So might microbes be affecting our weight, or even our brains? That sounds a bit sci-fi.
When it comes to obesity, there are several ways gut microbes might influence matters, including through appetite, production of gases, efficiency of using food, and impact on the immune system and inflammation.
When it comes to affecting mood, there are also several mechanisms. One is via the vagus nerve, a two-way highway that runs from our brain to various organs in the body, including the gut.
With the microbiome linked to so many conditions, does tinkering with it promise a whole range of new treatments?
It is worth being cautious: many studies show associations rather than cause and effect, and some are based only on studies in germ-free mice and have not been explored in humans. Even in mice things aren’t straightforward – effects are
not always the same for both sexes and can differ for different strains of mice.
And there are other factors to consider: “For obesity what it looks like is in different human populations, different kinds of microbes are involved in the differences between lean and obese humans,” said Knight.
Spector said: “I think everyone is right to be sceptical, and a lot of the links may just be that [microbes] are not necessarily the cause of [a disease], but they might be a secondary effect of it.”
Others say it isn’t surprising that our microbiome might be closely linked to our health. “All of human development and all the systems in the body have all evolved, or co-evolved, with our microbes,” said Cryan. “As humans we are very much human-focused and we feel that human cells and genes have primacy, but the microbes were there first.”
Pass the poo Does any of this actually affect patients?
Up to a point. The field has already led to advances in the treatment of
C difficile – an infection that causes serious diarrhoea and can prove deadly. Patients can now receive faecal transplants from a donor with a healthy microbiome to “reset” their inner community – a procedure that has been shown to rapidly cure the condition.
Some researchers, including Cryan, believe microbiome research could lead to the development of new mental health therapies. “We have coined the term ‘psychobiotic’ [by which we mean] a targeted intervention of the microbiome for brain health,” he said.
While that may be some way off, Cryan believes it will become routine for doctors to keep an eye on the makeup of patients’ microbiomes. “I think personally that bacteria- or microbiome-derived medicine is the future of precision medicine,” he said.
Let’s cut to the chase: what can I do to keep my microbiome in good shape?
This is where
prebiotics and probiotics come in: the former are substances, such as the fibre inulin, on which useful microbes can thrive, while the latter are microbes themselves that are thought to be beneficial for health, such as the Lactobacillus and Bifidobacterium species.
While both prebiotics and probiotics can be taken as supplements, whether you should shell out for them is another matter: there is little advice on which prebiotics or probiotics people should consume for a particular situation, and when it comes to probiotics it isn’t a dead cert that the microbes will colonise your gut when they get there, or
if they will offer benefits to already healthy people, such as preventing diseases. That said, if you are taking antibiotics or have IBS, there is some evidence probiotics might be a good idea.
“It is not clear yet whether you’re better off just having lots of yoghurt and other fermented foods or actually taking these formulations,” said Spector, adding that in general he recommends opting for tweaking your diet to get a dose of probiotics, since it isn’t clear which strains individuals should take. The same goes for prebiotics: “there is more variety in food in terms of the fibre, therefore more variety in the microbes,” he said. “Ideally you combine a prebiotic and a probiotic: something like sauerkraut or kimchi.”
The spotlight is on unpicking the mechanisms by which microbes are linked to human health. Among the conundrums is how and why the different strains of bacteria have different effects, while researchers are also developing studies to explore how the microbiome influences our response to food, and how different diets can tweak the microbiome. There is also a need to take more of the exciting findings from mouse studies and probe them in humans, preferably through randomised control trials.
Further reading: I Contain Multitudes, by Ed Yong Gut: The Inside Story of Our Body’s Most Underrated Organ, by Giulia Enders The Psychobiotic Revolution, by Scott C Anderson with John Cryan and Ted Dinan Follow Your Gut: How the Ecosystem in Your Gut Determines Your Health, Mood, and More, by Rob Knight
Illustrations: Pete Gamlen