As a nutritional therapist specialising in working with endurance athletes, people often come to see me seeking help with gastro-intestinal problems that they experience during training and, more commonly, racing. I have written briefly on this subject before with some practical tips to avoid GI problems while racing, most of which are appropriate for anyone competing in endurance sports. You can find these tips here.

I would now like to look at this issue, which affects so many athletes, in more depth. In the last few years, a number of studies have been published which suggest that there may be some additional strategies worth trying if you experience GI issues while training for or racing endurance events. Some of these are designed specifically to help mitigate the effects of strenuous exercise on the gut. Others may be appropriate if you suffer from chronic digestive health issues, which are then exacerbated during racing. In this blog, I am going to look firstly at the types of gut problems that may be experienced and why this occurs, and secondly at some strategies that may be worth trying to prevent these problems from affecting your race, based on the latest research.

gut health for endurance athletesGastrointestinal symptoms are estimated to affect 30-50% of endurance athletes. These are generally mild and of no risk to health, although they may have a significant adverse effect on race outcomes. Symptoms may relate to the upper abdomen, such as reflux/heartburn, burping, bloating, stomach cramps or pain, vomiting and nausea, or they may be lower abdominal in nature eg stitches, flatulence, urge to defecate, diarrhoea or intestinal bleeding1. Reduced blood flow to the intestines during intense exercise is believed to be one of the main contributors to GI symptoms, and this can be exacerbated by being dehydrated. This slows gastric emptying, the delivery of nutrients and fluid to the muscles, which often results in a distended stomach, gut discomfort and a negative effect on performance due to the inability to Take on fuel as needed. Racing in the heat and the duration of races may exacerbate this, as may the mechanical action of running, which puts a further stress on the gut. This may be why GI problems are most often experienced on the run, although being in the aero position on the bike for long periods, which might compress the GI tract, may also be a factor.

This reduced blood flow to the intestines is also thought to contribute to increased permeability of the gut wall, although it is not yet clear whether this necessarily contributes to GI symptoms. It is unlikely to be the only contributor. However, it is an area where research has taken place and, if you do regularly experience gut issues, particularly when racing in the heat, it may be worth considering some of the strategies that have been shown to reduce intestinal permeability. Before looking into those strategies, I’ll take a look at what intestinal permeability is and why it might be a problem. Tight junctions between the cells that make up the gut wall control what passes through it into the blood stream, for example glucose molecules. The stress of heat and oxidative damage during exercise both cause disruption to the proteins that form these tight junctions, such as zonulin and occludin, making them more permeable so that substances that do not normally cross the gut wall, such as endotoxins produced by gut bacteria, may now do so. This in turn may trigger an inflammatory response, potentially resulting in gastro-intestinal distress, such as cramping, nausea, vomiting. Fluid is no longer well absorbed and may accumulate in the stomach or result in diarrhoea.2 Increased intestinal permeability may also occur outside of intense exercise and has been linked to irritable bowel syndrome (IBS) as well as various immune-mediated disease states. Athletes with a history of IBS may be more likely to experience GI issues on race day.

So, having examined why GI issues may happen, let’s consider what steps you might be able to take to prevent them:

1. Train the Gut

If you only experience GI issues during racing or some training sessions, in addition to following the basic pre-race nutrition strategies which are outlined in the final section of this blog, it may well be worth “training your gut” so that it is better able to handle nutrition and hydration on race day, working on the basis that the gut is an adaptable organ. Here’s a summary of the steps that you can take from a recent paper on the subject by leading sports nutritionist Professor Asker Jeukendrup3:

  • Train with relatively large volumes of fluid, to train the stomach to handle it.
  • Train immediately after a meal to get used to exercising with food in the stomach
  • Train with relatively high carbohydrate intake during exercise, to learn to tolerate this, eg 60-80g/kg (or up to 60g/kg if you currently use c 30-40g/kg)
  • Develop your race nutrition plan and practise it on long rides and brick sessions
  • Generally increase the carbohydrate content of your diet, to improve the capacity to absorb and oxidise it.

With the first three recommendations, you would not need to do this every session but perhaps once a week for each recommendation. If done consistently over a few weeks, you may see an improvement in GI symptoms next time you race.

2. Experiment with a gluten-free diet

If you are someone who suffers from chronic digestive health issues (ie not just while racing), you may want to consider making dietary changes over a longer period. It has become increasingly popular in recent years for athletes to experiment with gluten free diets, which cut out all sources of wheat, rye and barley. The results of an online survey among 910 athletes who did not have coeliac disease (an auto-immune where you must strictly avoid gluten) found that 41%, including some world and Olympic medallists, followed a gluten free diet 50-100% of the time and 84% of these respondents indicated GI symptom improvement with the removal of gluten.4 But it is important to note that 57% were self-diagnosed as ‘intolerant’ to gluten. It is suggested that you consult a health professional before removing gluten from your diet to ensure that it is really necessary and, if so, that your nutrient needs are being fully met after the change. It’s also worth being aware of a recent study by the same authors as the survey which showed that a short-term gluten free diet (1 week) in non-coeliac competitive endurance cyclists showed no benefits for performance or GI symptoms versus a diet containing gluten.5 However, it was only a short period eating gluten free and the subjects also did not have a history of IBS, so the result might have been different if cyclists with IBS had been studied and given a gluten-free diet for a longer period. It may be worth trialling a gluten free period prior to a race. I suggest at least three months. See another one of my blogs here for further details. 

