I don't often write about food issues here – I have a recipe blog for that. But sometimes I find something so egregious that I feel compelled to write something. My wife suffers, quite badly, from Celiac disease. And, as with any auto-immune condition, it is always a running battle where the symptoms change and you're always in responsive mode.
Lately my wife has been having odd stomach issues and we finally managed to correlate it to mushrooms. Our whole kitchen is gluten free and the few gluten based ingredients we do have are physically stored, and used, on a different floor of the house. So we knew it wasn't a normal cross contamination issue. We also hadn't eaten out in weeks so that was off the table as well. A bit of googling turned up this Very Well post:
Answer: There's no question that plain mushrooms ought to be gluten-free — after all, they're a fresh vegetable, right? But lots of people report getting glutened after eating fresh mushrooms … enough people that it's worth looking into how mushrooms are grown.
In fact, once you learn how they're grown, you'll understand why many people react, especially those who are particularly sensitive to trace gluten.
You see, mushroom spores are grown either directly on gluten grains, or on a medium that's derived at least in part from gluten grains. Rye is used most commonly for this purpose, but growers also use wheat and occasionally on a combination of the two grains (barley, the third gluten grain, doesn't seem to be in common use for growing mushrooms). And this cultivation method leads to gluten cross-contamination on the finished fungi.
It should be noted that the article goes on to say that the presence of gluten in mushrooms is below the 20 ppm standard that defines gluten. But my wife is admittedly very, very sensitive to ingredients and she can taste the presence of things that I can't so it doesn't surprise me that she has problems with mushrooms. But, even so – what have things come to when a vegetable (ok a fungus) – in its "natural" state – contains gluten?
This only emphasizes to me just how very, very careful you have to be when you have celiac or any food related issue – what you think you're buying may not be exactly what you expect.