According to the WHO, diarrhea is currently defined as the passage of at least three loose or liquid stools per day. Recognizing that the joint presence of 2 criteria (frequency + consistency) is necessary, the frequent passing of formed stools (that is, of normal consistency) does not constitute diarrhea, nor does the passing of loose stools at a normal frequency, such as with breastfeeding babies.

However, while common in mild forms, diarrhea can be life-threatening. It was responsible for 1.6 million deaths in 2016, mainly among malnourished or immunocompromised children, or people living with HIV. ¹ The main cause of these deaths was severe dehydration associated with fluid loss due to the repeated passing of stools.

Most cases of acute diarrhea are due to infections from pathogenic bacterial, viral or parasitic microorganisms.^1-3

What is bacterial diarrhea? 

Diarrhea is defined as the passage of at least three loose or liquid stools per day, and infectious diarrhea is diarrhea caused by infection by a pathogen (virus, bacteria, or parasite) ^1-3. If the pathogen is a bacterium, the diarrhea is said to be “bacterial”. 

Which bacteria are responsible?

The bacteria involved are often⁶:

• Shigella, responsible for an illness called shigellosis and for 212,438 deaths worldwide in 2016,
• and Vibrio cholerae, responsible for dreaded cholera epidemics and for 107,290 recorded deaths in 2016, generally in poor populations without access to clean drinking water.

However, these are not the only ones. The bacteria Salmonella or Escherichia coli sometimes make the headlines in Western countries when they contaminate foodstuffs (raw ground meat, cheese, etc.) and necessitate much-publicized product recalls from supermarkets.

How do bacteria cause acute diarrhea?

In bacterial diarrhea, as in infectious diarrhea in general, it all begins with a confrontation between a pathogen (in this case a bacterium), transmitted by contaminated food, polluted water or contact with a sick person, and the host (our body). However, the dangerous nature of bacteria varies from one bacterium to another, and behind one name, there are actually many types of bacteria. For example, there are over 2,500 different salmonella strains of varying degrees of aggressiveness⁷. The same applies to Escherichia coli: not all E. coli are pathogenic, and among those that are pathogenic, there are many different strains^2,8.>

The outcome of the confrontation between the bacterium and the body depends on complex equilibriums, which largely involve the gut microbiota. Thus, animals lacking in intestinal flora are very susceptible to bacteria responsible for diarrhea: a micro-dose of certain strains of salmonella is enough to cause a fatal infection, while a dose 100 to 100 million times stronger is necessary to kill mice with intact gut microbiota⁹. The science behind this difference Without microbiota, the immune system is immature and thus unarmed against an invasion of pathogens, which are also able to freely settle in with no other bacteria occupying the territory⁹.

When the bacteria get the upper hand, they secrete specific toxins, which explains why each bacterial diarrhea is different^2,10:

• the toxin from Shigella destroys the cells of the digestive tract lining and thus causes severe diarrhea containing blood and mucus; <
• the toxin from Vibrio cholerae disrupts the absorption and secretion of ions and water in the digestive tract, causing a very liquid diarrhea rich in ions;
• the different types of E. coli produce different toxins: Enteropathogenic E. coli causes persistent liquid diarrhea (usually among infants), while Enteroinvasive E. coli causes diarrhea containing bile and mucus, etc.

Acute bacterial diarrhea caused by an infectious agent creates a major intestinal dysbiosis². It will take the microbiota several weeks to recover its equilibrium, which, in some cases, never returns to its initial state^2,11.

How to prevent bacterial diarrhea?

Due to the dangerous nature of Shigella, there is hope of a vaccine which could eventually not only prevent the 200,000 or so deaths caused annually by this bacteria, but could also reduce the need for antibiotics and the emergence of resistance that makes treatment ineffective. Several vaccines against Shigella are currently undergoing development, but none are yet officially approved¹².

Another research avenue: the gut microbiota. In humans, a “healthy” gut microbiota is considered a means of preventing cholera¹³<. In addition, probiotics are seen as a means of limiting the severity of some bacterial infections²: the probiotic yeast Saccharomyces boulardii could facilitate the restoration of gut microbiota in children suffering from acute diarrhea¹⁴; the probiotic E. coli (non-pathogenic) inhibits the formation of biofilms by pathogenic bacteria, including pathogenic E. coli²; a trio of specific strains of Lactobacillus, Bifidobacterium and Streptococcus reduces the duration of bloody diarrhea (dysentery) and hospitalization².

To these preventive measures, of course, are added the advice of hygiene and social distancing, which apply to all types of infectious diarrhea (see boxed text). In the specific case of food-borne infections (salmonella, E. Coli), care should be taken to cook food properly (E. coli is killed by heat, which explains why contaminated foods are often raw or undercooked meats).

How to treat bacterial diarrhea?

