exercise-induced diarrhoea

If you occasionally have trouble with your digestive system when you exercise, you're not alone. Scientific surveys reveal that up to 30 per cent of all runners suffer from abdominal cramping at one time or another, 50 per cent feel an occasional urge to defecate, 30 per cent actually have to defecate, and 25 per cent experience diarrhoea during or shortly after races.

What causes all this digestive-system mayhem during running? Running itself may not be the prime culprit. Research suggests that 15 per cent of runners who experience intestinal problems when they train may actually be suffering from irritable bowel syndrome. Another 15 per cent may be lactose intolerant, which increases the risk of diarrhoea. Finally, 25 per cent may be eating diets which are overly high in fibre.

Even so, that still doesn't explain why the other 45 per cent of defecation-prone runners feel the need to evacuate on the run. Perhaps the most popular theory to account for these cases of 'runners' trots' is that exercise decreases 'intestinal transit time' - the amount of time it takes for food to pass from the mouth to the rectum. Although this theory seems reasonable enough, supportive evidence is somewhat lacking.

One scientific investigation was able to show that individuals who began a regular exercise programme decreased their average intestinal transit time from 35 to about 24 hours. Unfortunately, dietary records weren't kept, so it's possible that the subjects adopted healthier, higher-fibre diets as they became fitter, a change which would have speeded up transit time even if no exercise were completed. In later research in which the food intake of exercisers was tightly controlled, exercise had no effect at all on gut transit time.

Reduced blood flow?
Another possible explanation for exercise-related diarrhoea is that strenuous activity reduces blood flow to the digestive system - sometimes by as much as 80 per cent (blood which would ordinarily flow to the intestines is diverted to muscles and to the skin for cooling). This loss of blood may induce a dysfunctional state in the intestines, increasing the risk of diarrhoea and cramping.

Various lines of evidence support this 'blood-flow hypothesis'. For one thing, when exercisers become dehydrated, the incidence of digestive-system problems increases, often fairly dramatically (in one study, 80 per cent of dehydrated athletes experienced digestive-system complaints). The problem is that, as dehydration develops, extraordinarily small amounts of blood are allowed to trickle towards the intestines, making it difficult for them to function normally. To reduce the risk of dehydration and gut maladies, it's probably wise to consume about 12 ounces of fluid immediately before potentially dehydrating exercise, along with three to four swallows every 10 minutes as exercise progresses (carrying a water bottle with you can be a pain, but it can also keep you out of trouble).

However, not everyone is convinced that blood-flow restriction is the prime cause of digestive upsets during exercise. For one thing, blood flow to the intestines is reduced during many vigorous activities, including running, swimming, cycling and cross-country skiing, yet only running seems to be linked with a high risk of diarrhoea. It' s possible that the upright posture and mechanical jarring and jiggling of the contents of the colon during running are all that is necessary to stimulate an increased frequency of visits to the porcelain throne.

What to do about it
What are some practical ways to prevent digestive-system problems during exercise? If you suspect that you have a lactose intolerance, you might try out some lactose enzyme pills (available over-the-counter at most chemists). In addition, caffeine should be avoided before and during exercise since it can act as a cathartic.

If you're troubled with diarrhoea during races, a low-residue (low-fibre) diet during the 24 hours before the race might be helpful. The only problem here is that low-fibre diets are often high in protein and low in carbohydrate, so they might interfere with your pre-race carbohydrate loading. This could lead to decreased performances in your marathons and half-marathons. For these longer events, you might consider carbo-loading with one of the highly concentrated, commercially avail able sports drinks. You should try this out first during your training, however, and not on the eve of an important race.

Finally, getting into the habit of evacuating your bowels at a regular time each day can help you avoid the problem of trying to run with a fairly full colon. If you can't seem to get into a routine, the consumption of warm liquids or a light meal (such as a small bowl of easily digestible hot or cold cereal) can often help. Putting something in your stomach allows you to take advantage of the 'gastro-colic reflex', in which food placed in the gullet naturally stimulates the large bowel to empty itself. Don't forget to allow enough time between eating and exercise to take advantage of this phenomenon.

As a last resort, some athletes have been helped by taking an over-the-counter medication called loperamide about 30 minutes before they begin exercising. Another medication called dioctahedral smectite may be even more effective (one study determined that the latter drug eliminated digestive-system problems in 72 per cent of triathletes who were prone to diarrhoea and cramping during exercise). Unfortunately, most prescription anti-diarrhoeal drugs contain chemicals which have a negative effect on sweating, so they represent a poor choice.

('EXERCISE-INDUCED DIARRHOEA: When to Wonder,' Medicine and Science in Sports and Exercise, vol. 26 (5), pp 523-526, 1994)