I usually drink about a liter of water after waking up every morning and go running (2x/week 5k and 1x/week 10k+) shortly (15-20 mins) after. I never feel any discomfort due to the full stomach. But recently I was warned this is "bad"; are there any physiological reasons behind this, or merely a matter of convention?
Edit (after answers): As I found out recently, this sort of thing (running on a full stomach, e.g. shortly after lunch) is done often enough in various military academies/bootcamps. So it is really unlikely to be physically harmful.