Quite common and usually not serious, bacterial diarrhea often clears up on its own. Attention should be paid, however, to the risk of dehydration among the very young, the elderly and immunocompromised patients. Loss of water and electrolytes (sodium, potassium and chloride ions) should be compensated for:

• orally (with oral rehydration solutions, or ORS);
• or intravenously in the most serious cases.

Health professionals may also recommend certain probiotic strains to treat acute infectious diarrhea in children. These good bacteria reduce the duration of diarrhea, and/or the duration of hospitalization, and/or the quantity of stools (ESPGHAN 2023).

Key points to remember about bacterial infectious diarrhea

• Diarrhea is defined as the passage of at least three loose or liquid stools per day, and infectious diarrhea is diarrhea caused by infection by a pathogen (virus, bacteria, or parasite). Due to the resulting dehydration, diarrhea was responsible for 1.6 million deaths in 2016, mainly among malnourished or immunocompromised children or people living with HIV^1-3.
• In cases of infection by a bacterium, diarrhea is said to be “bacterial”. The bacteria Shigella (shigellosis) and Vibrio cholerae (cholera) are the deadliest in poor countries without access to clean drinking water. In Western countries, Salmonella and Escherichia coli are associated with food contamination, while C. difficile is the main bacterium responsible for nosocomial infectious diarrhea in adults⁶.
• Sometimes, the gut microbiota manage to counter the infection; at other times, the bacteria kills the gut microbiota and causes diarrhea of which the characteristics (presence of blood, presence of mucus, passing of a large amount of liquid, etc.) depend on the type of bacteria involved ^2,1.
•  Prevention of viral diarrhea rests above all on good hygiene practices (washing and cooking of food, hand-washing, social distancing, etc.). Probiotics could have a place in the prevention of certain bacterial infections ². Lastly, researchers are working on the development of vaccines against Shigella ¹².
• treatment of bacterial diarrhea focuses on fighting dehydration.Antibiotics can be prescribed by health care professionals for people at risk (infants, the elderly, immunocompromised patients) or when the infection spreads from the intestines to other parts of the body ⁷. Probiotics can also be prescribed to reduce diarrhea and hospitalization time(ESPGHAN 2023).

What is parasitic diarrhea?

Diarrhea is defined as the passage of at least three loose or liquid stools per day, and infectious diarrhea is diarrhea caused by infection by a pathogen (virus, bacteria, or parasite). ^1-3 If the pathogen is a parasite, the diarrhea is said to be “parasitic”.

Which parasites are responsible?

Intestinal parasites can be classified into two main categories : ^6-8

• protozoa, organisms made up of a single cell and measuring less than one millimeter, among which the best-known are Giardia intestinalis (responsible for giardiasis), Entamoeba histolytica (which causes the dreaded amebiasis), Cyclospora cayetanensis (cyclosporiasis) and Cryptosporidium spp (cryptosporidiosis);
• helminths, organisms made up of multiple cells, better known under the name worms, of which the most well-known is undeniably Ascaris lumbricoides.

Infections by these 2 types of parasites are common: 350 million people are estimated to be infected by the 3 most common types of protozoa, and 895 million by helminths transmitted through soil. Due to the globalization of food production, international travel and waves of migration, protozoa infections are now more common in the West than worm infections. ⁶

How do parasites cause acute parasitic diarrhea?

In parasitic diarrhea, as in infectious diarrhea in general, it all begins with a confrontation between a pathogen (in this case a parasite), usually transmitted by contaminated food or polluted water, and the host (our body). However, parasitic infection does not necessarily lead to symptoms such as diarrhea. Infections by the protozoan Entamoeba histolytica are usually asymptomatic, but can provoke an invasive disease of the large intestine in immunocompromised patients.⁷ The same applies in the case of worms: a mild infection often goes unnoticed, while the presence of a large number of worms can cause a whole series of symptoms such as growth disorders and problems with physical development. This is because the worms feed on the tissues of the host, leading to intestinal blood loss and hindering absorption of nutrients. ^8,9 

Each parasite has its own habits : ⁷

• among protozoa, Giardia intestinalis infects the upper part of the small intestine and can cause, 6 to 15 days after infection, acute diarrhea that is very liquid, while Entamoeba histolytica attacks the large intestine and can cause bloody diarrhea with mucus ;
• the worm Ascaris lumbricoides infects us through the ingestion of its eggs, which may be present on improperly washed food (fruits and vegetables) or in polluted water. Its eggs release larvae into the digestive tract. These in turn pass through the wall of the digestive tract and reach the lungs via the bloodstream, where they may cause their first symptoms (early pulmonary phase). This is just the start. The larvae climb the trachea up to the esophagus and slide down the esophagus and then the digestive tract, where they turn into adults (late intestinal phase, with diarrhea, mild abdominal pain, weight loss, nausea and vomiting), which lay eggs that are eliminated in stools.

How to prevent parasitic diarrhea?

