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Runner’s Gut: Food and Hydration are Major Culprits

Updated: Feb 17, 2020

The best names are reserved for this awful experience: Runner’s gut, runner’s trots, and runner’s diarrhea.

You’re amped to continue reading, I know.

During an intense workout or run, athletes may experience bloating, gassiness, burping, vomiting, heartburn, cramping, or diarrhea—all of which negatively impact one’s performance and confidence surrounding fueling and exercising. Plus, it’s no fun to feel a grumble and then panic when quickly searching for a toilet (or a bush if you’re exercising outdoors!).

Those most at risk are endurance runners, and many that I’ve worked with avoid eating and drinking prior to a long run—they choose to run on empty rather than fueling accordingly.

Clearly, this isn’t an ideal situation. So let’s dig into the why and what to do about it.

An Overview: Why Does Runner's Gut Happen?

  • Mechanical: Running increases pressure on the stomach and leads to the jostling of its contents.

  • Blood flow shifts: Blood contains oxygen that the muscles need. When exercise begins, more blood is shifted away from the gut and towards the muscles.

  • Nutritional: The type of food and timing of the pre-event meal can cause gut distress.

Like a Bustling Construction Zone, Blood is Diverted Away from Where Construction Workers Need it to Be

When the body senses stress, the fight-or-flight mechanism is triggered. Stress can be related to anxiety about a race (e.g., stomach is in knots), beginning a training session, or being chased by an ax murderer. The brain doesn’t know the difference.

At rest, oxygen piggybacks on red blood cells to be delivered to the gut to promote digestion and absorption. When intense exercise occurs, as much as 80% of the blood normally being delivered to the gut is diverted to the muscles because they need extra oxygen to function. This means that undigested food tends to sit in the stomach like a brick.

So as the intensity of exercise decreases, most often the risk of runner’s gut does, too.

The reduction in blood flow may also impact the esophagus (i.e., the tube running from the throat to the stomach). The pressure between the valve that lives at the bottom of the tube and the top of the stomach may be reduced—meaning that stomach contents and acid may escape upwards, resulting in reflux and heartburn.

What’s Food Got to do With it? Lots.

Hypertonic Carbohydrates: Think Concentrated Sources of Sugars

Flashback to picking teams for soccer in grade school.

Captains take turns picking kids until all are chosen and the teams are equal in number. The body mimics this by making sure the bloodstream and the gut contents are equal in composition. When a team of 20 enters the small intestine and the bloodstream is only a team of 10, the latter releases water into the intestine to dilute the team of 20. Dilution is the body’s way of evening out the teams.

Unfortunately, more water than usual is now entering the intestines—meaning the risk of fast-moving watery stools are likely.

Like the team of 20, hypertonic beverages are those that are more concentrated with sugar than what exists in the bloodstream. For instance, a beverage containing more than 6-8% osmolality* from carbohydrates is considered to be hypertonic.

However, even if a sports gel or package of chews are within the 6-8% range, the risk of runner’s gut also increases if consumed without adequate fluid.

*To calculate osmolality, divide the grams of carbohydrate by the beverage portion in milliliters and multiply by 100. For instance, a 591-milliliter bottle of green Gatorade contains 34 grams of carbohydrates:

( 34 grams / 591 milliliters ) * 100% = 5.8%.

Lastly, rather than thinking more is better when it comes to carbohydrate dosing during exercise, consuming different types of sugars may reduce discomfort. For instance, the lining of the small intestine has doors that only allow certain sugars in. Glucose enters through door A and fructose door B. Once door A is overwhelmed, no more glucose can enter and it sits in the gut (no good!). Consuming multiple transportable carbohydrates (e.g., glucose plus fructose) allows for a combination of sugars that may fully enter their doors, avoiding leaving some behind in the gut.

Keep in mind this doesn't mean always aiming for a combination of carbohydrates. Rather, first maximize the 60 grams of glucose and then add up to 30 grams of fructose.

Fiber, Fat, and Protein: Great for Every Day Eating, Not for Running Snacks

The timing of and type of food within the last meal before exercise matters a lot. Foods that leave the stomach slowly include those high in:

  • Fiber (apple skins and whole grains).

  • Fat (avocado and nuts).

  • Protein (Greek yogurt and eggs).

