The Ultra Gut Impact: How to solve Runners Stomach.

The Ultra Gut Impact: How to solve Runners Stomach.

So you're fully up to speed, with what’s happening inside your digestive system during an ULTRA or or an Ironman endurance race, right?

It's surprising really that the very part of our body that provides crucial fuel to enable 5, 8, 10, 20, 30, 40hrs of exercise - is so poorly understood. There's even a bigger lack in clarity of what's happening inside the Gut when we put our body through these endurance feats. Then there's almost a total void on connecting the dots between exercise-induced gut health, and our systemic health.

So, maybe we shouldn't be surprised that nausea and GI distress (runners stomach) continues to be the #1 cause of DNF at ULTRA endurance races. But, beyond finishing endurance races, there's probably an even more serious sinister concern regarding exercise induced Gut health impacting our longer term health and lifestyle longevity.

Don't get me wrong - I'm an active advocate for the potential health-giving effects and longevity, that endurance sports brings to human beings...but it’s also good for us to be fully aware of what's happening beneath the skin, beneath the fascia, beneath the muscles, deep in the abdomen...and inside the Gut.

It's not about the Gut, but the impact of the Gut on everything else.

As a boy back in the early 80s' my dad took me to a seminar by one of the fathers of modern naturopathic medicine, Dr. Bernard Jensen. Besides my deep-interest in endurance sports from an early age, this seminar would set my mind on a career of study and interest in nutritional medicine, and more specifically a foundation of how gut-health affects systemic health. Thirty years later, with quantum leaps in medical science, diagnostic technologies, and genomic sciences - we are more certain now than ever on the relationship between gut-health and the well-being of human beings.

As of 2014, scientific literature pegs the microbial count inside the gut, as 10 times that of the overall cells that make up the human real terms, we likely have around 10 trillion cells that make up the human body, whereas we have over 100 trillion cells in the gut.

While on one level, we are directly concerned about the impact of extreme endurance exercise can have on the gut - but more important is understanding how impacting this gut micro-biome can directly affect the systemic health of the athlete. I’ll write more on this, but recently I published a short paper on the link of gut health to mental state in endurance athletes.

The ULTRA Gut Impact.

OK so firstly - when we say the Gut, we are talking about the (on average) 8 meter long hollow (should be) tube, running inside us, from the mouth to the butt. Different parts of this tube do different things -from the physical break down of foods, to chemical and enzymatic break down of foods, to absorption-assimilation of nutrients/fuel, to the production of hormones, reactive-oxygen species and chemical by-products, both good and bad. Under resting conditions, the gut is served well by nervous, hormonal, and circulatory systems, plus fluids, electrolytes, fiber and bacteria to fuel it to do its thing.

But in ultra extreme endurance exercise (think 4 to 40 plus hours), this well-orchestrated machine can be thrown into chaos - where structural, nervous, physiological and chemical norms go pear shaped and havoc reigns.

Understanding a little of what's going on will help a lot in training and racing ULTRA endurance events. So here's the story.

Exercise, the blood shunt from the Gut

The first area we'll talk about is the massive shift and shunt of blood in the body as a response to exercising muscles. This shunt of blood can happen fairly quickly with exercise, but is further intensified with prolonged exercise common in ultra-endurance sports.

During exercise, blood flow through exercising muscles can be 15-20 times greater than at rest. Where does this blood come from? Do we just produce more blood during exercise – well, of course not.

Exercise triggers (nervous and hormonal) mechanisms which dilate the capillaries that are typically closed at rest. Maybe only one-quarter of the capillary network is open during rest, but during exercise this can increase to 100%.

Conversely, exercise triggers nervous responses, and hormones such as epinephrine cause vasoconstriction of blood vessels feeding the internal organs. As muscles drive greater blood return through the veins during exercise, the heart has to step up in stroke volume and rate to keep up - and with this blood pressure rises. So the bottom line here is that there is a dramatic shift of blood away from the internal organs to the muscles, in order to keep up with the demands of exercise. Now when this is transient, and the intensity low, the body can maintain a baseline of blood flow through the organs (like the gut) and there's no issue. But extended periods of reduced blood supply to the gut, is more seriously concerning...and this brings us onto the second concerning-factor, heat.

Core temperature, and Endurance Exercise on Gut Permeability

Exercising muscles generate significant elevations in heat, inside muscle tissue, the skin (dilating capillaries, aimed at cooling the body), and also within the core of our body. Under normal healthy conditions and 'normal ambient temperatures' our core body temperature is 37C (98.6F) - and moves within a 1C range between morning and night. Exercise (similar to a fever from an infection), high ambient temperatures and humidity, can push our core body temperature up three, even four degrees C. If the core temperature rises further, heat stroke and even damage to the brain can occur.

