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The finding of increased fecal fat in the stool when measured over a 2-3
day period is almost always a sign of GI problem, not irritable bowel
syndrome. There are literally 100's of causes of steatorrhea( thats the
medical term) some have normal mucosa, some do not. A normal d-Zylose
test implies it is some  problem with normal mucosa that is causing fat
malabsorption
but not always. Pancreatic or liver problems can cause fat malabsorption
and have nornal biopsies of the intestine.
This is  a very difficult area of diagnosis as there are so many causes.
Not Medical advice
Joe Murray