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What are the Main Causes of Gastrointestinal Problems?

GI problems are very common in endurance athletes. Understanding the different causes and symptoms is the first step in knowing how to prevent them.

Some athletes always get gastrointestinal issues when they train or compete, while other athletes eat and drink what they want and never have any problems. The symptoms athletes experience are very individual - some feel bloated, some get diarrhea. So, what cause these issues? If we understand the exact causes, we may be able to prevent them.


What are the Main Causes of Gastrointestinal Problems Infographic


Glucose and muscle function

Unfortunately, we don’t completely understand gastrointestinal symptoms and why they occur (for a summary of symptoms and their prevalence in athletes see this previous blog). One of the reasons we know so little about them is that symptoms are often specific to race situations and are very difficult to reproduce or simulate in a laboratory. We do have some laboratory studies and these give at least some insights. There are also some field studies that can help us a little. What we have learned from these studies is that there are generally 3 categories of causes of gastrointestinal stomach problems

  • Physiological
  • Mechanical
  • Nutritional


Physiological causes

Blood is normally pumped around the body to supply oxygen and nutrients to different parts of the body and to remove waste products. With exercise, blood flow to the working muscles is increased as well as blood flow to the skin for cooling purposes. This means that there may be less blood going to the gut. In fact, studies show that blood flow can be reduced by as much as 80%. Such low blood supply can compromise gut function. This has been suggested as a cause of GI symptoms such as cramping. In addition, anxiety can result in neural stimulation and in hormone secretion which in turn can affect gut movement. This in turn can result in incomplete absorption and loose stool.


Mechanical causes

GI issues can also be caused by impact or it can be related to posture. For example, in runners GI-problems may be the result of the up and down movements. The repetitive high-impact mechanics of running may cause damage to the intestinal walls. The bouncing of the gut during running is also thought to contribute to lower GI symptoms such as flatulence, diarrhea and urgency. Estimates of the incidence of occult blood (blood in feces) after a race range from 8% to 85% mostly because of the wide range of race distances in various studies. The longer the distance, the greater the number of runners with occult blood. As many as 16% of runners in one study report having bloody diarrhea on at least one occasion after a race or hard run. The mechanical trauma suffered by the gut from the repetitive bouncing of running in combination with reduced blood flow to the intestine (and oxygen deprivation of some parts of the intestine) are probably the cause of the bleeding. Presence of bloody bowel movements after an endurance event raise the possibility of severe health risks. Posture can also have an effect on GI symptoms. For example, on a bicycle (especially in aero position), upper GI symptoms are more prevalent possibly due to increased pressure on the abdomen as a result of the cycling position. "Swallowing" air as a result of increased respiration and drinking from water bottles can also result in mild to moderate stomach distress.


Nutritional causes

Although GI problems may occur completely independently of nutrition and, in fact, may occur in an athlete who avoid eating before and during a race. However, it is clear that nutrition can have a strong influence on gastrointestinal distress. Fiber, fat, and protein intake have all been associated with a greater risk of developing GI-symptoms. Also, fructose is known to cause problems, although when ingested in combination with glucose, this can actually improve GI-comfort. Dehydration, especially severe dehydration, will increase the risk of problems and will make symptoms worse. One study demonstrated a link between nutritional practices and gastrointestinal complaints during a half–Ironman distance triathlon. Gastrointestinal problems were more likely to occur with the ingestion of fiber, fat, protein, and concentrated carbohydrate solutions during the triathlon. Beverages with high density (hypertonic beverages with osmolarities >500 mOsm/L) seemed especially to be responsible for some of the reported complaints. The intake of dairy products may also be linked to the occurrence of gastrointestinal distress. Mild lactose intolerance is fairly common and could result in increased bowel activity and mild diarrhea. Although some risk factors have been identified, it is still unclear why some individuals seem to be more prone to develop GI-problems than others. To minimize gastrointestinal distress, all these risk factors must be taken into account, and milk products, fiber, high fat, and high protein must be avoided 24 hours before competition and during exercise. You can find a detailed discussion of preventative measures in this blog.


Related Articles

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  • de Oliveira EP, Burini RC, Jeukendrup A. Gastrointestinal complaints during exercise: prevalence, etiology, and nutritional recommendations. Sports Med. 2014 May;44 Suppl 1(Suppl 1):S79-85. doi: 10.1007/s40279-014-0153-2.
  • Rehrer NJ, van Kemenade M, Meester W, Brouns F, Saris WH. Gastrointestinal complaints in relation to dietary intake in triathletes. Int J Sport Nutr. 1992 Mar;2(1):48-59. doi: 10.1123/ijsn.2.1.48.


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