But again, little is understood about how exercise and exercise-induced heat-dehydration further lowers blood supply to the gut, and more specifically creates an 'ischemic' (little-to-no oxygen -from blood) situation for the gut. Again, if this was for a small transient period - it might be manageable, but when the situation lasts 8, 12, 20, 30, 40 hours - that's another matter.

The scientific community is now well published with papers measuring the specific changes to the intestinal tissue integrity, as a direct result of heat generated from prolonged exercise. The tight junctions in the intestinal wall, can effectively break down with the heat from exercise, distressing the GI tract and increasing gut permeability.


Studies have shown how food molecules (peptides from proteins, inflammatory endo-toxins in the gut) which, under normal rest conditions are not seen in the blood stream, suddenly increase in presence in the blood, post-exercise. Other studies have photographed the dramatic damage to intestinal mucosa (gut lining) as the tissue suffers from repeated reductions in blood supply over multiple days.


So now you have a combined situation of reduced blood supply to the gut, and heat build-up in the gut - leading to a direct threat to the integrity of the gut lining.

But the threats and damage from an endurance sports lifestyle don’t stop there.


Simple Sugar/Fructose adding further, Gut inflammation

We have already published articles that summarize the concern over intense simple-sugar ingestion by endurance athletes, and the potential health risks this places on the athlete and the longevity of their lifestyle-sport.  But let’s come back to the Gut, and focus on the sugar (sucrose) and fructose impact to the gut alone.

In a recent study, documented in the Journal of Nutrition and Diabetes, it was shown that excess free fructose consumption, creates a fructose-reactive state in the gut resulting in the formation of advanced glycation end-products (pro-inflammatory mediators) in the gut. Furthermore medical studies, even from over 10 years ago, began showing that 3 in 4 tested people have a fructose-intolerance triggering digestive disturbances like pain, bloating, flatulence, and diarrhea.

Foods high in fructose? Dates, syrups, cereals, energy-gels, sports-drinks, energy-bars, dried-fruits, Colas, ‘sports-drinks’. If you’re an endurance athlete you’ll know all these foods have been consistently included in packaged endurance foods for the past 30 years. 

We then see the food industry attempting to bypass this by adding artificial sweeteners to foods – including endurance food energy foods. But this only further complicates the problem, as these too are well published in terms of their impact on gastro disturbances like gas, bloating and flatulence.

And now to add insult to injury, we see careless sports nutrition marketeers, mixing refined sugar carbs with ‘Band-Aid’ natural compounds, to help mask the symptoms of sugar-induced gut damage.  


Minimize the ULTRA Gut Impact - The Big 3

  •   Avoid high fructose/sugars fuels and drinks, in training-racing: Take a look at your training-racing fuels and do your homework on what you’re throwing into your body.  Commonly they're high in sucrose, fructose.  Move to mixed starch-fat complex fuels in HIIT and racing. Download the Right Fuel Right Time Fueling Guide to read up on better options, and to learn about how optimize your fat-carb oxidation, to minimize carb intake - and thereby reduce runners stomach.
  • Take L-Glutamine:  To further enhance your body’s resistance to exercise (heat) derived digestive disturbance, include L-Glutamine in your training (SFuels TRAIN), racing (SFuels RACE+) formulas and post recovery (SFuels Revival) from heavy HIIT sessions or racing. A number of studies have shown how L-Glutamine directly minimizes the heat-shock impact, to the digestive system, and muscle-tissue inflammation.
  • Keep cool, Keep hydrated:  The very nature of endurance exercise will raise our body temperature. But using cool water over our body, and in drinking cool-cold water will help to reduce the known effects of exercise (heat)-driven digestive disturbances.  Obviously you need to become well-sensitized to your own sweat output and fluid intake needs – but make sure you test and document this (weight before/after training, minus total fluid intake of training)…and do this for a variety of temperature/humidity conditions so you know how your body needs/processes water.

The Finishing Comment –

An endurance athlete can train for over a year in the preparation for a major ultra-endurance race. It’s amazing how common runners stomach is in this sport.  Not because of under-training, not because of muscle soreness or tiredness – but simply because of the gut impact, or runners stomach.

Follow the Big 3 above and minimize the Ultra Gut risks, and raise your chances of finishing and posting your new personal-best times.




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