Psyllium is the most common soluble fiber used by J-pouchers. Not everyone does well with it, and some find it gassy, but it does tend to thicken and bulk up the stool. I’ve used it every day since my surgery 17 years ago. Alternatives to psyllium include Benefiber and CItrucel, and some folks do better with one of those, while others do best without soluble fiber supplements. Some doctors apparently recommend taking these with smallish amounts of water, but I think that’s bad advice, and I advise using the amount of water recommended on the label. Psyllium develops a gel-like consistency in water when it sits, and I think that’s exactly what it does to a J-poucher’s stool. I didn’t get good results with psyllium capsules, since they didn’t mix well with the rest of the stool.
Insoluble fiber is challenging for some J-pouchers to process, though many of us are just fine with it. Think of raw veggies, skins, etc. It tends to come out looking rather like it went in. It can be helpful for people with colons who are prone to constipation (prunes!), but I don’t think insoluble fiber is particularly beneficial to J-pouchers, though the fruits and veggies are themselves nutritionally beneficial. I eat plenty of fruits and veggies, but not as a way to intentionally consume more insoluble fiber. It seems like a bizarre recommendation from that nutritionist.
Here’s the Wikipedia distinction, but like most general advice it assumes everyone has a colon: