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Suppose humans have developed the technology to travel between star systems. This might be some science-fiction method like warp drive, jump drive, peanut butter drive, hyperspace drive or whatever, or something vaguely scientific like the Alcubierre drive. The specifics of the means of propulsion doesn't matter here.

Now, a spacecraft carrying a number of humans-as-we-know-them has settled into orbit around an extrasolar planet, and the people onboard have determined that the environment on at least a part of the planet's surface is suitable for human life (about right oxygen content, not too hot, not too cold, ...). The next step is to determine if humans could actually live on that planet, as opposed to just briefly visiting.

One of the things that often get handwaved away in science-fiction is the ability of colonizers to eat and process foodstuffs found on other planets. Given how many different possibilities there are for how amino acids could be combined, and how those can form protein chains, it seems dubious that humans would be able to pick up some alien foodstuff that has evolved on some far-away planet, eat it, and derive any significant (if indeed any) nutritional value from it. We just haven't had any reason to evolve that ability.

Would humans be able to derive nutrition from alien foodstuffs? If yes: Why? If no: What would be the main complicating factors?

I'm tagging this reality-check because we don't know much of anything about what alien foodstuffs might actually be like, but bonus points for answers drawing validly from known sciences. (Basically, as much as possible, treat this question as if it were tagged science-based, or hard-science if you are able to.)

I believe this is not a duplicate of Compatible biochemistry, or not? (hat tip to Frostfyre for finding that one) because that question appears to posit that the biochemistry is compatible and asks if that implies that there is biochemistry compatibility both ways (alien life being able to eat Earth biomass implying that Earth life should be able to eat the alien biomass), whereas this question asks about biochemistry compatibility in the first place.

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    $\begingroup$ You already won the exoplanet lotto. Might as well double down. $\endgroup$
    – Kys
    Jul 19, 2016 at 18:37
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    $\begingroup$ Wouldn't they bring the required resources like seeds with them? A more realistic question may be whether or not Earth crops could grow in that planet's soil. $\endgroup$
    – GrandmasterB
    Jul 19, 2016 at 20:59
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    $\begingroup$ If the planet is livable, breathing and temperature and such, the easiest thing would be to just plant seeds you brought from home. Failing that, gather up a bunch of stuff and reduce it down to a paste of carbs and sugars, chemically, hopefully having checked for poisons, form it into tasteless nastiness. $\endgroup$
    – Seeds
    Jul 19, 2016 at 21:02
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    $\begingroup$ @Seeds However, this might pose an ethical problem - is it a good thing to introduce our species to an alien planet? They might very well take over the entire world over time and cause an ecological catastrophe. $\endgroup$
    – Luaan
    Jul 20, 2016 at 8:57
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    $\begingroup$ Simply landing on a planet will introduce uncountable microorganisms to the environment, planting wheat isn't going to make a huge difference. Either the existing life adapt, or they don't, the only other option is to never leave earth. Additionally, if the existing plant life isn't compatible enough for us to eat, then it won't be compatible enough to cross pollinate (if it even uses that reproductive strategy) so it will be very simple to control what we plant. $\endgroup$
    – Seeds
    Jul 20, 2016 at 15:02

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The answer is "it's possible, but VERY unlikely". There's a lot that goes into this, but the short answer is that there's no guarantee the proteins that developed on this planet will be compatible with our biochemistry due to a little bitch of a thing called chirality. And even if you get lucky and find a planet with compatible proteins (possible but definitely not probable), there's a lot on our own planet we can't eat, and it's very likely that most of the food there would contain elements or chemical compounds poisonous to use due to having evolved under different circumstances than life on Earth.

That being said, it's not entirely a lost cause. As long as you've tested the food and are certain it's not poisonous, even alien organisms with reverse chirality proteins can provide you with simpler nutrients like sugar and carbohydrates (maybe fat too?). You still won't be able to digest parts of it and you won't get any of those life-sustaining proteins, but there's no reason an alien fruit couldn't be as sweet and juicy as Earth fruit. Alien ethanol (a.k.a space hooch) is also possible with some fermenting and proper processing.

So, can you eat alien food? Yes and no. Assuming it's not poisonous, you can eat it and maybe even get some simple sugars and carbs out of it, but don't expect to get any of your daily protein from it (or even completely digest it for that matter).

