Nutrition Gut Issues

For those who have major gut, read large intestine, issues, bad enough that you seek medical or nutritionist advice/supervision. Can you tell me what is working for you. What you can tolerate the night before, morning of and during a long (3+ hours) race/workout.<br><br>
Does anyone know if using Immodium can impair nutrient absorption over a long distance race.<br><br>
I am still having difficulty nailing nutrition, I know what I can tolerate, I just can't figure out what sends me over the edge into gut distress. I think heat even moderate temps makes a huge difference, but can't seem to find a pattern with anything else.