Prevention rests above all on hygiene advice which applies to all types of infectious diarrhea.

Another avenue inspired by an observation: different people react differently to parasites; some are asymptomatic while others suffer from acute diarrhea. The gut microbiota can explain this clinical variation: for example, in Bangladesh, low abundance of the bacterium Megasphaera before and at the time of detection of the parasite Cryptosporidium has been associated with parasitic diarrhea in infants, which suggests that the gut microbiota may play a role in the severity of cryptosporidiosis. ¹⁰

If we look at helminths, the connections between these worms and gut microbiota are also the subject of multiple studies. Complex interactions exist between the two, but also with the host's immune system. However, intestinal worms should not necessarily be avoided: in small doses, they could have beneficial health effects, such as resistance to other digestive system pathogens or even the prevention of allergic ailments.¹¹

How to treat parasitic diarrhea?

As with all types of diarrhea, parasitic diarrhea exposes the very young, the elderly and immunocompromised patients in particular to the risk of dehydration. Treatment thus focuses primarily on treating the symptoms, namely, fighting the loss of water and electrolytes (sodium, potassium and chloride ions):

• orally (with oral rehydration solutions, or ORS);
• or intravenously in the most serious cases.

When a health care professional suspects that a parasitic infection could be the cause of diarrhea (e.g. if a patient has recently returned from the tropics), they will usually request examination of a stool sample in order to identify the possible parasite. However, it is not always so simple. The parasite may not be present in all stools (which would require multiple samples), and the presence of a small quantity of a parasite does not necessarily mean that the culprit has been found. This makes parasitic infections such as giardiasis potentially difficult to diagnose, and the treatment decision also depends on the history and symptoms of the patient. ¹²

Key points to remember about parasitic infectious diarrhea

• Diarrhea is defined as the passage of at least three loose or liquid stools per day, and infectious diarrhea is diarrhea caused by infection by a pathogen (virus, bacteria, or parasite). Due to the resulting dehydration, diarrhea was responsible for 1.6 million deaths in 2016, mainly among malnourished or immunocompromised children or people living with HIV. ^1-3
• In cases of infection by a parasite, diarrhea is said to be “parasitic”. There are 2 types of intestinal parasites: protozoa, measuring less than one millimeter, among which the best-known are Giardia intestinalis, responsible for giardiasis, and Entamoeba histolytica, which causes the dreaded amebiasis; and helminths, better known by the name worms, of which the most well-known is undeniably ascaris (roundworm). ^6-8
• In the great majority of cases, the presence of digestive parasites is asymptomatic and not severe, possibly even beneficial (immunity). Sometimes, however, they cause diarrhea, of which the characteristics (presence of blood, presence of mucus, significant amount of liquid, etc.) depend on the parasite. ⁷
• Prevention of parasitic diarrhea rests above all on good hygiene practices (washing and cooking of food, hand-washing) and on the development of access to clean drinking water. Microbiota are also the subject of multiple studies, since they may explain the variation in symptoms and severity from one person to another. ¹⁰
• The treatment of parasitic diarrhea focuses on fighting dehydration. If the parasite is identified, the prescription of medication can be considered. ⁹ Systematic deworming programs are also proposed. ⁸

What is viral diarrhea?

Diarrhea is defined as the passage of at least three loose or liquid stools per day, and infectious diarrhea is diarrhea caused by infection by a pathogen (virus, bacteria, or parasite). ^1-3  If the pathogen is a virus, the diarrhea is said to be “viral”.

Which viruses are responsible?

While rotavirus is the deadliest (210,000 estimated deaths in 2016 among children under the age of 5), it is far from the only virus responsible for acute diarrhea or even death: adenovirus, norovirus, sapovirus and astrovirus each kill between 17,000 and 37,000 young children every year. ⁶

Note: SARS-Cov-2, the code name of the virus responsible for the COVID-19 pandemic, can also cause diarrhea. However, the frequency of these cases is difficult to quantify. Figures vary widely from one study to another, with some researchers reporting an incidence rate of diarrhea in 2% of cases, others in 50%! ⁷

How does a small virus cause acute viral diarrhea?

In viral diarrhea, as in infectious diarrhea in general, it all begins with a confrontation between a pathogen (in this case a virus) and the host. The outcome of this confrontation depends on complex equilibriums, which largely involve the gut microbiota: in mice, for example, certain bacteria seem to be able to prevent or even cure a rotavirus infection. ⁸ The same virus will thus have a different effect depending on the state of the gut microbiota of the infected person. A less diversified microbiota would be more susceptible to the virus. ¹²

When the virus gains the upper hand, the process is always essentially the same: the virus infects the cells that line the small intestine and causes lesions that hinder the absorption of fluids. ⁹ The result is very liquid diarrhea (but without the presence of blood) which may be accompanied by other symptoms such as vomiting, nausea, abdominal cramps and fever. ¹⁰ This is the famous viral gastroenteritis, which reappears every winter. Particularly severe in the case of infection by rotavirus, which causes more pronounced symptoms, it is usually accompanied by dysbiosis of the gut microbiota, unsettled by the invasion of this virus. ^11,12

How to prevent viral diarrhea?

Given the danger of rotavirus for frail individuals, preventive vaccines have been developed to reduce its mortality rate in young children. It is estimated that the vaccination of infants against rotavirus prevented 139,000 deaths in children under the age of five in the period 2006 to 2019. This figure is considered to be both significant yet low considering the hopes pinned on the vaccination: the vaccine seems to be less effective in low- or moderate-income countries (which are paradoxically the most affected).

Among the factors that might explain this lower effectiveness is the composition of the body’s microbiota, which affects the immune response to the vaccination in the gut. ^13-16 To this preventive measure, of course, are added the advice of hygiene and social distancing, which apply to all types of infectious diarrhea.

How to treat viral diarrhea?

Very common and usually not serious, viral diarrhea can nonetheless lead to severe dehydration in the most frail individuals (infants, malnourished children, immunocompromised adults, the elderly). Treatment focuses on fighting the loss of water and electrolytes (e.g. replenishing lost sodium, potassium and chloride ions):

• orally (with oral rehydration solutions, or ORS);
• or intravenously in the most serious cases.

Health professionals may also recommend certain probiotic strains to treat acute gastroenteritis in children. These good bacteria reduce the duration of diarrhea, and/or the duration of hospitalization, and/or the quantity of stools (ESPGHAN 2023).

Key points to remember

• Diarrhea is defined as the passage of at least three loose or liquid stools per day, and infectious diarrhea is diarrhea caused by infection by a pathogen (virus, bacteria, or parasite). Due to the resulting dehydration, diarrhea was responsible for 1.6 million deaths in 2016, mainly among malnourished or immunocompromised children or people living with HIV. ^1-3
• In cases of infection by a virus, diarrhea is said to be “viral”. The rotavirus virus causes the most deaths, followed, far behind, by adenovirus, norovirus, sapovirus and astrovirus. ⁶
• Sometimes, the gut microbiota manage to counter the infection; at other times, the virus kills the gut microbiota and causes watery diarrhea, which may be combined with other symptoms  such as vomiting, nausea, abdominal cramps and fever. This is viral gastroenteritis. ¹⁰
• Prevention of viral diarrhea rests on the vaccination of infants against rotavirus (less effective in low- or moderate-income countries), access to clean drinking water, and hygiene and social distancing practices which apply to all types of infectious diarrhea. ^13-16
• The treatment of viral diarrhea focuses on fighting dehydration. Certain probiotic strains can be prescribed for the treatment of acute gastroenteritis in children (ESPGHAN 2023). Antibiotics, on the other hand, are ineffective against viruses and can also worsen the diarrhea.

Physical activity and gastrointestinal distress

Physical activity is, in its nature, putting stress on the body, particularly if it is intense or prolonged. While moderate amounts of exercise have a positive effect on gut permeability and inflammation ¹, sustained and intense exercise can have deleterious effects on the gut functionality ², in agreement with gastrointestinal distress and disorders being common, especially among endurance athletes. ³

Since the gut microbiome is linked to the gastrointestinal physiology ⁴, the gut microbiome could potentially play a role in gastrointestinal distress and physiological responses to exercise, influencing athletic performance. 

Effects of the gut microbiota on endurance exercise

Pointing towards a more direct effect of the gut microbiota on endurance exercise, a landmark study published in Nature Medicine in 2019 ¹⁰, found that marathon runners have elevated levels of Veillonella. The researchers found that lactate, formed during prolonged endurance, was transported from the circulation into the intestine where it was metabolized by Veillonella into propionate.

When the scientists fed mice the lactate-utilizing Veillonella microbes or administered propionate via intracolonic infusion, the endurance of the mice improved, as reflected by increased treadmill run performance. ¹⁰ Intriguingly, the study proposed that gut microbes may directly enhance athletic performance. Another recent study also reported that the gut microbiome is involved in stimulating gut-derived signals to the brain affecting motivation for exercise in mice. ¹¹

These discoveries suggest that by targeting the gut microbiome, athletes could potentially improve their athletic performance.

Gut-centric dietary strategies to influence athletic performance

An obvious way to target the gut microbiome is through the diet, since microbes feed on dietary components reaching the colon. Traditionally, many sports-centric dietary strategies focused on high protein, high carbohydrates, low fiber intake, and food avoidance. While such strategies may be sufficient to support host metabolism, restore glycogen stores, and reduce gastrointestinal distress during exercise ¹², the lack of dietary fiber could over time be detrimental to the athlete’s gut microbiota and bowel habits. ¹³

In contrast, gut-directed dietary strategies including adequate dietary fiber from a variety of foods, probiotics, and prebiotics, could support a diverse gut microbiome ¹³ and contribute to regular bowel habits. ⁴ The intake of dietary fiber and prebiotics fuels the resident gut microbes as they serve as substrates for the microbes. Once these indigestible components reach the colon, the microbes ferment them into gas and short-chain fatty acids.

While too much gas can cause discomfort and bloating, the resulting three main short-chain fatty acids – acetate, propionate and butyrate are associated with colonic health and integrity. These molecules can act locally in the gut, affecting our intestinal barrier function, motility, nervous system, and immune system, as well as in distant organs, affecting host metabolism. ¹⁴

To nurture our invisible microbial coach in the gut, scientists and athletes need to develop gut-centric dietary strategies that, on the one hand, reduce gastrointestinal symptoms during exercise and, on the other hand, support a diverse gut microbiome, stimulate colonic fermentation, and promote regular bowel movements — aspects that may all be key for athletic performance.

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Bacteria, viruses, fungi (including yeasts) and even parasites: a whole flora, known as the “gut microbiota”, inhabits our digestive system.

And it's all for our own good: the intestinal microbiota aids digestion, contributes to the maturation of our immune system, protects us from pathogens and intestinal toxins, and so on. This list of benefits is far from exhaustive, as the gut microbiota has many other strings to its bow, including when it comes to sport: this invisible coach could help us in our efforts, improve our times and motivate us for training sessions!

The prospects opened up by the discovery of these links between microbiota, the digestive system and sport are immense: can we optimize athlete performance through a personalized approach to microbiota?

Dive into the heart of a microscopic world to discover this bidirectional relationship with our muscular and mental performance.

Dive into microbiota and sport exhibition

Find out more about the link between microbiota and sport 

Interested in this topic? Explore it by consulting our dedicated content. 

Expert opinion

Dr. Henrik Roager is an Associate Professor and researcher at the University of Copenhagen. He leads the Microbiome & Metabolomics research group, focusing on how gut microbiota contributes to digestion, health, and athletic performance. Watch the video to know what Dr. Henrik Roager has to say about the contribution of microbiomes in sports.

To find out more

Discover our selection of articles on the health and microbiota benefits of sport.

Do athletes have a specific microbiota?

A number of studies have looked closely at the microbiota of top-level athletes in an attempt to identify any specific characteristics, or even the bacteria associated with their performance. Generally speaking, athletes (sidenote: Athlete A competitive sportsman or sportswoman who strives for a high level of performance through training.
Source: Rousseau AS. Nutrition, santé et performance du sportif d’endurance / Nutrition, health and performance of endurance athletes. Cahiers de Nutrition et Diététique. 2022 eb ;57(1) : 78-94 ) ' flora does indeed seem different.

First of all, the gut microbiota of athletes is more diverse. The athletes’ cardiorespiratory fitness, and more specifically their maximum oxygen consumption (VO2max (sidenote: VO2max A criterion specific to each athlete, VO2max is the maximum quantity of oxygen that the body can extract from the air and transport to the muscle fibers to meet their needs during exercise. The higher the VO2max, the better the performance. When this criterion is low, sporting ability is limited, and specific training will be needed to boost it. ) ), seems to be linked to the diversity of their gut microbiota. ²

Secondly, the microbiota of athletes is richer in beneficial bacteria (Bifidobacterium, Lactobacilli, and Akkermansia) and produces more valuable short-chain fatty acids (SCFAs (sidenote: Short chain fatty acids (SCFA) Short chain fatty acids (SCFA) are a source of energy (fuel) for an individual’s cells. They interact with the immune system and are involved in communication between the intestine and the brain. Silva YP, Bernardi A, Frozza RL. The Role of Short-Chain Fatty Acids From Gut Microbiota in Gut-Brain Communication. Front Endocrinol (Lausanne). 2020;11:25. ) ) ^3,4. However, there are major differences based on the sport and the athlete. For example, in the microbiota of the aforementioned rugby players, several bacterial taxa are over-represented, starting with Akkermansiaceae. ¹ In marathon runners and cross-country skiers, the gut flora is richer in bacteria from the large Firmicutes family (which includes “good bacteria” lactobacilli) and less rich in Bacteroidetes. The result is a favorable ratio between these two bacterial families (F/B ratio) associated with a higher VO2max, a key factor in performance. ⁵

Prevotella, bacteria associated with better physical performance, are also over-represented in marathon runners. ⁵ The same was true of professional cyclists in the US: Prevotella abundance increased with the number of hours they trained per week. ²

However, the big question that remains is whether athletes’ specific microbiota is the cause of their extraordinary performance or the consequence of their intense sporting activities, combined with a very specific diet. The answer is probably a virtuous circle involving a bit of both.

Is modulating the gut microbiota an ideal strategy for improving athletes’ health and performance?

A direct consequence of the relationship between the gut microbiota and sporting performance is the temptation for athletes to optimize their gut flora, whether through diet (to nourish their bacteria) or probiotics. ⁶

On the dietary side, research is beginning to take up the subject, focusing in particular on the benefits of a high-fiber diet for athletes’ microbiota and digestive health. ³ After all, athletes tend to opt for pasta over lentils and salad. However, their fiber intake should be proportional to their high caloric intake: 14 g of fiber per 1,000 kcal per day should help promote gastrointestinal health and performance. Furthermore, fiber intake should be avoided just before or after exercise, to avoid adding the effects of fiber (bloating, accelerated transit) to an already strained digestive system. ³

As for probiotics, the wide range of strains, doses, sports, and athletes makes it difficult to reach any conclusions. ³ Nevertheless, scientific studies have reported beneficial effects on general health, particularly immunity. In female swimmers, a probiotic yoghurt containing Lactobacillus acidophilus spp., Lactobacillus delbrueckii bulgaricus, Bifidobacterium bifidum, and Streptococcus salivarus thermophilus limits episodes of respiratory infection after competition. In rugby players, a probiotic containing several strains also reduces the frequency of upper respiratory tract disorders and gastrointestinal symptoms. Other studies show an improvement in immune function. ⁶

In terms of performance, certain experiments on mice seem promising. The most striking example is the bacterium Veillonella atypica, associated with the performance of marathon runners, which transforms a waste product of their muscles (lactate) into fuel (propionate).

By simply adding this bacterium to the digestive tract of mice, they become the new stars of the treadmill. ⁷ On the other hand, mice fed a diet devoid of fibers that are fermentable by bacteria drag themselves along the treadmill, losing muscle masse. ⁴ However, these results in rodents are preliminary and must be confirmed by studies on humans.

Is exercise good for our digestive health?

Like the rest of our body, our digestive system has everything to gain from physical activity (sidenote: Physical activity Any bodily movement produced by skeletal muscle contraction resulting in an increase in energy expenditure (EE) relative to resting EE”. Examples of physical activity include walking, cycling, active play, sports, housework, gardening, and DIY. Source: Caspersen CJ, Powell KE, Christenson GM. Physical activity, exercise, and physical fitness: definitions and distinctions for health-related research. Public Health Rep. 1985 Mar-Apr;100(2):126-31. ) . When we exercise in moderation (at less than 50% of maximal oxygen consumption or VO2max (sidenote: VO2max A criterion specific to each athlete, VO2max is the maximum quantity of oxygen that the body can extract from the air and transport to the muscle fibers to meet their needs during exercise. The higher the VO2max, the better the performance. When this criterion is low, sporting ability is limited, and specific training will be needed to boost it. ) ) :

• gut transit receives a boost;
• the mucous membrane lining the digestive tract is left in better condition.

A scientific study has shown that, following three months of moderate physical activity, gastrointestinal motility (contractions of the muscles in the digestive tract required to move food through it) is improved, and as a result transit is accelerated, reducing the time that future stools spend in the digestive system and therefore the period of contact between any pathogens present in these stools and the gut barrier. The same is true after just one week of moderately intense cycling. In other words, sport (sidenote: Sport Structured leisure-time physical activity which may include physical exercise where participants adhere to a common set of rules (or expectations) and where a goal is defined.
Source: Khan KM, Thompson AM, Blair SN et al. Sport and exercise as contributors to the health of nations. Lancet. 2012 Jul 7;380(9836):59-64. ) is good for the digestive system, as long as you’re not obsessed with the clock and don’t push yourself too hard. ¹

The same applies to the gut mucosa that lines the walls of our digestive tract. Sport in moderation thus goes hand in hand with a healthy mucosa that perfectly fulfills its barrier function. ¹

Your microbiota needs exercise, so get your sneakers on.

Is this a direct consequence of good digestive health? Moderate exercise keeps your gut microbiota in top form. Physical exercise therefore ticks all the right boxes ¹ :

• it improves the composition and functioning of the gut microbiota, promoting the establishment of a rich and beneficial flora; 
• it promotes the synthesis of molecules that modulate immunity and others with antimicrobial properties that effectively protect against pathogen attack. 

These initial research findings should encourage you to get off the couch. Doing a little exercise leads to greater diversity in the Firmicutes bacterial phylum, contributing to a healthier gut environment. ² Sedentary teenagers who take up half an hour of moderate-intensity running, four times a week, have a modified flora (and reduced moodiness), with an increased abundance of Coprococcus and Blautia. ³ Make sure to exercise regularly: professional rugby players boast a healthy microbiota. ^4,5

Lastly, there appears to be a link with the intensity of practice ⁶ : the higher the level of martial artists, the more diverse and rich in beneficial bacteria their gut microbiota. 

Physical exercise is even said to have therapeutic value: at moderate intensity, exercise seems to effectively reduce irritable bowel syndrome (from which many endurance athletes suffer). Another reason to take up regular exercise. ¹

Exercise, but not to excess.

Burn calories, but not at any price. As with everything, too much can be harmful: 60 minutes of very intense endurance training (at 70% of VO2max (sidenote: VO2max A criterion specific to each athlete, VO2max is the maximum quantity of oxygen that the body can extract from the air and transport to the muscle fibers to meet their needs during exercise. The higher the VO2max, the better the performance. When this criterion is low, sporting ability is limited, and specific training will be needed to boost it. )  capacity) leads to abdominal pain, nausea, and diarrhea. ¹ Other factors, such as altitude, ambient temperature, poor hydration, and age, also appear to influence levels of discomfort.

Runners are twice as exposed as those practicing other endurance sports, such as cycling or swimming. Most affected are sticklers for discipline, with the phenomenon 1.5 to 3 times more common among elite athletes (sidenote: Athlete A competitive sportsman or sportswoman who strives for a high level of performance through training.
Source: Rousseau AS. Nutrition, santé et performance du sportif d’endurance / Nutrition, health and performance of endurance athletes. Cahiers de Nutrition et Diététique. 2022 eb ;57(1) : 78-94 ) than among amateurs. ¹ 30%-50% of athletes suffer from digestive problems, which rises to 90% for those taking part in ultra-endurance challenges (sidenote: Ultra-endurance challenges Extreme events often lasting more than six hours and covering over 100 km (or much more), at times in difficult conditions. For example, the Ironman triathlon covers 226 km in total, including a 3.8 km swim, a 180.2 km bike ride, and a 42.195 km marathon on the same day; the Race Across America is 4,860-km bike race that must be completed in a maximum of 12 days; and the Iditarod Trail Invitational sees participants run, bike, and ski their way through 1,600 km of snow in Alaska. ) . ⁷ Images of marathon runners suffering from mid-race diarrhea are easy to find online.

What explains this epidemic of gastrointestinal disorders among athletes? The fact that their bodies dedicate all their energy to providing muscles with needed oxygen. Intense physical exercise causes the blood system to go into overdrive, with instructions given to immediately redistribute blood flow to the muscles, to the detriment of our gut and digestive system. At the same time, the body activates our sympathetic nervous system, which makes our heart beat faster when we’re scared, affecting transit. This dual mechanism explains the pain, nausea, and diarrhea. ⁷

The main concern is that the troubled digestive system causes the microbiota it hosts to become unwell. So, whether you’re an amateur or a professional, training too hard, or disproportionately to your physical level, can change the composition and function of your microbiota, which is known as dysbiosis.

The more intense the physical activity, the more rapid and profound the disturbance. ⁸ The result is increased gut permeability, where the membrane of the digestive tract can no longer act as barrier and border guard. Bacterial toxins and pro-inflammatory molecules can then enter the athlete’s body, potentially impacting overall health. ¹

Why is regular physical activity important for our health? Why is it so hard to get (back) into exercising?

A sedentary lifestyle (sidenote: Sedentary lifestyle “Waking behavior characterized by an energy expenditure close to the resting energy expenditure in a sitting or lying position.” Example: time spent sitting or lying down during the day outside sleep time, whether at work or school, on motorized transport, or during leisure activities, particularly in front of screens.
Source: OMS, Organisation mondiale de la Santé. Lignes directrices de l’OMS sur l’activité physique et la sédentarité: en un coup d’œil. 2020 Nov 25. 17 pages. ISBN: 9789240014862 )  seriously damages your health: insufficient physical activity is associated with a 20% to 30% increased risk of death.¹ Conversely, regular physical activity (sidenote: Physical activity Any bodily movement produced by skeletal muscle contraction resulting in an increase in energy expenditure (EE) relative to resting EE”. Examples of physical activity include walking, cycling, active play, sports, housework, gardening, and DIY. Source: Caspersen CJ, Powell KE, Christenson GM. Physical activity, exercise, and physical fitness: definitions and distinctions for health-related research. Public Health Rep. 1985 Mar-Apr;100(2):126-31. )  is associated with multiple muscular, cardiorespiratory, cardiovascular, bone, and weight management benefits.¹ However, it’s not always easy to get (back) into it.

Practicing an endurance sport (sidenote: Sport Structured leisure-time physical activity which may include physical exercise where participants adhere to a common set of rules (or expectations) and where a goal is defined.
Source: Khan KM, Thompson AM, Blair SN et al. Sport and exercise as contributors to the health of nations. Lancet. 2012 Jul 7;380(9836):59-64. ) , whether running, cycling, swimming, or aerobics, requires a number of physiological adaptations from the body, and not just in the muscles. Prolonged physical activity:

• induces a loss of water and electrolytes (notably sodium and chlorine) through sweating, which is designed to cool the body; 
• depletes reserves of glycogen (sidenote: Glycogen A form of carbohydrate (“sugar”) storage in the body, mainly in the liver and muscles. ) , used to feed our moving muscles; 
• increases inflammation due to the stress of exertion, etc.

While this is all for a good cause, it can be quite a challenge for the body; but here the gut microbiota may come very much in hand.

Does microbiota help improve our sporting performance?

Incredible as it may seem, recent scientific studies have shown that our gut microbiota helps us overcome the hydration, energy, and inflammatory challenges posed by sporting activity.

As you’re well aware, for sustained physical effort, hydration is key. In fact, certain gut bacteria may help transport fluids and solutes across the gut barrier, thereby maintaining hydration.

Another essential ingredient for sport and performance is energy. Here once again our gut flora gives us a boost, by helping muscles that have run down their glycogen reserves during endurance sports. Bacteria in the gut microbiota ferment fibers that our body is unable to digest, extracting valuable short-chain fatty acids (SCFAs (sidenote: Short chain fatty acids (SCFA) Short chain fatty acids (SCFA) are a source of energy (fuel) for an individual’s cells. They interact with the immune system and are involved in communication between the intestine and the brain. Silva YP, Bernardi A, Frozza RL. The Role of Short-Chain Fatty Acids From Gut Microbiota in Gut-Brain Communication. Front Endocrinol (Lausanne). 2020;11:25. ) ) that serve as an emergency fuel for muscles during exercise. ²

The effects of this energy boost are far from negligible: SCFAs provide more than 10% of humans’ daily caloric requirements. ³ The benefits of SCFAs go much further, since they promote the storage of glycogen reserves in muscles prior to exercise, delaying the time when the body requires emergency fuel. ²

Lastly, SCFAs are also thought to reduce inflammation induced by intense physical effort. ^{2, 4, 5}

At times, the mechanism is particularly ingenious, with bacteria transforming a waste product of the athlete (sidenote: Athlete A competitive sportsman or sportswoman who strives for a high level of performance through training.
Source: Rousseau AS. Nutrition, santé et performance du sportif d’endurance / Nutrition, health and performance of endurance athletes. Cahiers de Nutrition et Diététique. 2022 eb ;57(1) : 78-94 ) ’s metabolism into a useful resource. This seems to be the case with the gut bacterium Veillonella atypica, which is associated with the performance of marathon runners. ⁶

How is this possible? When runners’ muscles have used up all their glycogen reserves, they start fermenting to produce energy, which produces a waste product called lactate (the cause of cramps). This is when good old Veillonella comes into play, by transforming lactate into propionate, which the muscles use as an energy source. Thus the athlete’s performance is naturally boosted. ⁷

However, to benefit from these “boost” molecules, you need the right bacteria in your digestive tract, and you must feed them appropriately. Without them, micro-organisms can even generate products that are harmful to our performance. ^2,4,7

It's a faithful relationship. For the third year, Biocodex Microbiota Institute is celebrating World Microbiome Day running with two objectives: raise awareness among lay public about the importance of microbiota and valorize microbiota research through the Biocodex Microbiota Foundation national grant winners.

A look back at the latest microbiota research rewarded by Biocodex Microbiota Foundation

They are scientists, professors, physicians specialized in different specialties (gastroenterology, pediatrics, neurology, cardiology, microbiology, pharmacokinetics…). They come from Portugal, Finland, Belgium, Mexico, United States… They have made major advances in the role of the microbiota on health and associated diseases… And they all have won the Biocodex Microbiota Foundation national’s grant!

Since 2017, The Biocodex Microbiota Foundation rewards national research initiatives which aim to understand the interaction between microbiota and different diseases. On World Microbiome Day 2022, in order to give visibility to researchers, the Biocodex Microbiota Institute gives the floor to the national grant winners through dedicated interviews.

What did the Biocodex Microbiota Foundation national research grant allow them to do? What impact have their research results on patient care? Available on the HCPs’ dedicated section, these testimonies give us a clear idea of the variety and diversity of research projects currently underway. These interviews will be activated via the Institute Twitter’s account during 10 days and Biocodex Microbiota Foundation’s LinkedIn by June 27st. Don’t hesitate to share and spread the good new!

About the Biocodex Microbiota Institute

The Biocodex Microbiota Institute is an international hub of knowledge dedicated to microbiota. The Institute educates the lay public and healthcare professionals on the importance of microbiota on healthcare and well-being.

About the Biocodex Microbiota Foundation

Since 2017, the Biocodex Microbiota Foundation has been working to improve science's understanding of the human microbiota. Each year, the Foundation contributes to the funding of global research on microbiota via grants awarded to innovative scientific research projects. Calls for projects are regularly launched on a specific theme related to the microbiota, with the most promising projects then selected by an international scientific committee made up of independent experts.

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