Tren used to make my pre-existing digestion issues even worse. Likely via it's strong androgenic and mild progestenic activity.
Hormonal fluctuations can affect gut motility and acid secretion as well as mineral balance (which can affect acid/base production during digestion and gut motility...)...BUT, when I got really sick (GI issues) and took the time to clean out and rebuild my GI system, I haven't gotten any tren or hormone-related heartburn symptoms.
Most peopel have pretty toxic guts, especially those who have eaten high carb diets for a while and especially those who use or have used acid suppressing meds. Bacterial overgrowth is highly likely.
I recommend about 800mg Magnesium per day. Most people are deficient, especially those taking calcium supplements (bad idea btw) and/or chewing calcium based ant-acids often. Take the mag in the evening (I like Natural Calm) and it will help keep your GI tract more motile, as well as calm you down a bit (this always helps on tren).
A diet more along the lines of John Meadows suggestions would, in my experience, help avoid a lot of GI issues associated with high food intake and high carb intake.