By Katherine Schreiber

Many people still believe that too much exercise isn't a bad thing—and that, if you're going to be addicted to anything, exercise is the healthiest drug of choice. A growing body of evidence is proving these assumptions wrong, however. One recent study adds to this pile of proof by demonstrating a link between excessive physical activity and impaired gut functioning.

A team of Australian researchers led by Ricardo Costa systematically reviewed research into exercise-related gastrointestinal dysfunction and injury conducted over the past 20 years. Costa's team found that as exercise intensity and duration increased, so too did the risk of exercisers' guts being damaged or otherwise impaired. The damage appeared to arise from injuries to intestinal cells, causing the gut to leak disease-causing molecules normally contained within the intestine into excessive exercisers' bloodstreams. People who experience this so-called "leaky gut" often feel nauseous, have aches and pains in their stomach and bowel area, and experience cramps, bloating, and gas.

Flickr | jpalinsad360
Source: Flickr | jpalinsad360

Exercising for two or more hours at 60 percent VO2max seems to be the tipping point for exercise-related gut issues to occur, as Costa et al. found. (VO2max is a term for how much oxygen a physically active person is able to utilize during intense exertion. 60 percent of VO2max means that a person is exercising at 60 percent of their maximum capacity to utilize oxygen—in other words: at a moderate to strenuous pace. VO2max varies based on age, fitness levels, and sex. More conditioned athletes can perform without feeling the need to stop at higher VO2maxes.) High temperatures also appear to raise the likelihood that exercisers will experience such problems.

Costa et al. are careful not to dissuade people who have no history of gastrointestinal issues from ever engaging in strenuous exercise—nor do the researchers recommend that individuals struggling with gastrointestinal issues avoid exercising to be on the safer side. They do, however, caution exercisers to be aware of the potential risks inherent in pushing oneself too hard for too long (i.e. for more than two hours, especially in heat). And while they recommend that folks with GI problems stay away from extreme exercise (and avoid exercising in extreme weather conditions) Costa et al. actually point to evidence in their review that low to moderate levels of physical activity can help individuals struggling with GI issues to manage their symptoms.

This research adds one more entry to the often-overlooked but still growing list of downsides to excessive exercise. Other negative consequences of too much time spent exerting oneself include heart problems, impaired immunity, increased risk of injury, overtraining syndrome, loss of menstrual cycle in women (which may court osteopenia or osteoporosis).

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