In my Science Fantasy galaxy, I have several humanoid alien species with blood based on different elements, including iron (essentially human blood), copper (like Vulcans from Star Trek with green blood), and varying made-up metallic elements.

After reading a brief line about humans not being able to eat animal meat with incompatible amino acids in a novel, I realized that some parts of my world might have scientific realism issues where I don't want there to be - of course, it is Science Fantasy. Still, I would rather be able to explain certain things at least plausibly with science as opposed to just saying "magic stuff" and moving on.

Namely, the issue is with food and food consumption. Imagine that an individual with iron-based blood (a human for all intents and purposes) was to eat meat from an animal that has copper-based blood (or vice versa). Would this individual...

    1. Be able to safely eat such meat, assuming they were essentially the same as a human?
    1. Be able to eat the meat, but only if they had some kind of organ that allowed them to process the different blood type?
    1. Not be able to eat the meat at all (and if so, how serious would the consequences be if they tried)?

Relatedly, what about plants? Would an alien with copper-based blood be able to eat fruits/vegetables/grains/nuts that are functionally identical to those found on Earth?

  • 2
    $\begingroup$ I think the incompatible biologies would be a much bigger problem than a different base for the blood. Also being able to eat something without ill effects and being able to extract calories and nutrients to subsist on it are different things, which one are you going for? $\endgroup$
    – Bubbles
    Commented Jun 19 at 3:56
  • $\begingroup$ @Bubbles Mostly extract calories and nutrients, but based on the answers here, I see that this is not as much of an issue as I thought it was - given that half of the aliens have blood based on elements that don't actually exist, a good deal of the science in the world is halfway out the window as it is, leaving the question as more a creative issue - I am not going so deep into the science as to make the incompatible biologies relevant, so the question is more of how much I want to suspend disbelief. $\endgroup$
    – TGA_1
    Commented Jun 19 at 14:01
  • $\begingroup$ what you use to make blood has no affect on what you can eat, there are dozens of oxygen carrying molecules in earth life but they all eat the same 4 basic macromolecules. blood evolved waaay after digestion. $\endgroup$
    – John
    Commented Jun 19 at 20:59
  • $\begingroup$ Lobsters have copper based blood. $\endgroup$
    – DJohnM
    Commented Jun 20 at 3:55

6 Answers 6


Your question is incomplete

I think you're putting too much stock in the novel you read — or perhaps you're not telling us enough about the context of the statement. At a guess, the author was probably trying to use a cool conjecture of physionomy to foil a crisis in the novel. Writing fiction is much, much more than just being "scientifically real" (which I don't put a lot of stock in, anyway...). Why am I making a big deal of this?

Because the point that author was making only has meaning if eating the food was for sustenance.

I've an older sister who once was visiting the home of a family she didn't know well. She was invited to drink something hot and was asked if she wanted some milk to cool it. When my sister said yes, the wife took up her baby's bottle and squirted milk into my sister's cup. "And, oh Mom... I drank it!" declared my sister in a letter home.

In other words, WHY someone is eating something matters to the context of your question. Humans can eat all kinds of things without dying or even getting particularly hurt — and yet derive not a single mole of nutrition from the eating. Why would we do something like that? Perhaps as a courtesy to the alien family we're visiting... a family that doesn't really understand our dietary needs and is only trying to be polite.

What works in your favor is that chemistry isn't alien

Sometimes we think DNA and such are mysterious and magical. It's unlikely, we think, that the DNA we find on Earth would be found on a planet with an independent evolution. Consequently we believe that we couldn't eat the plants grown on that planet. Indeed, we suspect they'd be poisonous.


It's certain that we know that some types of chemistry are beneficial to the body, some are detrimental, some require massive amounts to be one or the other, and yet others have no effect at all. And that's just here on Earth.

We also know that the chemistry that makes up Earth is the same chemistry that's all over in the Universe. Does that mean there couldn't be an ammonia-based lifeform? Or silicon-based? Or something else? No... only that their chemistry must conform to the properties of the Periodic Table of the Elements just as everything must do here on Earth.

Which means that your quest for scientific realism is, if you'll excuse my frankness, misguided. Your iron-based-blood creature will have trouble eating meat from a copper-based-blood creature if you want it to be so. Nothing more. Nothing less. Because that's the way it is in real life.

Still not convinced?

There are humans who are lactose-intolerant... and those who can freely benefit from consuming it. There are humans who are allergic to peanuts... and those who can freely benefit from consuming them. There are humans who are allergic to water, and yet they need water just to live. The physiology of nutrition and digestion is not in any way binary. Some people need a lot of protein to be healthy, others need very little. Some people can't gain weight to save their souls in heaven. Others gain ten pounds looking at a picture of chocolate cheesecake. You can make yourself tolerant to some poisons by training your body to them. Others can't eat some foods because of what those foods look like, taste like, or the texture of the food — the issue having nothing to do with nutrition.

