a lot, actually! yinglets are resistant to a wide range of common natural toxins, owing to their home swamps in canon being so full of toxic flora and fauna. they apparently adapt to new toxins of those types fairly rapidly, as well, and sequester what their bodies can't process, making them toxic to most things that'd try to eat them
this is partly the result of their immune systems, toxin-processing organs, and the "hell microbes" (as Val often put it) that are in their guts, which help to break down toxins in their food, as well as breaking down most things they eat. think of it like the microbiome we have in our own guts, but way more powerful, and as far as I can tell, present before their small intestines (since otherwise it's unlikely they'd be protected against toxins they eat, so easily)
it'd be easier to list what they can't eat. and that's... anything that contains glutens!
it's not just wheat, to be clear. wheat contains gluten, yes, but for whatever reason, people try to claim other true grains (like oats, rye, barley, rice, and corn) somehow are "gluten free." they're not; they just don't contain wheat gluten, which is important to folks with celiac disease
according to Val, all true grains are a problem for yings, because when eating stuff made with them, the gluten in the food forms globs of protein that their hell microbiome can't break down quickly enough. it's like a blind spot in their guts. so the microbes keep wadding up on top of the clumps of food, which can cause clogs. to prevent this, yinglets rapidly evolved an emetic response* to all grains, which can only really be fought by liquefying the food (getting it so wet it turns into soup, or drinking it in a form that is already liquid, like beer)
of note, Val has also claimed (outside the comic) that potatoes are off limits to yinglets, due to their high starch content. however, beans and carrots and other such foods wouldn't be, and I honestly can't find a way to reconcile this, so I choose to ignore it. besides the lack of any way to make sense out of it, accepting this would mean denying them one of the most yingy foods possible! it's a root of a plant that is almost entirely toxic, and makes crunchy snacks that would be heaven for them. why prevent them from enjoying that?
well, I think in Val's case, it's because that means in the hypothetical world where Kassen got ahold of a baked potato or some crisps or fries, he'd get to suffer, not being able to eat them. and while I get the humor there, I just can't accept that
let yings have taters!
so yeah. to summarize: yings can eat just about anything. they really enjoy mollusk meat, but it's far from the only thing they eat on the regular. they just can't eat true grains, because of the glutens inside 'em
but, well, give 'em some probiotics tuned for this problem, and they could probably eat those just fine, too!
*put less clinically, eating grains makes them throw up








