Why does running make me go to the bathroom?

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I’m training for a half-marathon but have noticed an odd setback: Every time I run more than a few miles, I get diarrhea! Why does this happen, and can I prevent it?

My athletic patients often describe having to pause mid-run to answer an urgent call of nature. Compared with those in other endurance sports, runners seem to experience worse gastrointestinal symptoms – and younger female runners are at higher risk.

Moderate to intense exercise has several effects on the gut, including a painful “stitch” in the side of the abdomen, nausea, diarrhea and even bleeding, whether microscopically or in plain sight.

The latter happened to Yohann Diniz, a French race walker, at the 2016 Rio Olympics. Diniz was the favorite to win the men’s 50-kilometer event. About halfway through the race, he began to appear ill. Diniz then had a bowel accident and, even scarier, it seemed to be bloody. Diniz collapsed, but with some help, got back up and finished in eighth place. (Diniz’s story has a brighter sequel: He continued to compete afterward and remains a world record holder.)

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The effect of exercise on the gut

Exercise speeds up the rate at which our colon pushes contents forward, which is why I advise my patients struggling with constipation to get up and move each morning. However, exercise can also relax the normally tight sphincter connecting the esophagus to the stomach and delay how quickly the stomach empties. These other changes may lead to worsened reflux and nausea.

Studies have found that, when we’re maximally exerting ourselves, the blood flow to our guts can fall as low as 20 percent of its baseline as our body shunts blood elsewhere, such as to the leg muscles. In rare cases, low blood flow can lead to a condition in which the colon doesn’t get enough oxygen, or ischemia, and this itself triggers inflammation and bleeding. Fortunately, these episodes, if they occur, often resolve on their own once the exercise is halted.

One study found that 12 percent of long-distance runners had pooped their pants while running. Fecal incontinence can be because of a combination of factors: the urgency of a spasming colon coupled with the mechanical pressure of running that can make it harder to control the anal sphincter. If this is an issue you’re familiar with, talk to your doctor, as you may benefit from pelvic floor physical therapy to strengthen those muscles.

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How to minimize GI problems while exercising

No one wants to see all their hard work go … down the toilet on race day. Here are my tips as a gastroenterologist to minimize that risk:

Avoid meals two to three hours before exercise. Especially avoid high-fat or high-fiber meals, as these can take longer to digest and can potentially aggravate nausea and acid-reflux symptoms. Some people load up on carbs the night before their sporting event. This is fine in healthy people, because the carbs will have been emptied from their stomachs well before morning.

During long workouts, skip energy drinks, coffee and fruit juice. Take small, frequent sips of hypotonic fluids instead. Hypotonic fluids, such as many sports drinks, can help quickly replace fluid and electrolyte losses from sweat. But hypertonic fluids – such as sugary carbonated beverages, energy drinks and high-fructose juices – should be avoided. These draw fluid into your gut from your bloodstream and can worsen your chances of dehydration and ischemia.

Practice hydration during training – not just on race day. One study of more than 1,000 long-distance runners found that gastrointestinal symptoms were worse among those who did not practice hydration during their training.

Avoid coffee right before the race. Coffee is a potent trigger of colonic contractions. (Not the best thing when you only have access to portable toilets.)

Don’t take NSAIDs for aching joints and muscles. Nonsteroidal anti-inflammatory drugs such as ibuprofen taken before exercise can increase damage to the lining of the gut, which is implicated in blood loss, at a time when your GI tract is already under stress. Instead, try acetaminophen (Tylenol), ice packs or topical creams.

Get a good night’s sleep, and relax the morning of the race. Sleep disturbances and anxiety, especially the morning of a race, are associated with increased gastrointestinal symptoms.

The stress of travel and sorting out race logistics can trigger diarrhea. Waking up before the crack of dawn, panicking that your ride won’t make it, hunting down the booth with the race T-shirt: None of that is fun. If you can, get in early to sort out these details calmly and to take some time to yourself before the big moment.

Use data smartly. If you have a smartwatch or other device that measures your heart rate, check whether you tend to feel nausea, cramping or other gastrointestinal symptoms when your heart rate goes over a certain threshold. With higher-intensity exercise, your gut may not be getting enough blood flow. Use that data to recalibrate your training goals to a safer range.

Lastly, there’s often discussion about the best form of carbohydrate intake to improve performance during a race: solids, gels or liquids. There isn’t enough data about which form may be the most beneficial to the gut, but some limited studies suggest that liquid forms may be the most gentle for people who frequently experience bowel symptoms during exercise.- – –

What I want my patients to know

Physical activity helps us stay regular and is associated with a 7 percent reduced risk of colorectal cancer. Still, I don’t advise pushing your body past its limits to reach a goal that may not be healthy for you. When you feel nauseated during exercise, that’s your body sending you a message. Pause and see whether you’re up to continuing. When you have fecal incontinence or bleeding, that’s your body asking you to stop what you’re doing. Please heed that call.