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This question originally appeared on Quora. Answer by Matan Shelomi.

Firstly, all humans can digest gluten. All. Anything we cannot metabolize, such as cellulose (also known as dietary fiber, a very important part of your diet!) passes through us and gets pooped out. Cellulose is found in all plant cells: if we only ate things we can digest, we’d all be carnivores!

Gluten is a protein. We have proteases for it. We can break it down. We just can’t break it down completely: one of the pieces, named gliadin, does not get broken. This is not improper digestion, that’s just how we digest it. Lots of foods only get partially broken down. However, according to this Science paper, gliadin reacts with an enzyme called transglutaminase that, in Celiac patients only, triggers an autoimmune response, due to a gene mutation in these patients. For non-Celiacs, it gets pooped out and does not trigger the response. The article notes that swallowing some bacterial protease enzymes that digest gliadin further could make gluten safe for celiacs, just as lactase enzyme pills helps lactose intolerant people eat dairy (working pills of these do not exist at this time, but some fraudulent products are sadly being sold as such). However, there is nothing improper about us not breaking down gliadin. That’s just what we do, and it’s normally not a problem.

Structural Basis for Gluten Intolerance in Celiac Sprue

Another paper found that gliadin causes zonulin production, which increases intestinal permeability to large molecules, and that this happens in all people. It has nothing to do with good or bad metabolism, however, as this happens to everyone even if they get zero symptoms; nor does that article mention if any other foods have the same effect. Zonulin was only recently discovered, so we don’t know much about it yet, nor do we know how accurate the past work is. In any case, whether a food produces it or not does not mean we cannot digest the food, just that some people react badly to the normal breakdown product. The fault lies in the mutation in those who feel ill, not in the food or in the human digestive system.

Nothing here says we cannot digest gluten. The papers instead describe what happens when we do.

We can all metabolize gluten. If we couldn’t, then it would pass through us all with no effects! But just as some people have food allergies, some are autoallergic to gluten’s natural digestive product, gliadin. Gluten has been consumed by humans and other animals for millennia and is perfectly healthy for most, and there’s nothing improper about us not breaking it or any other food product down 100%, but Celiac patients do react to it differently, so for them it is best worth avoiding.

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