbiology by Professor Lesley Hoyles at Nottingham Trent University.
In June, for instance, an influential paper that had claimed we could diagnose someone with cancer based on bacteria in their blood had to be withdrawn, after other researchers found problems with the methods.
One reason for the confusion is that microbiome researchis still in its infancy. So different scientists around the world are using different methods, different statistical tools, different chemical reagents, and so on. It’s unsurprising they sometimes get different answers.
Another issue is that the science is so complex. People may have a thousand different species of bacteria in their gut, not to mention other microorganisms like viruses and fungi, many of which have only just been discovered.
Just this month, scientists in California announced that the results you get from analysing a faeces sample probably changes depending on whether it was the morning or evening that they …er…obtained the sample.
Although it was based on work in mice, they concluded that if the same results are seen in people, the first faeces produced after the usual overnight fast will have different bacteria levels from those obtained after a long day’s eating. “It’s literally that what you put in has an impact,” explained Professor Hoyles.
If confirmed in human studies, this would make all previous studies that have ignored the time of sample collection suddenly look less trustworthy.
These kinds of reliability problems are not unique to microbiome science but the field does seem especially prone to them.
What should consumers do?
Researchers are making progress at standardising their methods, but in the meantime, where does this leave people wanting to use the science of gut bacteria to improve their health?
The first piece of advice that many microbiologists would give to the public is that we should lower our expectations. For most people there is not going to be any off-the-shelf supplement or probiotic that will be a magic bullet to cure any particular health problem.
Consumers should be particularly wary of faeces testing kits where people are given a score for the healthiness or otherwise of their gut bacteria, because the science is not yet ready, said Professor Diane Hoffmann at the University of Maryland.
Earlier this year, Professor Hoffmann published a report for the US government-funded National Institutes of Health, which found that test kit manufacturers are making health claims that are not supported by evidence. “We just don’t know what a healthy microbiome is yet,” she told me.
How about supplements? There is little good evidence that anyone who is generally healthy with a balanced diet needs to take probiotics, said Professor Hoyles. But if anyone does like to take them, she recommends choosing ones made by the large well-known companies, which tend to have done more research to support their products. There is some evidence that they can help shorten bouts of “traveller’s diarrhoea”, she says.
Professor Hoyles also recommended reading the labels on probiotic packets with a sceptical eye, as some firms include vitamins in their products, which lets them make health claims based on the effects of the vitamins.
A much cheaper way of nurturing your gut bacteria than buying pricey probiotics is just eating more fibre, in the form of wholemeal bread, cereals, and peas, beans and lentils. When bacteria in the colon digest fibre, they release a postbiotic called butyrate, which has multiple beneficial effects on health, including providing energy for colon cells, said Professor Hoyles.
“There’s a big body of evidence around butyrate and its importance to intestinal health – it’s one of the few metabolites where the mechanistic data are there,” said Professor Hoyles. You could say, then, that fibre is the original prebiotic.
Having said that, I’m not going to use this newsletter to give out health advice, as I’m not a doctor. Plus, everyone’s body is different, and if you have found a way of eating that suits you, I’m not going to try to dissuade you from it.
In fact, I would love to hear from you if you are a fan of probiotics – or if you’re not. And if there are any other controversial science topics you’d like me to tackle in future newsletters, please drop me a line.
Other things I’ve written recently
Another of my favourite topics in medicine is when there is genuine debate among doctors over how best to treat a particular condition. A classic example is the disagreement among medics about how intensively they should treat people with high blood pressure.
Compared with doctors in other countries, GPs in Britain tend to let people continue with higher blood pressure before prescribing medicines to lower it, especially in older people, who are more vulnerable to side effects from such drugs. Now a new study has questionned that approach.
I’m watching this week
This week I have been captivated by My Octopus Teacher, currently showing on Netflix. I’m fascinated by the question of what kinds of consciousness other animals possess, and how much of a meeting of minds there can be between humans and animals, such as, for instance, my pet cat Milo.
With their very different nervous systems to our own – octopuses arguably have nine brains, with a central one plus a small one in each arm – these creatures have minds that are even harder to fathom than those of other mammals. But over a year, film-maker Craig Foster forges a surprisingly intimate bond with a small common octopus living in a kelp forest off the coast of Cape Town, South Africa.
This is i‘s science newsletter with Clare Wilson, a subscriber-only newsletter from i. If you’d like to get this direct to your inbox, every single week, you can sign up here.