Performance from the Inside Out Part 12:

Healthy Gut and Balanced Gut Microbiome Minimize Risk of Gut Symptoms

By Lillian So Chan with Manny W Radomski, PhD



Healthy Gut and Balanced Gut Microbiome Minimize Risk of Gut Symptoms

It is hard to go on racing with a runner’s gut or an abdominal cramp.

GI symptoms occur in as many as 30–50% of endurance sports participants. GI symptoms are more common among runners, occurring more often in women and younger athletes. Symptoms are often associated with dehydration or meals two to three hours before activity.

In runners, lower gut symptoms including diarrhea and rectal incontinence (“runner’s trots,” 14–38% incidence rate), urge to defecate (24–54%), rectal bleeding (up to 28% of marathon runners and 84% of ultramarathon runners), and abdominal cramps (up to 68%) are more common than upper gut symptoms such as GERD (gastroesophageal reflux), nausea, vomiting, and stomach pain.

As physical activity intensity and duration increase, there is evidence for increased gut injury and dysbiosis:

    • compromised permeability and gut wall barrier integrity
    • alteration in gut microbiome composition and function
    • endotoxemia (gut bacteria toxins translocated to blood circulation)
    • impaired gastric emptying
    • slowing of small intestine transit with poor nutrient absorption
    • accelerated colon flow and poor water absorption (resulting in diarrhea)
    • excessive activation of gut immune response and inflammation
    • significant reduction of splanchnic blood flow to gastrointestinal organs including the liver (flow can decrease more than 50% during exhausting exercise)
    • mild hepatic (liver) damage (normal function highly dependent on adequate blood supply)
    • stomach or rectal bleeding


Researchers found that during moderate exercise of intensity less than 75% VO2max, gastric emptying seems to occur at a similar rate as during rest. One hour on a treadmill at 71% VO2max does not affect nutrient absorption in the gut.

But, for example, at 90% VO2max, stationary cyclists experienced exercise-intensity-dependent GERD. About thirty seconds of maximal workout on an exercise bike can make many people physically sick. Runners experience more gut symptoms, including GERD, than cyclists.

Exercise stress duration of two hours or more at intensity VO2max 60% or higher seems to be the threshold beyond which significant gut distresses manifest, regardless of fitness status.


Healthy gut and balanced gut microbiome help reduce GI symptoms.


Many endurance-sports-induced gut symptoms are related to physiological changes resulting from the activity. These changes include alteration in gut nervous system activity, circulating gut hormones, gut blood flow, nutrient and electrolyte absorption in the gut, intestinal secretions, and gut motility. After an endurance event, these changes in healthy athletes are normally transient and reversible with adequate rest, bowel rest, and hydration.

Studies have found that improved gut absorption, which is generally associated with improved tolerance of fluids and foods during endurance exercise, reduced the chances of gut distress symptoms.

Short-chain fatty acids (SCFAs) produced by gut microbes influence a range of processes important for healthy gut functions. These include host-microbes immunity signaling, colon acidity, gut motility, gut permeability, gut wall lining cell proliferation, gut transit flow, water absorption and reabsorption, and stool consistency.

Not all athletes in endurance sports experience gut symptoms. As expected, the risk for gut symptoms is lower in those who are generally healthy and those with healthy gut functions.


References and Links

See the Full List of References for this Series

Table of Contents



About the Authors

Lillian So Chan is the founding editor of WellnessOptions, a print magazine and website, and author of the book WellnessOptions Guide to Health published by Penguin Books. With over thirty years of experience in journalism and editing, Lillian has established unique editorial directions for several award-winning publications. She has worked for Maclean’s, Canada's largest news magazine, and served as a Governor and Deputy Chairperson of the Board of Governors at the Simon Fraser University, British Columbia, Canada.


Manny W Radomski, PhD
is the former Director General of the Defence and Civil Institute of Environmental Medicine (DCIEM) of Defence Canada. He was a professor in the Departments of Physiology and Community Health in the Faculty of Medicine, and in the Faculty of Physical and Health Education at the University of Toronto, Canada.

He served as Scientific Advisor to the Chief of Air Staff, Defence Canada; Board Director of the Canadian Defence Research and Development Executive Committee; member on the NATO Research and Technology Agency’s Human Factors and Medicine Panel.

He is the former Editor-in-Chief of the Undersea Biomedical Research Journal and serves as a referee for the Aviation, Space, and Environmental Medicine Journal.

He has published on diving and aerospace medicine, human performance and protection, stress endocrinology, sleep, tropical medicine, and circadian disorders. Manny is a co-editor of WellnessOptions magazine and journal.

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