Less atmospheric pressure at altitude allows stomach gasses to expand. Expanding gas needs a place to go, so....
The most persistent altitude symptom that I have seems to be heartburn. I never, ever, get heartburn in my day to day life, but I get it all the time at altitude. i think it has something to do with atmospheric pressure as well.
I've been thinking about that but less preasure is not enough to cause so much of it. If we assume that the gas behaves as an ideal gas, then its behavior is described by this equation:
p is pressure (in pascals, not PSI), V is volume (in m^3, not cubic inches, feet nor miles), n is amount (in moles, unit new enough that nobody was able to screw up yet), R is a constant and T is temperature (Kelvin, not Fahrenheit nor celsius)....
All n, R and T are constants (assume now no additional gas forming and body temperature is constant) and preasure changes from 83 000 Pa in Denver to 61 000 Pa on a 14er, which means volume increases 83 000/61 000 which is 1.36 times. So from 1 gallon of the gas in Denver, you have just 1.36 galon of gas on the summit of a 14er. In my opinion, not enough. I think that for some reason, the bacterias in our intestins start to work like crazy and increase the production. Maybe the body somehow supports them as they may help extract more nutrition or something like that...But I thought that somebody with medical education may know what exactly is going on..
I also sometimes suffered heartburn high in the mountains.