Running In The Summer: Are You Still Tight In The Intestine?

15. August 2017

Runners are familiar with gastrointestinal symptoms such as diarrhoea and nausea. If runs are very long, they can have more serious consequences. Leaky-gut syndrome increases the permeability of the intestinal wall: bacterial lipopolysaccharides and toxins find their way into the bloodstream.

Sport promotes health, this is undisputed. First and foremost, the muscles and cardiovascular system benefit from regular training. Nonetheless one should not exaggerate, as a recent meta-study shows. Intense training lasting over two hours, especially in the heat, apparently increases the permeability of the intestinal wall and the number of microscopic injuries in the intestine.

Extreme sports can lead to a leaking gut

Endurance sports events such as marathon races are attracting constantly increasing numbers of participants. Yet a lot of runners during or after the run have to battle with gastrointestinal complaints such as diarrhoea or nausea. The frequency of symptoms increases the longer the run is and the higher the temperatures involved.

Leaky-gut syndrome promotes subclinical inflammatory conditions

The critical limit was found by Australian nutritionists led by Ricardo Costa from Monash University in Melbourne when doing a meta-study involving the following parameters: anybody running longer than 2 hours with less than 60% maximum oxygen intake damages his or her bowel massively – regardless of individual fitness status. The long-lasting high volume strain increases the permeability of the intestinal wall, so that bacterial lipopolysaccharide (LPS), toxins and undigested substances are able to find their way into the bloodstream. This condition is called leaky gut or “permeable intestine” and can have serious consequences. Once in the bloodstream, in being foreign to the body these substances activate the immune system – a subclinical inflammation (silent inflammation) occurs, something in turn related to numerous chronic diseases.

Extreme sport changes blood flow in the intestine and intestinal motility

Gastro-intestinal ailments due to extreme sports are the result of complex, multifactorial interactions between the digestive tract and blood circulation, the immune system and the enteric nervous system. Roughly summarised, two different paths can lead to an extreme sports-induced leaky-gut: on the one hand there is altered blood flow during the sport in order to maintain muscular performance. The missing blood in the intestine can initiate a visceral hypoperfusion, which is followed by a visceral ischaemia. These processes damage the epithelial cells of the intestine by breaking the epithelium, or by destroying either the tight junctions or the proteins which regulate the tight junctions. On the other hand, stress hormones and the activation of the sympathetic nervous system may alter intestinal motility. These processes are expressed during extreme sports events as nausea, burping and bloating. The symptoms are often increased when the athletes eat or drink during training.


Two different paths can lead to leaky-gut in extreme sportsmen. Source: Costa, RJS. Et al. Aliment Pharmacol Ther. 2017 Jun 7.

Binding Protein I-FABP Measure For Damaged Intestinal Epithelium

The measure of damage to the intestinal epithelium used by the scientists was defined in terms of the blood concentration of the fat-binding protein I-FABP (intestinal fatty-acid binding protein). The small cytosolic protein is released as soon as mature enterocytes are injured or destroyed. Its concentration in the blood correlates to training-associated visceral hypoperfusion and the resulting ischaemia.

Moreover the exchange of various sugars, as well as the concentration of lipopolysaccharide in the blood, can be used as a measure of the permeability of the intestine and the expression of leaky gut. With respect to these parameters however, the outcomes of experiments carried out as part of these respective studies varied so strongly that a direct comparison was not possible.

Leaky-gut associated with numerous chronic diseases

Once endotoxins or other toxic substances have passed the permeable wall of the intestine, they rapidly enter the bloodstream and are distributed throughout the body. The immune system attempts to fight the invaders through inflammatory reactions and antibodies. This can have far-reaching consequences. Leaky-gut syndrome has already been associated with numerous chronic diseases in scientific studies. These diseases include for example rheumatism, psoriasis, eczema, irritable bowel, chronic fatigue syndrome, diabetes, celiac disease, multiple sclerosis, heart disease, migraine, autism and Parkinson’s disease. How long leaky-gut syndrome caused by extreme sport actually lasts is not yet clear. Initial studies have calculated this to be between one day and one week.

How can leaky-gut be avoided in extreme sports?

There is potential benefit to those involved in extreme sports in carrying bacteria which form short-chain fatty acids such as butyric acid. This is because these short-chain fatty acids are needed by the intestine to feed its epithelial cells and to keep the tight junctions functional. They also help in repairing small injuries to the epithelial cells and therefore could – when taken in through food or in probiotic form – be helpful companions for extreme sportsmen. The study situation in this area is nevertheless still so limited that it is not yet possible to provide clear recommendations on bacterial strains and on duration of ingestion.


The bacterial intestinal colonisation in the athlete is proportionally related to the extent of leaky-gut. Source: Costa, RJS. Et al. Aliment Pharmacol Ther. 2017 Jun 7.

Further measures to prevent leaky-gut being induced by extreme sports as according to the Australian scientists include:

  • Sufficiently drinking yet without drinking too much: athletes who started their training with normal hydration had less gastrointestinal symptoms than athletes who were dehydrated or over-hydrated, as over-hydrogenation can also lead to gastrointestinal symptoms. Normal hydrogenation should be maintained during training as far as possible.
  • Carbohydrate consumption: athletes who consumed 15 g carbohydrates before exercise and then every 20 minutes during a run at 60% maximum oxygen intake and 35 degrees Celsius had better parameters than athletes who drank only water during the same period. The consumption of gluten does not appear to influence the extent of intestinal ailments with extreme sports.
  • FODMAPs: anybody who as an extreme sportsman has a FODMAP-reduced diet evidently has fewer gastrointestinal symptoms than other athletes. This diet form, in which the amounts of fermentable oligosaccharides, disaccharides, monosaccharides and polyols are strongly reduced, is already a familiar part of dietary recommendations for irritated bowl patients.
  • Extreme athletes should abstain where possible from taking NSAIDs, because they promote leaky-gut syndrome. If intake of nonsteroidal antirheumatics is absolutely essential, they should definitely not be taken immediately before training.

The most effective method of avoiding gastrointestinal problems during and after exercise is a moderate workout. The Australian scientists’ meta-analysis showed that only extreme athletes who for instance had completed an ultramarathon of about 90 km or a 100-mile cycling race showed signs of a endotoxemia – as greatly increased lipopolysaccharide (LPS) concentrations in the blood. Athletes who trained under two hours had only slightly increased LPS values. Levels increased with training units that lasted longer than 2 hours (at 60-70% of maxV O2) and/or at high heat of more than 33 degrees Celsius. Those who exercise for a period shorter than 2 hours and at temperatures below 30 degrees will relieve their intestines – and presumably have more fun in sports.


Systematic review: exercise-induced gastrointestinal syndrome-implications for health and intestinal disease.
R.J.S. Costa et al.; Alimentary Pharmacology and Therapeutics, doi: 10.1111/apt.14157; 2017

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