Week five of your marathon training. You’re confident and feeling good, because up to this point everything has gone perfectly. Your shoes are on, your GPS watch is synced up and you’re ready to hit the road for your longest training run yet.

The first few miles go great—until you feel a slight grumble in your stomach. You try to ignore it, but it becomes a cramp and then something more—something that tells you to stop running or else something bad might happen. Your long run suddenly becomes a search for the nearest public restroom.

What caused your stomach to rebel against you? The general answer: gastrointestinal distress, more commonly known as gastrointestinal distress. If this has happened to you, you’re not alone. Over 45% of runners experience gastrointestinal distress at one point or another during marathon training. (See How Simple Pre-Race Eating Led to Chris Legh’s Breakthrough.)

Exercise Symptoms of Gastrointestinal Distress

• Abdominal cramping
• Gas
• Burping
• Heartburn
• Nausea
• Urgent need for a bowel movement
• Diarrhea
• Vomiting
• Gastrointestinal bleeding (severe cases)

As you can see, the symptoms range from mild to severe, but one thing remains the same: GI distress is uncomfortable and can quickly turn a great run into a run cut short.

What Causes Running Gastrointestinal Distress?

Much research is still being done on GI issues, but we do know of a few culprits.

Digestion happens easiest when you are resting. The body sends extra blood to the gut to help the process go “smoothly.” Exercise can disrupt digestion, because more blood flows to the muscles and less to the stomach. Also, the gut goes through mechanical trauma from the body bouncing up and down with each step. Foods that stimulate the gut, such as fiber and caffeine, can produce discomfort. Finally, exercise triggers a hormone boost that signals the stomach to move its contents along. The more intense your run, the more likely you are to experience distress.

Rest assured though. Some of the things that lead to GI distress can be controlled—especially hydration. Losing just 2% of your body weight from fluid loss can lead to GI troubles. For a 150-pound person, that’s just 3 pounds.

How to Avoid Running Gastrointestinal Distress

There are several things you can do to keep your gut calm and collected during runs. One bad experience should not prevent you from continuing your training. Eliminate unwanted running emergencies and banish gastrointestinal distress for good with the following tips. (See also Time Your Fueling for Peak Performance.)

Avoid anything new before running

Don’t use the day of a run to try a new protein powder, restaurant or gel. Stick with foods that you know settle well in your stomach. For many people they can range from toast with nut butter to eggs and bacon.

Eat 1-2 hours before running

This will give your stomach plenty of time to digest your food.

Avoid high-fiber foods 1-2 days before long runs

Fiber stimulates the intestines. If you feel that high-fiber foods affect your run, avoid things like  beans, broccoli and grains.

Hydrate

Aim to drink 64 ounces of water each day, more on days when you are running. Drink 6-8 ounces before starting a run and drink frequently along your route. If your run will last longer than an hour, drink a 50-50 mix of water and a sports drink to replenish lost electrolytes. (Learn how to Individualize Your Hydration Schedule.)

Stick with “stomach-friendly foods”

These are foods low in fiber, low in fat, low in sugar (no donuts), and devoid of artificial sweeteners (which have a laxative effect).

Promote pre-run bowels

Try to go to the bathroom before beginning your run. Stimulants such as coffee or tea can help.