More than 100 million people in the United States are expected to travel at some point from this Christmas to New Year’s Day—and each and every one of them will take roughly 100 trillion intestinal microbes along for the ride.

Among the various other things influenced by these gut bacteria—such as eating habits, for example—they also help control how much, or how little, a person poops. For many travelers, “how little” is the operative phrase: By one estimate, as many as 40 percent of people experience constipation while they’re away from home, partially because their gut bacteria’s reaction to the change of setting.

“Any time you leave your general habitat, it’s throwing your gut microflora off balance,” says Brooke Alpert, a New York–based registered dietician. Sometimes, that begins before you reach your new destination: In some people, the very act of traveling from point A to point B can cause constipation. Movement stimulates the gut, so sitting on a plane or in a car for long periods of time can cause the intestines to clog; ignoring the urge to go while in the air or on the road can also make it more difficult once you finally sit down on the toilet.

Time differences can also pose a problem. Many people have a normal bowel-movement routine, pooping at regular intervals throughout the day. But when jet lag or a new time zone shifts that schedule ahead or backwards by a few hours, it can mess up that routine, causing constipation.