Runner’s gut

Dom Cadden

So you like your runs, but you don’t like the trots – no-one does! ‘Runner’s gut’ can range from an uncomfortable bloating feeling to nausea or abdominal pain, or that horrible feeling that there’s an upside-down volcano about to erupt from your lower bowel. This commonly occurs during races of 10km to marathon, and when that feeling comes, you know your race is off to a bad start (or a bad shart?). Fortunately, with some experimentation, the issue can often be fixed for good.


This condition is not unique to runners, but runners are considered to be twice as likely to have bowel discomfort compared to other athletes. In part, these problems in the lower gastrointestinal (GI) come from the vibrations going up through the body and the jostling of the GI tract and its contents. In a 2008 study of more than 1200 runners in a long-distance race, 45 percent reported problems like cramping and diarrhoea during the event.

A lack of blood flow to the gut (ischaemia) is the main cause of nausea, abdominal pain, vomiting, and diarrhoea. Without the wind flow that you get cycling or the cooling water you get swimming, high intensity running can cause the body’s temperature to increase to as high as 41°C. Add dehydration to heat stress (in addition to the jostling of the GI tract) and an overall decrease in blood volume leads to a further reduction blood flow to the gut.


Women and those prone to nervous anxiety are more prone to running-related gut issues. Sports psychologists can help the latter, gender re-assignment surgeons can help the former (well, you never know…). It’s also something that may pop up after years of running – elite athletes are 1.5 to 3 times more likely to have gut issues compared to recreational athletes, probably because of the higher intensity of activity. Young athletes are also more at risk, again because of higher relative intensity, they are more susceptible to dehydration and their gut is less conditioned to the jostling and digestion on the run.

For people with irritable bowel syndrome, lactose intolerance or fructose malabsorption, gut problems (especially the need for a toilet) can be triggered while running. Everyday foods such as wheat, apples, pears, onions and garlic can trigger the bowel alarm while running, as can some of the carbohydrate chains found in many endurance drinks, gels and powders.


Dehydration will cause gut issues, but runner’s gut is often much more than dehydration – although it can be one of multiple factors. A water loss of more than 2% (e.g. a 1.4kg loss in a 70kg person) increases the risk of nausea, vomiting, diarrhoea and other GI problems during exercise. Consequently, it’s important to start your run well hydrated, but there are some fluids to avoid pre-activity. Drinks that are more than 10% carbohydrates (e.g. fruit juices, soft drinks, caffeinated ‘energy’ drinks) just before or during activity are bad news for people vulnerable to runner’s gut because they draw extra water into the bowel. Small to moderate doses of caffeine (75-200mg) are unlikely to alter hydration levels enough to have an effect, while any drink with bubbles can cause bloating that can lead to gut discomfort. Water and sports drinks that uncarbonated are more like 4–8% carbohydrates (e.g. Gatorade™) are generally well tolerated.

Longer endurance athletes might take in food or energy drinks while they run, which puts their GI tract under more strain. Too much carbohydrate intake during moderate intensity exercise can cause problems – it’s often suggested that athletes can only digest around 250 to 300 calories per hour (about 65 -75g of carbs), although this varies with the individual and factors such as increased intensity or heat. Excess osmolality in the gut can also upset it. This is where the ratio of carbs to fluid is too high. Generally, most athletes’ guts can only tolerate 6 to 8% osmolality during moderate intensity activity. When the osmolality is too high (and you have to account for solid food, too), it will draw water into the gut and cause a bloated feeling due to a delay in emptying food out of the stomach. There’s some evidence suggesting that maltodextrin and fructose may work better than glucose at the same osmality.


Dietitians tend to recommend that your pre-race meal be eaten 2-4 hours before the event, depending on the composition of the meal, because you need enough time for the meal to leave the stomach. For most people, 90-120 minutes will be enough if the meal is high in carbohydrates, but it’s good to be on the generous side before a race because anxiety can slow down the emptying of food from the stomach. High fibre, fat and (to a lesser extent) protein pre-exercise tend to cause an increase in GI discomfort because they are slow to empty from the stomach.

There are also some foods that activate the GI tract and these are best avoided in the four hours before your run. These include foods containing lactose (i.e. dairy), sorbitol (sugarless gum), and caffeine. Some athletes also say they have gut issues the day after drinking alcohol (they might have other problems, too!) or when they’re taking vitamin C problems or after eating legumes, spicy food or lots of fruits and vegetables (especially with skins left on). Often people will tolerate some of the factors above well and maybe not tolerate others so well – treat this as a checklist to see what works for you.

If you still really have gut trouble, try low-fibre, carbohydrate-rich liquid meals as the last meal before an event – there are some good powder mixes commercially available or you can make your own smoothie.


  • White bread sandwich or roll with banana & honey
  • Porridge made with reduced fat milk/soy milk and fruit juice
  • English muffins or crumpets with honey or jam
  • low-fibre breakfast cereals such as cornflakes, rice bubbles, Nutri-Grain with low fat milk and bananas
  • Low fat creamed rice and tinned fruit
  • Pasta or pizza topped with a simple tomato sauce (no fatty meats such as bacon, sausages, salami)
  • baked jacket potato and creamed corn
  • Low fat cereal or muesli bars
  • Soy smoothies made with banana, berries