Celiac disease was identified by (and its discovery is attributed to) Aretaeus of Cappadocia in the 1st century AD. He was an impressive diagnostician, working chiefly from observation, rather than via experimentation. He supposedly determined the sweetness of urine in people with diabetes mellitus ('mellitus' meaning sweet) by observing that ants would clamber for the urine of those patients.

With regards to Celiac Disease, his diagnosis was also based on observation. His patients with Celiac Disease would have frequent diarrhea with significant quantities of undigested food. Those undigested bits were consistently grains.

As with modern medicine, poop tells a health care professional a great deal. They never tell you in premed that 30% of all your conversations for the rest of your professional life will be about poop. If you work in oncology, you'll have daily conversations with outpatients in which you ask them for their poop description.

We obviously have different methods of establishing a more secure diagnosis nowadays. A doctor isn't going to hand you a bulgar wheat salad and say, 'eat this and then poop into this bag in 30 minutes.' Or maybe they do. I'm not a diagnostician. I'd expect that there's a bit more at stake now than then. Aretaeus could get a diagnosis wrong, but people were grateful for the effort. A health care professional gets it wrong today and there are lawsuits and board hearings.