I appreciate that you don't have problems with these foods, and I assume other prebiotic-sensitive ME/CFS patients probably will not either; but it doesn't really make sense why prebiotics within foods should cause no issues, but when the same prebiotics are taken in pure form, this does trigger problems. If you take leeks for example, these contain 3 to 10% inulin (ref: here). A couple of large leeks weighs around 400 grams, so if you were to eat two leeks, you would be getting 12 to 40 grams of inulin. That's quite a bit of inulin, probably more than you would take if you were supplementing with pure inulin power. I take inulin daily, and usually take 2 heaped teaspoons of pure inulin power, which is around 10 grams. One explanation might be that pure inulin powder is more accessible to the gut bacteria than inulin found in food items. However, this is just guess. The whole thing seems a bit of a mystery.