So what's reality, really?

Far too many people forget that the fundamental truth of science is the necessity — the necessity — that facts must be proven through repeatable empirical experimentation. Mathematical prediction is not science. A clever YouTube video or fun insight in a novel is not science. And what's real is much more forgiving than the current fad of scientific realism permits.

Unless you plan to change chemistry in your universe, the answer to your question is, "it's believable that one can eat the other." Notably because anyone who disagrees can't show a repeatable experiment to prove you wrong. Of course, they couldn't do that if you'd chosen another of your three bullets, either. But that's both beside the point... and the point.

Trying to build a fantastic world pursuant to what we know about science today (because it won't be entirely true according to what we know about science tomorrow) is noble — but good worldbuilding is more concerned with consistency and suspension of disbelief, and if you can't suspend it using the Scientific Method, then suspend it with conviction that your idea it great.

  • $\begingroup$ I'm sorry... H2O intolerance is a real thing? $\endgroup$
    – No Name
    Commented Jun 19 at 20:48
  • 1
    $\begingroup$ @NoName sure... for example, there was a well-documented outbreak of it on the Titanic $\endgroup$ Commented Jun 20 at 4:38
  • 1
    $\begingroup$ No I saw, and read. I was just surprised that an offhand joke from Finding Nemo actually has basis in reality $\endgroup$
    – No Name
    Commented Jun 20 at 5:09
  • 1
    $\begingroup$ @Bobson "pretty much nothing would stop the physical act of eating from being possible" reminds me of the French(?) guy who ate an airplane. $\endgroup$
    – Michael
    Commented Jun 21 at 14:47
  • 1
    $\begingroup$ @Michael I believe you're talking about Michel Lotito. $\endgroup$
    – JBH
    Commented Jun 22 at 4:27

Ultimately, we have one example of a tree of life and associated amino acids and proteins, so a totally alien biochemistry is unlikely to interface well with Earth biochemistry, fullstop, regardless of the molecule that carries oxygen (or whatever they use!) in their blood.

So, broadly, the answer is likely "no". However...

There is an example on Earth

The medically important Horseshoe Crab famously has blue blood because it uses hemocyanin (a copper-based oxygen transporter) in place of hemoglobin.

Horseshoe crabs routinely eat the same things as hemoglobin-employing sea creatures. So if one ignores all the other biochemical issues, copper-based blood would be no barrier to enjoying human food. (But the other issues are way more likely to be an issue than the oxygen transport molecule.)

Edit: To more fully answer the question as asked, humans also can and do eat horseshoe crabs. They are not generally viewed as delicious, but we humans are pretty well known for our habits of trying to eat everything that doesn't immediately kill us. So the copper/iron divide is traversable from either direction.

  • 1
    $\begingroup$ However if the alien blood was based on arsenic, mercury, or even plenty of necessary elements (like sodium or potassium - although they're probably too reactive to make sense, or selenium) you'd have a problem. In general elements in the same group of the periodic table as necessary ones are likely to have issues in large quantities, even if necessary in trace amounts $\endgroup$
    – Chris H
    Commented Jun 19 at 8:32
  • $\begingroup$ @ChrisH - this is true, but the question as written was copper-based vs. iron-based. $\endgroup$
    – jdunlop
    Commented Jun 19 at 8:59
  • $\begingroup$ "and varying made-up metallic elements." so not just copper. Those made up elements (a concept I struggle with TBH) are worthy of a footnote at least, which is what I try to offer $\endgroup$
    – Chris H
    Commented Jun 19 at 9:25
  • 1
    $\begingroup$ A bigger example on Earth are the insects which have ichor (blood-equivalent) also based on Copper. They are edible, modulo cultural conditioning. $\endgroup$
    – nigel222
    Commented Jun 19 at 10:29
  • 1
    $\begingroup$ Nevertheless, copper toxicity is real. Humans also constantly regulate the iron in their bodies, and some is continuously lost. A diet exclusively of copper-based food would kill a person eventually. $\endgroup$
    – Wastrel
    Commented Jun 19 at 14:36

Its a definite maybe

One problem would be deficiency diseases if something needed here only as a trace element is required in orders-of-magnitude larger quantities by the aliens. For example Molybdenum or Chromium. This could be overcome by supplementation.

Another problem might be toxicity. Here, hexavalent chromium is now known to be a serious cancer hazard to humans. Bad news for eating on an alien world where evolution found a positive role for hexavalent Chromium.

Here on Earth we can eat things with Copper-based blood and vice versa. But that is very probably co-evolution. There are some obscure marine worms with Cobalt-based blood. I have no idea if we've tried eating them, but fish in the same environment probably do. Unless they signal their toxicity in some way (like black-and-yellow strips on terrestrial insects, but there's no light in the abyss).

