That darkness broods upon the deep is confirmed by some tummy-related facts: our gut hosts around 100 trillion bacteria; they comprise 200 to 1,000 species; and their combined weight in the colon is about 2kg.
Perhaps unsurprisingly, and despite being carefully sequestered away down there, our microbial community of gut flora – or microbiome – influences, and is influenced by, events going on elsewhere in the body. For example, Carl Zimmer has recently speculated on the possible role of the microbiome in dementia and autism.
And in a 2017 editorial published in the British Journal of Sports Medicine Irish researchers cited the fact that the results of multiple studies suggest a relationship between our gut microbiomes and inflammatory conditions such as rheumatoid arthritis, spondyloarthropathies and gout. In addition, bacterial nucleic acid "of oral and gut origin has been found in blood serum and joint fluid in these conditions, suggesting a compromised gut barrier is a contributing factor."
But what is the relationship between our microbiome and exercise?
In 2014, researchers from Cork and Dublin were the first to report that exercise is associated with gut microbial diversity in humans.
They recruited 40 elite professional Irish rugby players, 46 controls – 23 with a low body mass index (BMI) and 23 with a high BMI – who underwent blood, nutritional and faecal analyses. The researchers not only found that the athletes had lower blood concentrations of inflammatory markers than controls, but also that athletes had more diverse microbiomes than controls, correlating with fitness and dietary protein consumption in the athlete group. Interestingly, the microbiomes both of athletes and the low BMI control group – but not the high BMI group – were enriched with the bacterium Akkermansia muciniphila.
Although the primarily Cork-based researchers took care not to equate increased exercise with improved microbiomes, a year later they nonetheless speculated that it may be tempting to view increased microbial diversity as a possible benefit of exercise. Lead author of the paper is Dr Orla O'Sullivan, a computational biologist at the Teagasc Food Research Centre and a funded investigator in APC Microbiome Ireland.
Dr O’Sullivan’s team considered the question of whether people’s gut microbes could be “trained” through exercise to resemble that of an elite athlete, “with increased protein intake or a combination of both […]an intriguing prospect.” Is this possible?
"We observed only subtle variation in the gut microbiome in an eight-week intervention," O'Sullivan told The Irish Times. "We hypothesised that it's fitness and not exercise that is important for the health of your microbes. An eight-week exercise programme was unable to replicate a lifetime of chronic exercise that an athlete partakes in. Basically, training for a few weeks isn't going to replicate an athlete's gut microbiome."
An American review published this year highlights ways exercise could alter a person's gut microbiome. For example, around 70 per cent of the body's immune cells are found in gut-associated lymphoid tissue, and animal studies have found that exercise influences the gene expression of these cells, "downregulating pro-inflammatory cytokines and upregulating anti-inflammatory cytokines and antioxidant enzymes".
Exercise may also impact the integrity of the mucus layer lining the gut; this layer supports the growth of beneficial bacteria.
Another possibility is that exercise training may favourably alter the circulation of bile acids. For example, one study found that when mice with high blood concentrations of cholesterol were given access to a running wheel for 12 weeks, they secreted more bile acid compared with sedentary mice: “Bile acids are potent regulators of gut microbiota community structure, and an absence of these molecules is associated with significant alterations in gut microbial communities (ie gut dysbiosis). Thus, changes in the bile acid pool could significantly shift the gut microbiome with exercise.”
Faecal test
Looking to the future, is it feasible that a faecal test might be used to gauge the fitness either of a non-elite athlete or a patient? O’Sullivan remains cautious, pointing out that the field of microbiome research is only in its infancy: “Whilst we know a lot, there are still a lot of unknowns. For example, there are microbes which we know are beneficial to human health, and those that are detrimental, but until we understand more about causation, for now faecal tests aren’t suitable. In the future though I feel they will be an integral part of medical examinations and will guide patient treatment.”
What advice would O’Sullivan offer to those who might perhaps be planning to boost their athletic performance but are wondering what role their microbiomes may have to play in achieving this aim? “What we have learned from microbiome research,” O’Sullivan says, “is that diversity is key: the more different types of microbes you have in your gut the better for your health. One of the main drivers of this is diet so the more diverse your diet is, the more diverse your microbes. And whilst we don’t yet fully understand the full impact of exercise, being fit is good for your overall heath and it’s never a bad thing to exercise.”
This exciting field of research not only confirms that many of the microbes we host not only confer health benefits but may be enriched through exercise.
