Every day we live and every meal we eat we influence the great microbial organ inside us - for better or for worse - Giulia Enders, Gut: The Inside Story of Our Body’s Most Underrated Organ
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It’s not hard to stumble across any number of articles and papers proclaiming the next wonder food to improve your gut health. As scientists increasingly discover the central role that gut bacteria play in our overall health, it’s tempting to latch on to these promises to try to revitalise everything from your weight to mental wellbeing.
But the science has a way to go before we know exactly what nutrition is best for your gut. BBC Future spoke to leading gut health and microbiome researchers to sift fact from fiction on gut health "wonder foods", probiotics, prebiotics and what changes to your diet could genuinely boost your gut health.
The interest in how to improve your gut health is so high because recent advances have begun to unpick how the microbiome affects many conditions beyond those affecting the digestive system. Studies have linked gut bacteria – known collectively as the microbiome – to changes in mood and mental health, tendency to obesity and to cardiovascular health. For people who want to maintain a healthy weight and mental health, the goal would be a way to "hack" their gut bacteria.
"The general belief is that a diverse gut microbiota is a synonym for health, since [these bacteria] are helping us produce nutrients and essential substances that our cells cannot," says Sonia Fonseca, a researcher studying the interactions between the diet, gut, microbiome and brain at the Quadram Institute. "So feeding our microbes with a diverse diet and creating a comfortable environment for them seems the right thing to do."
But finding a way to hack your microbiome might be harder than it sounds. For one thing, like much of health research, even though a study finds a link between one food and an improvement in gut bacteria, it doesn't mean that food has caused the change in the microbiome.
"Many studies are based on finding correlations, which even sometimes are contradictory, but only a few are interested in explaining causation," says Fonseca. "That is the challenging part."
While headlines may be moving faster than solid science, some clear trends are emerging on things we can do to make a real impact on gut health, says Kevin Whelan, professor of dietetics at King's College London. The majority of evidence supports that there are four main ways to do this.
The first is taking foods or supplements with probiotic bacteria in them. These are bacteria that are generally thought to be part of a healthy microbiome – particularly common ones in supplements and "live" yoghurts (meaning they contain living bacteria) are called bifidobacteria and lactobacillus.
"In general, what we know is that if you take a probiotic yoghurt with those bacteria, we know it will increase the number of those strains in your gut," says Whelan.
That might seem like a logical plan – more healthy bacteria, healthier gut. But of course, that's just the start. It's not just about quantity of those helpful bacterial strains, it's about diversity.
"There are many thousands of different types of bacteria found in people's microbiomes. Each individual person might have 150-250 types in their gut," says Whelan. "What we know is that people with diseases generally have a less diverse microbiome – so they tend to be the ones closer to 150 than 250 types."
Verdict: Adding a few strains of bacteria to your microbiome through taking a probiotic probably won't boost your gut health diversity all that much.
Just one letter different, prebiotics are a source of food for probiotic bacteria to live off, such as inulin (easily confused with but very different from insulin) or galactooligosaccharides. These molecules are often indigestible to humans, so pass straight through the gut to where the bacteria are.
"Most research on prebiotics involves giving a sachet of what is mostly carbohydrates to people that only certain strains of bacteria can use, so those strains can flourish," says Whelan.
While taking a probiotic may be like planting a seed, taking a prebiotic is like nurturing it by giving it the nourishment it needs. But again, this approach comes up against the same limitations as taking a probiotic alone.
Verdict: "Prebiotics do not increase the diversity of the microbiome," says Whelan. "They will increase specific bacteria, but they won't increase the number of different types of bacteria."
Mixing it up
So microbiome diversity is probably not achievable by swallowing a whole range of supplements. But there are ways to improve diversity by focusing on the foods you eat.
"Have a look at the people around you," says Whelan. "You'll find some people will have the same lunch every day. And in the evening, three or four different main dishes, and they will eat that for a whole year – bar going out occasionally."
Even if your habitual diet is balanced, with plenty of fruit, vegetables, whole grains and so on, having a predictable routine is not likely to do much good for a diverse microbiome.
"Dietary diversity is about challenging the concept of constantly eating the same thing," says Whelan. "For example, if you have fish regularly, make sure it isn't always salmon. Make sure you have wholegrains regularly, but not just wholegrain bread."
Verdict: Evidence for the efficacy of specific foods is always questionable, but eating a varied and diverse diet of healthy foods is likely to lead to an equally diverse and healthy microbiome.
Studies are also pointing to possible benefits from fermented foods such as kombucha and kefir. These foods, which have been made traditionally for thousands of years, have been studied in the field of "psychobiotics", which is when ingesting bacteria of a particular type has a positive mental health effect. Some studies have shown that people taking fermented milk products had lower levels of cortisol, a stress marker, in their blood compared to a placebo control group, and also had a more diverse microbiome.
While these results are promising, studies such as this are often small – with two dozen or so participants – and so larger clinical trials are needed before we can be sure, says Fonseca. In general, evidence like this needs to be put in a wider context for it to make any sense.
"It has been claimed that people consuming fermented dairy products live longer, so let's say we can consider that fermented products are healthy – but eating only fermented dairy products is not a healthy diet, obviously," says Fonseca. "Everything must be considered in a context including not only diet but also factors such as lifestyle, medication, stress levels and genetics."
Whelan agrees that some people may be taking fermentation too far.
"We're suddenly aware in the last year that people have gone kombucha crazy on this whole microbiome trend. People are like, 'Oh this is a natural way to do it, people have been eating these foods for thousands of years.' But what people haven't done yet is researched whether it will actually change your microbiome," says Whelan, who is working on a review paper on this topic at present.
Verdict: Fermented foods may help boost your microbiome diversity – but the science hasn't up with this craze yet to say with certainty either way.
In general, when you see advice that says to eat particular foods or products to boost gut health, it's best to be sceptical, says Fonseca. "I would say that scientific studies are never that conclusive," she says. "People – including myself – get used to reading this type of news that is not accurate or even true, and they end up either being misguided or becoming sceptical about scientific discoveries."
There is a long way to go before we can say there is an optimal diet to boost gut health. But the basic building blocks are there to build a diet that at least gives you a good chance of a healthy, diverse microbiome. While supplements like probiotics and prebiotics might be of some help, one of the best things you can do may be to ditch your routine and try something new.
Source: Martha Henriques, How to eat your way to a healthy gut, BBC Future, April 24, 2020.