- Roberta Angeleanu
- BBC future
Our guts are full of life. Competing for space and food in our gastrointestinal tract are about 100 trillion bacteria, viruses, fungi and other single-celled organisms such as archaea and protozoa.
Its functions range from helping to ferment the dietary fiber in our food to synthesizing vitamins and regulating our fat metabolism.
They also help protect us from unwanted invaders by interacting with our immune system and influencing the level of inflammation in our gut or other parts of our body.
Less diversity of these intestinal inhabitants is observed in patients suffering from obesity, cardiometabolic diseases and autoimmune diseases.
Some diseases are associated with too much or too little of certain types of bacteria in our intestines.
Having low levels of one of the most common bacteria in the gut of healthy adults, the so-called Faecalibacterium prausnitziiassociated with inflammatory diseases.
Many factors, including our genes, the types of medications we take, the stress we face, whether we smoke and what we eat can interact to change the balance of microorganisms in our gut.
The composition of this internal community, known as the microbiota, is highly dynamic.
But just as a simple lifestyle can change our gut microbes, so too we can make decisions that help them prosper in a healthier way.
Eating a varied diet that contains more than 30 different plant foods per week can help. A good night’s sleep and lower stress levels can also be beneficial.
Surprisingly, spending time in nature can also have a positive effect.
Perhaps even more surprisingly, however, exercise can also affect our gut bacteria.
Microbes “in shape”
We all know how good exercise is for our physical and mental health, but could an after-work jog be just what we need to keep our gut microbes in tip-top shape?
“Exercise appears to affect our gut microbes by increasing bacterial communities that produce short-chain fatty acids,” says Jeffrey Woods, a professor of kinesiology and public health at the University of Illinois who studies the effects of exercise on the human body.
“Short-chain fatty acids are a type of fatty acid produced primarily by microbes and have been shown to alter our metabolism, immunity, and other physiological processes,” adds Jacob Allen, an assistant professor of exercise physiology at the University of Illinois who works with Woods.
Over the past 10 years, animal and human research has helped unravel it how powerful this link is between exercise and changes in the gut microbial community.
For example, mice that were allowed to run on a treadmill whenever they wanted were found to have significantly lower amounts of the bacteria, the so-called Turicibacterthe presence of which is associated with an increased risk of bowel disease, said Woods and Allen, who led the study.
Mice that were sedentary or were gently pushed to encourage them to run had much higher amounts of these bacteria. (Forcing the mice to run is thought to have caused chronic stress that offset the benefits of exercise.)
The researchers found that exercise also seems to lead to higher levels of a certain short-chain fatty acid called butyrate, which is produced by bacteria in the gut through fermentation of fiber and is linked to a host of health benefits.
Butyrate is the main fuel for our gut cells, it helps control the gut barrier and regulates inflammation and immune cells in our gut.
In 2018, researchers in the US also found that transplanting gut microbes from trained mice into germ-free mice reduced the amount of inflammation in the guts of the mice that received the microbes.
But while these animal studies provide some clues about how exercise can change the balance of gut microbes for the better, we’re not mice.
So what does human research tell us?
There is no shortage of studies that show that moderate to intense exercise, such as running, cycling and training can potentially increase the diversity of bacteria in the gutwhich is associated with better physical and mental health.
Doing aerobics for just 18 to 32 minutes, along with resistance training three times a week, for a total of eight weeks, can make a difference.
Athletes also tend to have greater gut microbial diversity than sedentary people, although some of this may be due to the specialized diets athletes often eat.
But several studies show that a combination of exercise and diet can increase the number of Faecalibacterium prausnitzii and butyrate production in active women, often with improved bowel function.
People with low levels of this type of bacteria appear to be at increased risk of inflammatory bowel disease, obesity and depression.
Woods and Allen’s studies highlight that running for 30 to 60 minutes can affect the abundance of butyrate-producing bacteria such as Faecalibacteriumin the intestines.
In a study involving 20 women and 12 men of varying body mass indexes (BMIs), Woods and colleagues set out to determine whether six weeks of aerobic exercise could change gut microbes in older adults who had previously led sedentary lives.
They asked participants to do three sessions of moderate-to-vigorous intensity aerobic exercise per week, either running on a treadmill or cycling for 30 to 60 minutes.
Fecal and blood samples were collected throughout the study, with dietary controls to limit dietary changes in gut microbes.
Their findings show that “butyrate producers” increased in abundance with physical training, regardless of body mass index.
Along with the change in the microbial community, lean participants showed an increase in short-chain fatty acids, such as butyrate, in their stool samples.
Interestingly, when the study participants returned to their sedentary lifestyle for the next six weeks, the researchers found that the participants’ gut microbes returned to their original state.
This suggests that while exercise may improve the health of the microbial community in our gut, these changes are transient and reversible.
Another small study, published in 2019 by a team led by Jarna Hanukainen of the University of Turku in Finland, noted more specific changes in the gut microbes of 18 sedentary participants who had been diagnosed with type 2 diabetes or prediabetes.
Participants performed either high-intensity interval training or moderate continuous training (40-60 minutes of cycling) three times a week for two weeks.
The researchers noted that both training regimens increased The Bacteroidesa critical group of gut bacteria that play a role in breaking down sugars and proteins and prompting the immune system to produce anti-inflammatory molecules in the gut.
Reduced levels of these bacteria are associated with obesity and irritable bowel syndrome.
Hannukainen and his team also saw significantly lower levels of molecules that indicate inflammation in the blood and gut in the participants who exercised.
The researchers say their work has also shown this exercise specifically reduces gut bacteria that are associated with obesity.
Exactly how exercise leads to changes in the community of microorganisms that live in our gut is still unclear, though there are several theories, Woods says.
“Lactate is produced when we exercise, and this can serve as fuel for certain types of bacteria,” he says.
Another potential mechanism, he explains, could be through exercise-induced changes in the immune system, particularly the gut immune system, since our gut microbes are in direct contact with the immune cells in the gut.
Exercise also causes changes in blood flow to the gut, which can affect the cells that line the gut wall and in turn cause microbial changes.
Hormonal changes caused by exercise can also cause changes in your gut bacteria. But none of these possible mechanisms “has been conclusively proven”Woods says.
There is much more we can learn about how our physical activity affects the creatures that live in our gut, such as how different types of exercise and its duration can alter the microbial community.
It can also differ between individuals, based on your existing gut inhabitants as well as your BMI and other lifestyle factors such as your diet, stress levels and sleep.
But as scientists continue to uncover more secrets hidden in our gastrointestinal tract, we may find new ways to improve our health through the vibrant and diverse communities of organisms that call us home.
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