Stress, Gut Function and the Endurance Athlete

Getting to a race a realising the queue for the portaloo is longer than the supermarket queue in the height of lockdown can be a serious cause of stress for the already stressed athlete.

It is well documented that psychological stress can wreak havoc on gut function. Anecdotes about having to defecate before the start of (or even during) an endurance event are commonplace but when that urgency to go spills over into regular training and day to day life then the question needs to be asked – how stressed are you?


The vagus nerve is the communication link between the brain and the gastrointestinal tract, the so-called “brain–gut axis. What’s important to understand is that 80-90% of this communication is UPSTREAM (gut to brain) and 10-20% DOWNSTREAM (brain to gut) It oversees a vast array of crucial bodily functions, including control of mood and anxiety, immune response, digestion, and heart rate.

Overview over the basic anatomy and functions of the vagus nerve – Breit et al.


From a digestion perspective the vagus nerve controls the release of gastric acid and digestive enzymes. Chewing your food can only do so much to break down all the carbohydrate, fat and protein we chow down on an hourly basis. Stomach acid and multiple enzymes are responsible for breaking it down to the smallest pieces before the nutrients can be absorbed to fulfill their purpose. Protein in particular is a target for stomach acid and if the acid level is too low they can pass undigested and unabsorbed for utilisation by the body. 


Stress is a key disruptor to the link between the brain and the gut. That miscommunication can affect levels of stomach acid and digestive enzymes leading to an inability to digest food. Then when undigested food passes to the large intensities that when we can get bloating, gas and cramps etc.

Another consistent finding is that hormones released from the brain in response to stress can increase the natural ebb and flow of colon activity leading to an urgency to go. 

In others this hormonal activity can decrease the natural flow of the stomach leading to nausea, vomiting and reflux.


There is a very simple DIY test we ask our athletes to complete when they display signs of low stomach acid. Once that has been established we can design a program to minimise discomfort while dealing with the underlying issue.

We are all well equipped with the tools we need to override nerve-induced digestive distress.

Here are some simple tools for event days and during daily life to decrease the stress response which impairs our brain-gut communication:

  • Limit or avoid caffeine and other stimulants
  • Avoid dehydration to maintain optimum flow of blood and nutrients to the gut
  • Practice breathing exercises and/or meditation on a daily basis but in particular on event days
  • Ensure meal times are stress free zones and eat eat slowly and mindfully. 
  • Keeping meals away from your work space allows you to focus more on enjoying your food and gives you a break from work based decisions.

A lot of athlete’s don’t like to think they are particularly stressed. However if you pile together an 8hour work/study day, a 10-15 hour training week, travel to and from races (sometime across multiple countries) family life (or chaos!) there’s not a lot of room left for the mind to switch off. Until they understand this constant state of stress could be the cause of their digestive issues – that’s the only time they’ll actually agree to take a step back and re-evaluate.

And even if they don’t want to take 10 minutes to practice a box-breathing session – luckily there are temporary nutritional fixes to these digestive issues that we can integrate into an athletes program to minimize the GI discomfort without an dependency on restricting food or food groups.


Breit S, Kupferberg A, Rogler G, Hasler G. Vagus Nerve as Modulator of the Brain-Gut Axis in Psychiatric and Inflammatory Disorders. Front Psychiatry. 2018;9:44. Published 2018 Mar 13. doi:10.3389/fpsyt.2018.00044

Wilson, Patrick B., and Wilson, PhD, RD, Patrick. The Athlete’s Gut: The Inside Science of Digestion, Nutrition, and Stomach Distress. United States, VeloPress.