For general health purposes, these items are great. However, consuming snacks and meals high in these items close to or during exercise can increase the risk of runner’s gut because they take longer to digest and absorb (i.e., they sit in the gut not fully digested).

Running Dehydrated

Athletes running in the dehydrated state are more likely to report gut distress while exercising. Dehydration leads to the symptoms of runner's gut in multiple ways:

Some athletes may complain of gut issues after having consumed fluids, but this is more likely due to not typically drinking while running (i.e., not training the gut to tolerate fluids) or only drinking after the negative effects of dehydration have already begun to occur.

Running Hurts, but Anti-inflammatory Drugs may be Causing the Runs

Athletes reporting soreness from running or weight lifting may be popping non-steroidal anti-inflammatory drugs (NSAIDS)—or more commonly known as Aspirin, ibuprofen, Advil, Aleve, and Motrin, among others. NSAIDs are known to reduce the integrity of the intestinal wall, therefore leading to or worsening runner’s gut for some athletes.

Take-home Message: How to Minimize (or Even Prevent!) Runner’s Gut

For the stressed and anxious runner come competition day, they may do best by being referred to a sports psychologist. Although creating a ritual for eating, drinking, and sleeping prior to an event is helpful to ease anxiety, sometimes the stomach butterflies prevail.

If an athlete pops NSAIDs out of habit or isn't capitalizing on alternative recovery methods (e.g., stretching, an Athletic Therapist), opt for those solutions first. If NSAIDs are mandatory and you think they're contributing to gut distress, speak with a medical doctor.

Before Exercise

The further an athlete is away from beginning exercise, the larger and more complex a meal can be (see table below). Keep in mind that every gut is different, so some athletes may tolerate a slice of whole grain toast 4 hours out whereas others won’t.

Remember to hydrate! Four hours before exercising, opt for 5-7 milliliters per kilogram body weight (or 1 ounce for every 10 pounds of body weight). Avoid chugging large volumes of water, since excess fluid can remain in the stomach, resulting in a sloshy feeling while running.

Adapted from "Nancy Clark's Food Guide for Marathoners" by Nancy Clark and "Sports Nutrition: A Handbook for Professionals, Sixth Edition" by Christine Karpinski and Christine Rosenbloom.

During Exercise

Avoid hypertonic beverages. Aim for those with 6-8% osmolality*. Real world examples include:

  • A sports drink (e.g., Gatorade).

  • ½ cup (4 ounces) of pulp-free juice mixed with ½ cup of water.

  • 1 tablespoon of honey consumed with 1 cup (8 ounces) of water.

Hydrate! Your best bet is to weigh yourself before and after a run. For every pound lost, that correlates with two cups of sweat (or 16 ounces). On your next run, plan to consume the equivalent of fluid lost as sweat during the entire run. Note that as your running environment changes, as will your sweat loss. For instance, outdoor running in the heat and humidity will result in a higher sweat loss than the same run completed on a treadmill in an air conditioned gym.

Consider multiple transportable sugars. Since the intestine can oxidize ~60 grams of glucose every hour, adding 30 grams of fructose allows for a total of 90 grams to be absorbed (remember: maximize the use of all doors lining the intestine).

If you aren’t sure what types of sugars your item has, check out NutritionData.Self.Com (the hyperlink exemplifies honey. Scroll down to the carbohydrates box and click on the blue “More Details” button).

Be mindful of how much and what carbohydrates to consume: Regardless of an athlete’s body weight, formulas for carbohydrate intakes during exercise are listed below. After the first hour of exercise for well-trained athletes, aim to consume:

  • 30 grams of glucose per hour for exercise lasting 1-2 hours.

  • 60 grams of glucose for exercise lasting 2-3 hours.

  • 60 grams glucose plus 30 grams fructose for exercise lasting more than 2.5 hours.

Train the gut: An athlete practices running and works to improve their strength in the weight room—practicing nutrition is no different. Practice eating and drinking leading up to and during training sessions. Keep a journal of which foods and beverages work and which don’t.

*To calculate osmolality, divide the grams of carbohydrate by the beverage portion in milliliters. For instance, a bottle (591 mL) of regular Gatorade contains 34 grams of carbohydrate. So (34/591)*100% = 5.8%.


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