There's also the possibility of toxicity arising not from any single element but from alien biomolecules. An Earth example is Glyphosate (the weedkiller) which has low toxicity to animals but large toxicity to plants. It's an organophosphate. Phosphorus is absolutely essential to all life, but combine it the wrong way and you get deadly nerve gases.


Sure, no problem. We have plenty of animals with copper based blood (including horseshoe crabs, some lobsters and shrimp, some octopuses and quid, and countless insects), and not only we can eat them just fine, but they can eat us right back.

There are some more...exotic options like cobalt based blood, but these are extremely rare. The whole point of a metal-protein blood pigment is to bond with oxygen well and carry it: and that function works best with iron and copper. Most other metals either do not form such metal-protein pigments well, nor are they functional as oxygen carriers. They either do not bond well enough to be useful (so absolutely no gold-based blood, and likely no tin-blood either), or they react all too well (magnesium based blood would fail spectacularly). Iron based blood hits the sweet spot of proper oxygenation between Organism Suffocates and Organism Bursts in Flames. Copper is the close next.

you can probably have an organism with cobalt-based blood, though this is so inefficient I doubt you could scale one up to be bigger or more complex than a tiny worm.

So essentially, stick with iron and copper.


You can generally get around having to magically hand-wave everything by transfering the problem into your universe, instead of viewing it as something kind of outside the universe that you need to explain to the reader directly.

This means that if you are unsure whether humans could eat unobtainium food, you will not, as an author, talk directly to the reader (via generic paragraphs of just explaining how your universe works), but you will leave it to your characters to show it.

For example:

  • Bob, a wizened veteran, slaps the pink berry out of the hand of Max, his young padawan, and explains that this fruit smells great but will rot his skin away if eaten.
  • The crew of a spaceship stranded on an unknown-to-them planet with unknown biomass spends a nice few weeks of using up their food reserves and starving a little bit, before working up the courage to eat some plants or animals sneaking around their little base. All kinds of horrible or hilarious results ensue.
  • A high-tech space station shared between all kinds of alien species has separate areas for the separate needs (i.e., Oxygen-based air, Sulfur-based air, water-based, high-pressure, high-temperature and so on and forth). Where different species share the same air, the restaurants offer wildly different meals for each, and while a human eats their pizza, his alien business partner guzzles his smoothy with little bugs worming around.
  • As a special case, let one species be a common food item for the other...
  • Species can easily base their decisions on prejudice, religion, culture, or simply taste, and if you don't know how to explain something scientifically, you never need to do so.

And so on and forth. Just avoid the issue, your story will very likely be more entertaning for it.


As long as the life is generally carbon based it is likely they'd be able to eat most plants like humans could. Sugars, oils, alcohols, starches, and many of the trace minerals that come with them should be fairly universal. There's some molecules with a "left hand" and "right hand" twist to them that make one having some effect on the human body but the other inert. I don't recall any drugs in particular as examples but there are drugs that have left and right twists to them that will leave one effective and the other ineffective or far less effective. I may be mistaken but naturally produced chemicals like these will bias strongly for one kind of twist while artificially produced there's a near equal number of each.

A badly shaped protein, a prion, could cause bad things to happen. If the meat from some alien species has such proteins then it would be bad to eat. If the only difference is in the oxygen carrying nature of the blood then there could be heavy metal poisonings from eating too much. But then too much of anything is bad, too much rabbit meat is bad. Eating too much polar bear is bad, or rather too much of the polar bear.

There are plants that produce a kind of allergic response in humans while some animals eat them up without any harm. A plant grown in some soil could be safe but if grown in other soil could contain poisonous metals, a species that relies on these metals for their blood would not likely find this poisonous.

I'll agree at least in part with other answers presented. If you want what is scientifically based then use examples seen on Earth. If you want to add some story element then add some fantasy to do what feels right. There's enough variables in life as we know it that you can establish that maybe something is healthy or not because of some conditions of the soil the plants were grown in, there could be some proteins or vitamins in the meat that could be poisonous, some people could be allergic to something while others not, and so on. If you want a character to die from eating something then make that happen then find something that fits, make something up that sounds good, or maybe just leave it a mystery.

If the main variable you want a focus on is the differing metals in the blood then consider the different metals that fit and that primary concern is some kind of metal poisoning. Most everything posed by myself and others will have some kind of effective treatment, at least for humans. If there's characters concerned of poisoning because of a meal then perhaps they take some kind of treatment as a precaution, before or after consumption. Perhaps this treatment is also a food, drink, or seasoning that could be safely shared with the other species. Or maybe it is something offered and the other species politely refuses, both knowing that it might not be good for them, just so everyone is civil and pleasant over a meal. Think of people that might not like alcohol, have their glass filled regardless, but doesn't drink any or takes only a sip to be polite. They've technically "shared" the meal, which might be more important culturally/politically than consuming the meal, and all leave the table healthy.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .