Moderate exercise for a healthy gut microbiota

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