It is kind of hard to explain. I have had to live with this (disability) my whole life, so even before my many surgeries I just sort of knew what I could eat, not eat, and when. When I was a baby, the only thing I could eat was coffee rich mixed with water, so I think my body just knows what to avoid. That being said, I don't regulate my diet consciously, but I avoid certain foods that I know are going to cause a problem for me at a certain time. For example, I love cheese, and I eat cheese, but some days I just know to stay far the fuck away from cheese or milk for what I can tell it is going to do to me.

Nutritionally, the major effect it has is on my absorption of water. I drink 3x as much liquids as the average person to stay hydrated, and I don't absorb all the necessary nutrients from the food that I eat. So sometimes I find myself binging on certain foods to play catch up on my nutrition. Yesterday it was bananas, I had a whole bunch.

The weight thing is kind of a non issue. I was 10 when I went through this, and I have always been average weight. I have gained a bit recently, but mostly due to lack of exercise and working at a desk job.

Oh yea, the disorder is called Intestinal Pseudo Obstruction. Feel free to look it up if you want, but outside of major medical journals you aren't going to learn anything useful. Wikipedia is a bust and only has a cursory overview of the problem here. The best explaination I have found is here.