I'm a complete newbie to this forum (and only a three month old vegan!) but I thought I'd bump this topic. I saw that FODMAPs were mentioned a couple of times over on the IBS thread but, after my own experience, they may be worth looking into if your dietitian hasn't recommended an elimination diet already (although eliminating legumes isn't really an option as a vegan, unfortunately).
I saw a gastroenterologist a couple of years back when I started becoming vegetarian and my IBS flared up so bad I could barely eat. After an endoscopy and various tests I was diagnosed with IBS and NERD (Non Evident Reflux Disorder), prescribed with Pariet (Rabeprazole sodium) and sent off to the hospital dietitian. She put me onto a total FODMAP elimination diet for 10 weeks - she told me it can take that long for some of the fermenting agents to finally clear the gut - and then I slowly reintroduced individual food groups (ie. fructans, polyols) whilst keeping a food diary to see which ones were the culprits. At the time I was so desperate to sort it out that I just went back to being an omni so I could cut out all triggers ... everyone's different but by doing the elimination/reintroduction thing I found that my main triggers are:
- onions, leeks, shallots, garlic, snow peas, beetroot, some of the fancy lettuces, brussel sprouts and cabbage
- apples, pears, pawpaw and rockmelon
- consistent intake of gluten and mushrooms (a few servings a week is cool)
- artificial sweeteners
- physsilium husks and other fibre boosters
- palm oil. Not on the FODMAP list but this stuff makes me nauseous
I was lucky and those diet changes practically cleared up my problems - it's only since I've reintroduced legumes (even with the long soak with apple cider vinegar and cook with cumin seeds method) into my diet and gone vegan that I've started to have some problems again. On iffy days I've found it helpful to limit my vegetables to only a couple of safe varieties in each meal to make it easier on my gut, and supplementing with glutamine twice a day seems to help too. Bad attacks I can usually control/manage by reaching for the DeGas, Buscopan and peppermint oil pills and by drinking green tea with fresh ginger. The dietitian also told me not to drink with my meals (forget the reason) but that does sometimes help too.
A note on onions.. I've found I can cook with onions for the flavour boost, so long as I fish them out before they start to break down and become incorporated. Onion powder is just as bad for me as actual onions and I have to be careful to scrutanise food labels as stinking onion powder seems to be everywhere.
Not sure how helpful any of that will be but good luck in your quest to manage your IBS :) If you're interested at all, a full list of FODMAP foods can be found here.