Dr John P Wright
Updated: 20 July 2006
Anyone who has had the pleasure of partaking in equestrian sports will know the effect of relatively mild exercise on the horse’s gastrointestinal system. In man the cardiovascular and respiratory limitations on performance and the value of cardiovascular training in improving performance in aerobic sports is well recognised. The role of the gastrointestinal tract both as a limiting and sustaining factor in aerobic exercises is less well appreciated.
The spectrum of gastrointestinal effects of exercise is wide (Table 1). From the unlikely observation that swallowing decreases oxygen saturation in patients with chronic obstructive airways disease (1) and, by inference in exercising athletes, to the more serious rectal bleeding that may be due to ischaemic bowel (2).
The symptoms experienced by marathon runners are well described (3) with over 50% of runners at some stage of their careers reporting an urge to defaecate and just under half actually passing stools at least once while running (Table II). Abdominal cramps and anorexia are almost as common. This apparent high frequency of symptoms is in fact not necessarily the experience of each runner at each race as shown by a study where only one out of 86 patients actually complained of diarrhoea (2) (Table III). More serious, however, was the discovery of occult blood positive stool in 13% of the patients in this latter study. This did not appear to be related to the amount of training the runners had undertaken or the duration of their running experience. If blood loss is so common, what is the cause of this dramatic symptomatology ? A number of causes of occult blood in the faeces have been suggested including traumatic damage due to shaking of the bowel, a gastritis related to the stress of the exercise, and gut ischaemia. Splanchnic vessel constriction has been shown to occur in runners during severe exercise with visceral blood flow decreasing by 20- 30% of resting values (4). This decrease in blood flow appears to be related to exercise intensity and was unrelated to the state of fitness of the subjects. Although this would explain the positive occult blood found in experienced as well as inexperienced athletes (2) it is unlikely that splanchnic ischaemia would cause mucosal ulceration and blood loss. The relationship between exercise and splanchnic ischaemia is complicated by the observation that stressed baboons who are exercised have a greater decrease in splanchnic bloodflow than baboons that were only exercised (5).
Intestinal blood loss is one on the possible causes of “Runners anaemia” but haematuria, haemolysis and reduced erthropoeisis analogous to chronic disorder anaemia have also been implicated (2).
Reflux symptoms were reported in 11% of marathon runners (3).This contrasts with the 20% of patients with reflux that report that their symptoms become worse with exercise(6). The reasons for this are not clear. Delayed gastric emptying associated with high fluid volume intakes and in the case of cycling a position designed to potentiate reflux may all be important factors. The usual advise takes the form of advising running on an empty stomach and routine anti-reflux measures.
Gastric emptying and exercise have been known to be related for many years. In 1833 William Beaumont made the comment that mild exercise hastened and severe exercise slowed gastric emptying (7).This essentially summarises our knowledge to date(8). Exercise does not appear to have a marked affect on gastric secretion. In a study of 2 groups of athletes who exercised between 45 and 75 minutes after taking a meal, there was no change in the acid secretion in the athletes as compared to the controls(9). This is in direct contrast to a study on dogs with Pavlovian pouches(10). In these animals, gastric mucosal blood flow and acid secretion dropped quite markedly during exercise on a treadmill.
Gastric pathology may be produced by forced exercise in animals. In a study of rats exercised for three 45 minute periods interspersed with 15 minute rest periods, all developed gastric ulcers(11). The relevance of this to humans is uncertain. There appears to have been no work on the incidence of peptic ulceration in exercising athletes.
The effect of marathon running on bowel motility has also been quite difficult to understand. There appears to be an increase in gastric motility followed by reduction as time and severity of the exercise increases(8). An increased frequency of defaecation in marathon runners has been described as possibly due to a high fibre diet(12). This change in fibre intake was also thought to account for the tendency of exercise to normalize bowel function bringing those who were constipated or had diarrhoea back towards a normal stool(13). These changes in bowel function might be due to shaking of the sigmoid colon with more frequent emptying of the bowel rather than true decrease in intestinal transit time. This observation has resulted in 3 patterns of bowel function being recognised. Firstly, an early bowel action when starting exercise, secondly the diarrhoea that some athletes develop as they near exhaustion, and the final type of diarrhoea which occurs in patients who have collapsed(14).
Pancreatic function has not been well studied, but it would appear that exercise decreases pancreatic output in response to food and stimulatory hormones(10). Endocrine pancreatic function is also affected by exercise with insulin secretion decreasing in response to a falling blood glucose concentration associated with increased effort. As might be expected glucagon concentration conversely increases. Serum gastrin, motilin and VIP concentrations also have been shown to increase with exercise(12). The physiological role of these changes is not yet clear.
Finally, there appears to be a correlation to the amount of exercise taken and colonic cancer. In a study comparing sedentary workers to those with active physical employment, those who had been working at a desk for over 20 years had twice the incidence of colonic cancers as compared to physically active workers(15).
When managing patients complaining of gastrointestinal symptoms in relation to exercise, it is prudent to remember that conventional diagnoses are more likely than esoteric conditions associated with exercise. Symptoms of the irritable bowel syndrome often appear during exercise. In fact the physiological changes discussed above may exacerbate an underlying irritable bowel problem. Some unfortunate athletes may be forced to give up their sport because of uncontrollable symptoms during exercise.
Carbohydrate intolerances may account for some of the symptoms athletes experience. Reducing milk intake may bring quite dramatic improvement in some athletes with presumed lactose intolerance. Starch intolerances responsible for the 'splenic syndrome' may respond to reduced carbohydrate intake prior to exercise but generally results of such dietary manipulations are not rewarding.
In general exercise tends to make functional symptoms characterised by the irritable bowel worse. This may be overcome by standard advice on diet and gastrointestinal physiology but some athletes will be seriously limited to the extent of giving up competitive exercise.
OVERALL EXPERIENCE OF GASTROINTESTINAL SYMPTOMS IN 119 MARATHON RUNNERS
From Worobetz & Gerrard. 1985 (Ref 3)
HISTORY OF GASTROINTESTINAL SYMPTOMS IN 63 MARATHON RUNNERS
From Halvorsen et al. 1986 (Ref 2)