HASO Fact Check:
Just a disclaimer before I start. I love the humans are space orcs genre and are super interested in all the stories that have come out of it. But I've noticed a few "what if this, what if that" 's that I think would likely be very biologically inaccurate, and I'd like to offer an opinion on what I think it'd really be like. My intention isn't to be all "I'm actually right and you're wrong", but just to stoke some thought if anyone cares for a bit more of plausible scientific reality for their fun thoughts. In any case though, I think it's fun to explore all sorts of scenarios. Anyway, let's get into it.
Aliens eating human food, and vice versa. I've seen a lot of posts about aliens can't eat chocolate, or especially how spicy peppers are deadly to aliens but humans like them just fine. And potentially the other way around with the aliens liking stuff that would definitely kill a human. But in these scenarios (if realism matters to you) it's enticing to look at why this is, the biological reasons for it and how digestion itself actually works.
In very simple terms (actual biologists pls dont get mad at me) (tho I'm open to correction), digestion works by using a specific biology. Often specific enzymes in your body that break down whatever specific kind of food you're eating. If you're lactose intolerant, you lack a special milk-digesting enzyme. If you're a cow, you have special enzymes for breaking down grass (as well as 4 stomachs, and gut bacteria to help). This is why pandas have to spend a huge fraction of their day eating bamboo, because relatively not that long ago they used to be carnivores (hence the sharp teeth) and recently switched to eating plants, which they largely lack the biology for, so the nutritional value of each bite is proportionally less since they can't use it as efficiently.
But my point is, all of these creatures are on Earth. They've spent generations adapting to and specifically trying to fit themselves in to what the Earth has to offer. Not even accounting for the even longer time some of them had even further up the evolutionary chain. Your internal biology is specifically adapted for the specific things you're eating. Almost all animals don't have to adapt their internal layout significantly through their evolution because most of the stuff they're eating stays the same. And even in cases like the panda, there's this overarching structure of "Earth life" that's biology is relatively similar to each other, so switching over to plants is similar-ish enough to other stuff that the current biology can handle it.
But then take aliens. Their biology might be so different from ours, from all of Earth-life's that it's unthinkable. Not only will they not be able to just digest certain types of Earth cuisine, but it's highly likely that the biology they've been adapted to on their home world might be incompatible with ours. If their biology is substantially different than ours, then it's more than likely their food's is as well.
Like at the most extreme example, a plant isn't going to be allergic to chocolate. That's not what it eats. That's not how it eats. And that's just a plant, from Earth. Who knows tho, maybe chocolate will be deadly to them. In dogs it produces a certain compound that's toxic to them. And stuff that's deadly to you isn't constrained by biology, but stuff that's healthy, or even useful to you is.
Imagine like mirror life, cuz I think a few people have heard about that. Left life would be able to eat all the things from Leftworld, and right life would be adapted to specifically eat all the stuff from Rightworld. There'd be no reason for anything from Leftworld to have the adaptations to eat anything from Rightworld. And vice versa. It's possible that this means it's poisonous, but also just potentially that everything from that planet is indigestible to the inhabitants of the other world, just by default. On the basis of a too different biology, not procured for interaction.
And needless to say, it doesn't have to be "mirror" life, it can just be regular aliens. Humans need xyz macronutrients and micronutrients to sustain themselves. And all life on Earth is made of these substances that we can eat. By definition everything living on earth is food to us. But aliens might need abc macronutrients and micronutrients and their entire planet's biology might be made up of those.
And what's fascinating is that's how it is for their entire biology, not just in terms of food. The most deadly type of venom in the galaxy? Haha, nope, just on your planet. Lots of snake venoms cause blood clotting and your blood vessels fall apart and a bunch of other gross things. But it's designed specifically for things with blood. Earth things. Alien with no blood? No problem! And not only at that level, but the specific "cocktail" of stuff in venom is chemically designed to specifically interact with Earth chemistry to have these effects. Even if aliens have blood 90% similar to Earth life but just with different proteins or enzymes that the venom can't interact with, then it just straight up will have no effect. Not because they're special and happen to be immune to it, but just because that's the default. And why would the snake be adapted to kill things it's never interacted with before? Probably an Australian snake could kill something in Alaska, or maybe by random chance regular spit could be deadly or addictive to an alien, but the more different the design of the thing, the more likely it'll be to be incompatible entirely.
Looking around, I found a nice video about this (Homochirality: Why Nature Never Makes Mirror Molecules) by Steve Mould. I thought it was quite good. Talking about mirror life, a bit less sensational than recent YouTubers doom and gloom that it could end everything on Earth, and more just some interesting points. But he talks about "homochirality" that all Earth-life has the same 'mirroredness' and I think I like extending this idea. Taking this one step further in my example to "homobiology" that all Earth life is fundamentally compatible with itself in almost every way it could be, not just chirality. Along with aliens probably having their version of it too.
But in the end, I don't know if it really matters to people that much. If all the stories are just for stories sake and fun ideas without really care for being super scientifically accurate, then that's cool! I just wanted to give some people some more fuel for thought in interesting and plausible ways aliens and humans could interact in stories.
