Twenty years ago, scientists thought of the human gut as little more than a muscular tube for processing food, extracting nutrients and, yes, packaging up the waste for disposal. We knew it was home to bacteria, but they were mostly thought to be involved only in digestion, with the odd nasty popping up now and then to make us sick.
Today, that picture is utterly transformed. We now see the gastrointestinal tract, in particular the large intestine, as a diverse and dynamic ecosystem comprising trillions of microbes, their genetic material and metabolites they produce. This is collectively known as the gut microbiome, which actively influences almost every aspect of our physiology – from metabolism and mood to immunity and inflammation.
Immune virtuoso
“If you think of the human body like an orchestra, then your gut microbiome could be considered the conductor, communicating with and influencing all these different organs and systems,” says Dr Holly Neill, Science Manager for Yakult UK and Ireland.
It’s early days in terms of figuring out precisely how this virtuoso controls our health. But one of the most intriguing findings in the last few years, according to Neill, is the way it directs the immune system – and the impact that may have on how we age. Indeed, emerging evidence suggests that the secret to a longer, healthier life might lie in your gut microbiome.
Our awareness of the microbial menagerie within began in the mid-2000s, when faster DNA sequencing allowed researchers to identify microbes directly from stool samples. The diversity they discovered was astonishing: thousands of different species, many new to science. But the surprises didn’t stop there, because it quickly became clear that our resident microbes have an influence well beyond the intestinal tubes they call home via the metabolites they produce as they break down dietary fibre.
In one early set of experiments, researchers demonstrated that transplanting gut bacteria from obese mice to germ-free mice – with no gut microbiome of their own – led to the recipients rapidly gaining weight, even though there was no change in their diet. It was the first clear evidence that the gut microbiome calibrates metabolism, and we have since found that a reduced diversity of microbial species is associated with obesity, type 2 diabetes, irritable bowel syndrome, and many other conditions.
It has since become clear that the gut microbiome’s influence extends to the brain. Human studies have found associations between altered microbiome compositions and depression, anxiety and cognitive performance. This suggests that our colonic colonists can shape how we think and feel by producing metabolites that affect chemical signalling in the brain.
Perhaps the most exciting new development, however, is the growing recognition that there may also be a connection between the composition of your gut microbiome and how well you age. This has come to the fore thanks to studies showing that centenarians have a distinctive gut microbiome.
“There is one set of bacteria that are associated with healthy ageing, as they’re disproportionately represented in the fit and healthy centenarians,” says Professor Claire Steves, who studies ageing at King’s College London. “Then there’s another set associated with ill health and frailty in old age.”
Whether that means this particular microbial profile contributes to longevity, and that different compositions drive adverse ageing, is not yet clear. “Because there’s constant interaction between the host and the microbiome, it is hard to be sure about causality,” says Steves.
One plausible mechanism involves the short-chain fatty acids (SCFAs) produced by certain species of microbe, and particularly an SCFA called butyrate. This fuels and maintains the cells in our gut lining and plays a key role in regulating the training
of immune cells in the gut by modulating their production of inflammatory cytokines, signalling molecules that ramp inflammation up or down.
For most of our lives, this gut-immune axis works perfectly well, fighting off pathogens without sending inflammation into overdrive. As we get older, however, our gut microbiome changes: diversity goes into decline, says Steves, typically with a significant reduction in SCFAs like butyrate.
The hypothesis is that, as a result, the gut lining degrades and becomes leaky. This allows pathogens to escape and trigger immune responses, while the balance of
pro- and anti-inflammatory cytokines circulating around the body is disrupted. Together, this is thought to drive low-level systemic inflammation, otherwise known
as “inflammageing” (see diagram, left).
Restored ecosystem
For now, it’s just a hypothesis. But Steves points to several studies in older people in which dietary changes designed to restore a favourable microbial ecosystem in the gut resulted in reduced levels of inflammatory markers. “There’s actually quite good evidence that we can change the microbiome to alter inflammation status,” she says.
All this emphasises the importance of looking after our gut microbiome, and raises the question of how to cultivate the bacteria that may increase health span. There is no shortage of options but Steves recommends one simple dietary change: eat more fibre to nurture the gut bacteria that produce SCFAs such as butyrate. “We could all do with getting more fibre in our diets – that is a really easy way to improve your gut microbiome.” And it might just help you join the ranks of the centenarians.
1. Eat more plants
Eat a wide variety of plants, particularly those high in polyphenols and fibre that many species of beneficial bacteria feast on. Fermented foods
like yoghurt, kefir and kimchi may also boost microbial diversity.
2. Move yourself
Regular aerobic exercise, such as running and cycling, appears to increase the variety and abundance of beneficial bacteria in the gut.
3. Reduce stress
Gut microbes are sensitive to stress, which has long been associated with a leaky gut that allows bacteria to enter the bloodstream and
cause inflammation.
4. Sleep soundly
The microbiome has its own daily cycle, so
poor-quality sleep can disrupt its rhythms and take its toll on the diversity of the ecosystem in your intestines.
5. Hang out with friends
We pick up some of our microbes from contact with other people. Indeed, increased isolation may be part of the reason microbial diversity declines as we age.
Find out how to optimise your gut at: www.yakult.co.uk/gut-health
Yakult’s mission is to inspire happiness and well being for all. This dream was born out of its founder, the Japanese scientist Dr Shirota’s, belief that prevention of illness is better than cure, and that gut health is key to well being. Yakult began their journey in Japan in 1935. Dr Shirota’s vision still drives the company 90 years later, as Yakult’s team of research scientists continue their work on gut science.