Gut and endurance sport: runner’s gut, cramps and what helps

Healthy state

Exercise redistributes blood around the body. At rest a large share flows through the stomach, gut and liver, because digestion needs a lot of oxygen. During hard exercise the nervous system diverts blood to the working muscles and to the skin, which cools the body. At maximal effort, blood flow to the gut can drop by up to 80 percent. The gut copes easily with short and moderate sessions.

In sensible amounts, endurance exercise is good for the gut. Movement stimulates gut motility and helps with sluggish digestion. In one study, six weeks of endurance training shifted the gut microbiota Microbiome, gut microbiota The community of all microbes in the gut, mainly bacteria, plus viruses, fungi and archaea. Most live in the colon. More in the chapter “Building microbial diversity” towards more butyrate Butyrate A short-chain fatty acid and the main energy source for the cells of the colon wall. It strengthens the barrier and dampens inflammation. More in the chapter “Butyrate and other short-chain fatty acids”-producing bacteria, independently of diet. In irritable bowel syndrome Irritable bowel syndrome Recurring abdominal pain linked to bowel movements, without tests finding another cause. Today it is understood as a disorder of gut–brain interaction. More in the chapter “The gut-brain axis”, more exercise improved symptoms in a randomised trial Randomised trial A study in which chance decides who gets the treatment and who does not, for comparison. This makes results particularly reliable. , and at a follow-up about five years later participants were still better off than before.

The gut can also adapt to eating during exercise. Sugar is absorbed in the small intestine through transporters, and there are different kinds. Glucose uses a different route from fructose. With glucose alone, at most about 60 grams per hour reach the blood, because its transporter is then saturated. A mix of both uses two routes at once; in studies this allowed up to 65 percent more sugar to be used. Whatever is absorbed in the small intestine cannot ferment in the large intestine.

A well-known study found more bacteria of the genus Veillonella in marathon runners after the race. They feed on lactic acid (lactate), which builds up during hard exercise, and turn it into the short-chain fatty acid Short-chain fatty acids Acids such as acetate, propionate and butyrate that gut bacteria make from fibre. They nourish the gut wall and act throughout the body. More in the chapter “Butyrate and other short-chain fatty acids” propionate. Mice given these bacteria ran longer. Whether this works the same way in humans has not been shown. But the study shows that gut microbiota and training influence each other.

Disrupted state: causes and effects

Many endurance athletes know nausea, burping, abdominal cramps, sudden urgency or diarrhoea during exercise, known as “runner’s trots”. According to studies this affects 30 to 50 percent of athletes, and up to 93 percent in triathlons in the heat. Severe symptoms affect around 4 percent in marathons and up to 32 percent in Ironman races. Experts call it exercise-induced gastrointestinal syndrome. It becomes measurable from about two hours of exercise at moderate to high intensity, largely regardless of how fit someone is.

What happens in the gut is well studied. The cells of the small-intestinal lining get less blood and therefore less oxygen, and are briefly damaged. A protein from these cells (I-FABP) rises in the blood and serves as a marker of the damage. The gut wall becomes leakier for a few hours, bacterial components get into the blood and trigger an inflammatory response. At the same time the stomach empties more slowly, the small intestine slows down and sugar is absorbed less well. What is not absorbed ferments in the large intestine and causes gas, cramps and diarrhoea. In healthy people this usually heals within a few hours without lasting effects.

Heat makes everything worse. In an analysis of 132 two-hour runs at 21 to 37 degrees Celsius, the more body temperature rose, the greater the gut damage, inflammation and symptoms. Starting already dehydrated also causes more problems. After a sauna session with 3 percent weight loss, the stomach emptied more slowly during cycling, and the athletes had clearly more nausea and stomach cramps.

Runners are affected more often than cyclists, because the constant up and down shakes the belly. In a comparison under the same conditions, 44 percent had symptoms when running and 25 percent when cycling, but severity was the same. On the bike the upper abdomen is affected more, because the bent posture presses on the stomach. Heartburn is also more common when running. Measurements showed that the valve at the stomach entrance does not open more often during running, but when it does, acid is more likely to flow back, probably because of higher pressure in the abdomen and the movement.

Food plays a big role. Fibre, fat, a lot of protein and fermentable sugars (FODMAPs FODMAP Short for fermentable sugars and sugar alcohols that are poorly absorbed in the small intestine, such as fructans, lactose, fructose and sorbitol. More in the chapter “Doing low-FODMAP right”) shortly before exercise stay in the gut for a long time and ferment there. Sugar during exercise can also cause problems if the gut is not used to it. In a survey of 221 endurance athletes, sugar intake ranged from 6 to 136 grams per hour. Those who ate a lot had more nausea and bloating more often, but were also faster in the Ironman.

Ibuprofen and similar painkillers before exercise make the damage worse. In a small study with nine trained men, the damage marker rose almost twice as high after cycling with ibuprofen as after cycling without, and the gut wall became leakier. The researchers therefore advised against taking painkillers preventively before exercise.

Long-distance runners have iron deficiency more often. One reason is blood from the gut: after races of 10 to 42 kilometres, 20 of 24 runners had more blood in their stool than before, mostly invisible. On top of that, after hard exercise the body ramps up the hormone hepcidin for some hours, which slows iron absorption in the gut. Very rarely, the lack of oxygen leads to real inflammation of the large intestine with visible blood (ischaemic colitis), so far known only from case reports.

For people with irritable bowel syndrome or inflammatory bowel disease Inflammatory bowel disease Umbrella term for Crohn’s disease and ulcerative colitis: long-term, relapsing inflammation of the gut that needs medical treatment. More in the chapter “Immune system and inflammation”, studies suggest moderate exercise tends to help. Whether very long and hard efforts are safe for them has barely been studied. Those who already have a sensitive gut in everyday life often notice runner’s gut earlier and more strongly.