3. Undertake a Low FODMAP diet 

If you regularly suffer from GI problems, you may also want to try following a low FODMAP diet. FODMAP stands for Fermentable Oligo-, Di-, Mono-saccharides and Polyols. These are types of carbohydrate that may be fermented by bacteria in the small intestine, resulting in symptoms such as abdominal pain, bloating, diarrhoea and flatulence. Lactose (found in dairy products), fructose (found in fruit, honey and many sports nutrition products) and galacto-oligosaccharides (found in beans, lentils and chickpeas) are all FODMAPS – the amount in each specific food varies. Fructans, found in wheat, rye and barley, are also FODMAPs and it has been proposed that these, rather than gluten, which is a protein, may be the reason for improvements in some non-coeliac people’s GI symptoms when they cut out these foods, which also happen to contain gluten. The survey of 910 athletes discussed above also found that 55% of the athletes avoided one of more FODMAP categories, primarily lactose, and that 83% of those reported improvements in their GI symptoms.6 The FODMAP diet is complex and involves eliminating and then reintroducing foods, which I recommend you do with the support of a health professional. A 2016 review of 22 studies supported the efficacy of a low FODMAP diet in the treatment of functional gastro-intestinal symptoms 7 so it may be an avenue worth following with appropriate support.

To learn more about gluten free and low FODMAP diets in athletes, I recommend listening to the Endurance Planet podcast with Dr Dana Lis, who conducted both the survey and gluten free performance trial. You can access it here.

4. Try taking the supplements colostrum and zinc carnosine

There have been a number of studies in recent years looking at nutritional compounds which might to help reduce the intestinal permeability shown to increase with intense exercise, particularly in the heat. Two may be worth trying if you suffer from GI distress during races which might be linked to increased intestinal permeability: bovine colostrum, a natural source of growth factors, and zinc carnosine. A word of caution on using colostrum if you are subject to drug testing for banned substances: colostrum is not prohibited per se, however it contains certain quantities of IGF-1 and other growth factors which are prohibited and can influence the outcome of anti-doping tests. Therefore WADA does not recommend the ingestion of this product.8

Back in 2011, a study showed that colostrum truncated a rise in intestinal permeability by 80% following exercise, whereas intestinal impermeability in those taking a placebo rose by 250%.9 Five years later, the same authors conducted a follow up study which looked at the impact of both colostrum and zinc carnosine on heavy exercise induced increases in gut permeability, both separately and combined, and also compared with placebo. Gut permeability was found to rise three-fold after exercise. Taking either zinc carnosine or colostrum truncated the rise by 70% after 14 days of supplementation. But when the two were taken together, a 30% reduction was seen after just two days of supplementation.10 A 2017 double blind placebo controlled study then found that intestinal permeability was lower in athletes who took colostrum for 20 days than in those who took whey protein.11

But not all studies on colostrum are positive. For example, a 2014 double-blind, randomised, placebo-controlled study found that taking bovine colostrum for seven days before exercising for 90 minutes in 30 degree heat had no effect on the physiology or performance of highly trained endurance athletes, nor untrained individuals.12 It’s important to bear in mind that it is not known conclusively that endotoxaemia as a result of increased intestinal permeability contributes to GI distress during heavy exercise or exercise in the heat, but if you have a history of GI issues during racing, it may be worth trying colostrum as a strategy, taking it for at least two days in conjunction with zinc carnosine or for 14-21 days on its own prior to an endurance event.

5. Supplement with a probiotic-prebiotic combination, and possibly alpha lipoic acid and n-acetyl carnitine as well

Another strategy that may be worth trying is to supplement probiotics, the beneficial bacteria found in the gut that play a very important role in both digestive health and regulation of the immune system. A 2012 study 13 found that 14 weeks probiotic supplementation prior to an intense exercise test by 23 trained men reduced gut permeability compared with placebo. A 2016 study 14 then suggests that supplementing probiotics may be best combined with a prebiotic, eg the fibre fructo-oligosaccharide (FOS) which provides fuel to enable the probiotic bacteria to colonise in the gut, and possibly anti-oxidant supplements such as alpha lipoic acid and n-acetyl carnitine as well. In the study of 30 recreational long-distance triathletes who supplemented for 12 weeks prior to doing an Ironman event, it was found that the participants who took a probiotic-prebiotic combination had significantly reduced endotoxin levels six days after the race, while those taking placebo showed no reduction. A further group who took the anti-oxidants in addition showed lower endotoxins both pre-race and 6 days afterwards, suggesting that this might be the best strategy to help minimise GI issues during the event. It is also worth noting that only the placebo group showed an increase in intestinal permeability after the event. These studies suggest that pro-biotics, pre-biotics and anti-oxidants may help to maintain tight junction integrity in the gut wall during long duration exercise, although they do not look at instances of GI distress in the same subjects, so the link is still not conclusive. But it is another strategy to try.

6. Plan your diet for the 24-48 hours before an endurance event:

Finally, some basics for all athletes to help avoid GI issues in your races. Well established dietary guidelines for the period 24-48 hours beforehand include increasing the proportion of carbohydrate in your meals and snacks to help start the race with full muscle glycogen stores, avoiding high fibre foods (choose refined carbs such as white bread and rice instead of wholegrains), avoiding high fat foods and too much protein (which might slow gastric emptying), avoiding spicy foods and anything you haven’t eaten before, and avoiding caffeine if you find it causes loose stools. Plan what you will eat in advance as far as possible and make sure that you have tried out your pre-race dinner and breakfast in training. Drink regularly through the day to ensure you start the race pre-hydrated and add an electrolyte tablet to your water if you sweat heavily or it is forecast to be a hot day, in order to ‘sodium load’.

This has been a lengthy exploration of some of the recent research into ways of potentially avoiding gastro-intestinal distress during endurance events, but given the number of athletes affected, I hope it will have been of interest and that you have taken away some new ideas to try.

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



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8 WADA prohibited list – Q&A section:

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