One of the reasons people become constipated when they travel is that they get dehydrated, and one of the reasons people become dehydrated when they travel is that flying is dehydrating. So why is that?
First of all the air on a plane is dry. Typical humidity in US cities ranges from 35% to 70% or higher. Typical humidity in the cabin of an airplane is generally less than 20%. Dry air dehydrates you by drawing the moisture out of your body - mostly from your skin but also from your lungs.
Secondly, airplane cabins are pressurized but not to the pressures that most of us are used to living at. In fact, airplanes are pressurized to altitudes of 6000 to 8000 feet - well above sea level. At those altitudes there is less oxygen. Less oxygen means that you need to breath more – hyperventilate. Breathing faster makes you lose the moisture from your lungs, and breathing faster into dry air makes you lose even more water from your body.
Finally, the majority of people tend to have some level of anxiety on a plane. Anxiety also makes people hyperventilate and makes them sweat more. Both of these factors lead to dehydration, especially on a low humidity plane.
So pay the few bucks and make sure you get some water to bring on a plane. Staying hydrated will prevent the fatigue, dizziness and constipation commonly associated with it, and get your trip off to a much better start.