+1 to #16--it's all about diet and management.

I used to suffer bloating in-flight and retire to the restroom on an hourly basis to relieve the gas. Turns out I was becoming lactose intolerant; didn't notice it on the ground, but up in altitude it was much more noticeable. Once I cut out the milk, my discomfort went away entirely.

(I got a final, unambiguous confirmation once when upgraded on an AA transcon and was served their milk-and-cookie snack; the moment I drank the milk my stomach started responding. My lactose intolerance has since gotten worse so I notice it on the ground, too, but basically flying was my "early indicator." Lactose intolerance affects lots of adults gradually, but increasingly, after 30.)