Gastro-intestinal (GI) problems are common during endurance sports events, particularly marathons, ultra-distance runs and the run during a long distance triathlon. Typically, 30-50% of competitors in such events may be suffering from symptoms such as heartburn (reflux), bloating, flatulence, diarrhoea, nausea and abdominal cramps or other discomfort.1 Generally females are more likely to experience GI issues than males, and a recent study of 221 athletes competing in Ironman events demonstrated that those with the most serious GI symptoms were more likely to have a history of GI problems.2 Such symptoms may impair performance and even prevent you from finishing the race.

When you exercise at a high intensity, this leads to a redistribution of blood flow as your body’s priority is to nourish the muscle rather than to digest food in the gut. This leads to delays in emptying your stomach, which may result in feelings of fullness, bloating, stomach cramps, nausea and even vomiting. If you become dehydrated, as may occur particularly in the latter stages of a race, the effect is exacerbated. Another cause of GI symptoms is thought to be the ingestion of sugars in sports drinks. Some people may have issues in tolerating sports drinks with a higher concentration of sugars such as glucose or fructose. A 1992 study found that GI problems were more likely to occur with the ingestion of fibre, fat protein and concentrated carbohydrate sources during a triathlon. 3

The wide variation of results in studies of nutritional intake and GI problems during endurance events suggest that individuals respond differently to foods and nutritional products such as sports drinks and gels. If you do experience these types of symptoms during your events, you approach to resolving them is likely to be an individual one of trial and error. Working with a nutritional therapist may be helpful in identifying your personal triggers for GI distress.

Here are some actions that you can take in order to minimise the risk of experiencing GI problems in your next race:

  • Train with the products that will be provided at the race to check that you can tolerate them, and find alternatives you can carry if not.
  • Practice your race hydration strategy in training
  • Avoid fibre-rich and high fat foods both during the race and in the 2-3 days before it.
  • If you have a history of GI distress in races, consider avoiding gluten and/or dairy in the week or two prior to your race only.
  • If you are planning to use caffeine as an ergogenic aid, try it out in training first.
  • Eat your last meal at least three hours before the start of the race.
  • Choose a sports drink with less than 7% concentration of sugars (ie less than 7g per 100ml) and consider avoiding brands containing fructose.
  • Try stress management techniques such as meditation to reduce race day anxiety, which may contribute to GI problems.
  • Consider contacting a nutritional therapist to arrange for gastro-intestinal function tests or a carefully managed exclusion diet

If you do start to suffer GI symptoms during your race, slow down to allow your heart rate to drop and increase blood flow to your stomach, stop taking in further foods or sports drinks, and drink some plain water.

Jo Scott-Dalgleish BSc (Hons) is a BANT Registered Nutritionist, writing and giving talks about nutrition for endurance sportBased in London, she also works as a Registered Nutritional Therapist, conducting onetoone consultations with triathletes, distance runners and cyclists to help them eat well, be healthy and perform better through the creation of an individual nutritional plan. To learn more about these consultations, please visit


1 B.Pfeiffer. Nutrition- and exercise-associated gastro-intestinal problems. In Sports Nutrition from Lab to Kitchen. Meyer & Meyer Sport. (Maidenhead, UK.) 2010. Page 135-39.

2 Pfeiffer et al. Nutritional intake and gastro-intestinal problems during competitive endurance events. Med Sci Sports Exerc. 2012; 44(2):344-51

3 Rehrer et al. Physiological changes and gastro-intestinal symptoms as a result of ultra-endurance running. Eur J App Physiol. 1994; 64:1-8 cited in A Jeukendrup. Nutrition for Endurance Sports. J Sports Sciences. 2011; 29 (S1):S91-99