Endurance racing DNF, What’s the Number 1 Reason?  Research points to Runners Stomach.

Endurance racing DNF, What’s the Number 1 Reason? Research points to Runners Stomach.

You may get away with certain training approaches when you’re out there for five hours or less – but when your Ultra or Ironman race could be six, nine, twelve, twenty or thirty hours then don’t kid yourself – those approaches offer no guarantee of a finish.

Once again the reasons for DNF were shown again this year at the (2017) Western States 100 Endurance Run. While an impressive courageous run by Jim Walmsley, his race was unfortunately cut short by the reasons of DNF we bring out below. Jim’s winning day at the Western States will come – he is way too good, and too smart not to learn from this and course correct.

As the research is showing us, ultra-long endurance exercise places a whole new load on our system creating certain symptoms that dramatically raise the risk of a ‘DNF’ (Did Not Finish). Firstly the reasons for a DNF are NOT, what you probably think they are!

Here’s the research –

Competitors in two premier 100mile ultra-marathons, the Western States and Vermont 100, were invited to complete a post-race questionnaire. 500 athletes responded and provided data on reasons for their DNF (did not finish). Of all the reasons for not finishing these ultras, vomiting, and nausea, was the highest factor at 23% of all respondents. Commonly you will hear these symptoms collectively referred to as runners stomach. 

Since the early 1990s, researchers began noticing the higher prevalence of gastrointestinal distress during endurance events like triathlons, Ironman races, and ultra-marathons. The effects were not simply slower race times and DNFs – but even more serious conditions like bleeding in the gastrointestinal system, endotoxemia, and hospitalizations. So began a wave of studies during these races, to find the cause.

In 2012, two Ironman races, a half-Ironman, 1 marathon and a 150km cycling race, were used to evaluate 221 athletes, to examine the relationship of diet and gastrointestinal issues during an endurance competition.  Ironman athletes reported an average higher intake of carbohydrate (~65g/Hr.), versus the cycling and marathon events.  Higher carbohydrate intake was positively correlated with increased scores of nausea and flatulence (runners stomach).

One year later a team studied a group of ultra-runners competing in the Javelina Hundred desert ultra-endurance race where 89% of the group experienced nausea.  Interestingly, fluid consumption and the percentage of fat consumed was higher in runners without gastrointestinal distress.

Actually in 2015 the 161-km Western States Endurance Run was used to examine the incidence of gastrointestinal symptoms for finishers and non-finishers. Of the non-finishers surveyed 35% reported gastro-issues, with nausea (runners stomach) being the number one reason in over 90% of these athletes.   And here again in the 2017 Western States we see that no one is immune to this – including the elite pro runners, with Jim Walmsley’s stomach shutting down and causing him to pull out – after leading through the first 100kms.

In a recent study, pre-race dietary factors were evaluated in giving rise to gastrointestinal (gut) distress during the cycle-run legs of triathlons.  It was found the morning (of the race) calorie and carbohydrate (higher) intakes were modestly associated with upper gastrointestinal distress (vomiting, nausea), whereas caffeine intake had a correlation to lower gastrointestinal distress (abdominal cramp, flatulence 0 i.e. runners stomach).

In reviewing published scientific papers on this subject, a consistent theme is emerging on the amount and rate (intensity) of carbohydrate intake and also the type and ratio of carbohydrate being consumed.

The nutritional profile of high-volume sports bars, gels and drinks used in endurance races, shows noticeably high sugar (cheap to manufacture), low to no fat or fiber.  Additionally what stands out in these formulations is the very high simple-sugar ratios – and the types of highly intense sugars used, like, cane sugar syrups, fructose, maltodextrin, glycerin, rice syrups, honey and glucose syrups.

Eating one of these leading (cheap simple sugars) bars, is the equivalent to taking over 6 teaspoons of sugar.  Add to that drinking 500ml/15-20 ounces of sports drink – for another 6 teaspoons of sugar.  Now you are at 12 teaspoons.  Maybe an athlete could get away with this in a three hour race, but consuming these for 6 to 40 hours, would certainly raise your DNF risk, due to gastrointestinal symptoms and nausea.

Whole foods (bananas, fruits, sandwiches) have a balanced mix of macro-nutrients with far less simple sugars and at lower concentrations.  This coupled with some natural fiber results in a food that the gut can digest without the distress during racing and training.  If you want to know what’s actually happening physiologically to your Gut during an ULTRA or Ironman – read THE ULTRA: GUT IMPACT article here.

For high-intensity racing (or training) lasting less than 2 hours, you need not be too concerned with all this – since enough muscle glycogen is available to fuel working muscles.

The bottom-line is, for those of you competing in 5 plus hour endurance races who want to minimize the risk of a DNF from simple sugar carb fueling – replace the high sugar/carb products, train your body to burn fat for fuel.  Train and Race on these lower-carb, healthy fat products from SFuels to avoid Runners Stomach



  1. Gastrointestinal distress is common during a 161-km ultramarathon.  Stuempfle KJ, et al.  J Sports Science. 2015
  2. Nutritional intake and gastrointestinal problems during competitive endurance events.  Pfeiffer B et al. Med Sci Sports Exercise. Feb 2012.
  3. Dietary and non-dietary correlates of gastrointestinal distress during the cycle and run of a triathlon.  Wilson PB et al.  Eur J of Sports Science. June 2016.
  4. Association of gastrointestinal distress in ultramarathoners with race diet. Stuempfle KJ et al.  Int J Sport Nutr Exerc Metabolism. Apr 2013.
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