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    $\begingroup$ @Shalvenay: This is not accurate. L-Glucose was briefly marketed as an artificial sweetener, but abandoned in favor of cheaper substances. According to NASA, it is not even possible to discern the difference via taste. $\endgroup$
    – Kevin
    Jul 20, 2016 at 5:35
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    $\begingroup$ I imagine that some fitness food company would love a sugary but otherwise calory free fruit, that thing would be a killer product. $\endgroup$
    – DrakaSAN
    Jul 20, 2016 at 9:39
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    $\begingroup$ There are only two chiralities, so I don't see why it would be so unlikely. $\endgroup$
    – Christopher King
    Jul 20, 2016 at 12:58
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    $\begingroup$ @Kevin So Shalvenay is half accurate. L-Glucose is 0 calorie and tastes sweet, with only rare earth organisms able to metabolize it. This also disagrees with Schroeder's last paragraph: even somple sugars (like Glucose) could be indigestible. $\endgroup$
    – Yakk
    Jul 20, 2016 at 15:39
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    $\begingroup$ Chirality can also cause all sort of other unexpected problems - thalidomide being the prime example. One form of the molecule cures morning sickness. The other causes birth defects. www3.fed.cuhk.edu.hk/chemistry/files/chiraldrug.pdf So those alien reverse chirality proteins may do all sorts of unexpected stuff from getting you high to shutting down vital bits of your metabolism! $\endgroup$
    – DrBob
    Jul 20, 2016 at 16:29
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Most plant food on Earth won't sustain people. That's why people were hunters before farmers. Meat, at least earth-meat, is easy to digest in almost every form. Few plants have anything edible about them, as a defence mechanism. Consider the Apple Tree, it only produces apples specifically so that animals/birds eat them, walk somewhere else, and poop out the seed, thus propitiating their species. And this is only in the fruit season. The leaves, the bark, the sap, little to no nutritional value, and many actually have negative caloric values; they cost more to digest than they provide.

When people figured out that hey, there are a couple fruiting-body plants out there (corn, fruits, vegetables) that provide nutrition that they don't have to risk their lives for, we made sure to make A LOT OF THEM. In untouched nature they are very rare. Grass is likely the only edible thing you'll find in the fall in any place that has no people in it.

So on an Alien world, if it had cultivated a civilization, maybe they've spread the "edible" food (edible to them, whether that means to us I can't say), but otherwise the entire world is designed to sustain life -- to prevent being eaten. Carrots and the like survived by hiding underground -- that protected them from above-ground threats. So animals learned to dig and find them.

Many foods that do have edible mass, at least using Earth again for examples, contain poisons, UNLESS eating them is part of the propagation cycle. Consider Mushrooms for example. They don't need you to eat them to spread, and most all of them are poison to people.

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    $\begingroup$ If the plants of an alien planet had never encountered humans before, it's possible none of their evolved defense mechanisms would work on us. Humans could just as easily become an invasive species. $\endgroup$
    – DaaaahWhoosh
    Jul 19, 2016 at 19:30
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    $\begingroup$ A non-domesticated carrot is just a mundane root, btw. $\endgroup$
    – JDługosz
    Jul 19, 2016 at 20:38
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    $\begingroup$ Mushrooms are the fruiting body of mycelium. Eating a mushroom is equivalent to eating a flower or a fruit from a plant. The main part of the organism is usually a series of root-like fibers running through whatever material it's drawing nutrients from (e.g. rotting wood, soil, etc). $\endgroup$
    – Mar
    Jul 19, 2016 at 21:37
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    $\begingroup$ Edible plants around here in the autumn, not cultivated or bred by humans: Blackberry, beech mast, chickweed, mallow, sorrel, dandelion, elderberry, fat hen, rosehip, haw, hazelnut, mint, rowan berry, sloe (with some processing), silverweed, nettle. Some of those are leaves with little nutritional value, but there are a range of nuts and berries as well. You're imply wrong about there being next to nothing in the wild that's edible. $\endgroup$
    – Mike Scott
    Jul 20, 2016 at 6:50
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    $\begingroup$ @MikeScott Berries are a seed propagation mechanism, so it's not surprising they are edible. Nuts are "seeds" - they are "designed" to be protected by the hard shell, but we found ways around that protection. Of the rest, I don't think you could actually feed yourself - while they do have nutritional value, it's not quite enough for a human, and they're very hard to digest. Of course, specialised animals can and do feed themselves on plenty of plants. Edible isn't quite enough - you need something you can actually live on. And don't forget humans are incredibly poison tolerant. $\endgroup$
    – Luaan
    Jul 20, 2016 at 8:37
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Let's start by changing the question, and later apply the answer to this to the question as asked: Instead of a different planet, let's imagine our explorers are actually visiting our own planet Earth, but in a different eon. Say, the Triassic Age, to pick one at random.

Now, ask the same question: Can they eat the plantlife?

In theory, framing the question this way should make it easier to answer: We're removing all issues of alien DNA or chirality, etc.

But in my view, the answer would still be a resounding 'No'.

The Triassic Age pre-dates the appearance of flowering plants, so you would have no fruit. There's also no grass species yet at this point. So at a stroke, we've lost the vast bulk of plants that humans like to eat.

There are, of course, plenty of plants in the Triassic, but none of them are familiar, and most of them will be inedible. You would have to spend a very long time doing a careful analysis of the plants that do exist, analysing them for nutritional value and for toxins (and there are likely to be toxins that you haven't seen before).

Given enough time, it's possible you might find a few species that are edible; possibly including the distance ancestors of some of the food plants we know today. But it won't be easy; my guess is that your colony will not survive.

And this is on our own planet.

Phrase the question on a different planet, and the issues become orders of magnitude more difficult. There may be out-right compatibility issues such as chirality or alien DNA. But even if the local life forms are completely compatible with us in that respect, they are still likely to have evolved their own entirely unique toxins and bio-defences to avoid predation, and as we've never had to deal with them, humans will be totally unable to cope.

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    $\begingroup$ Hi Simba. I added a point to your first paragraph that you do apply the answer to the question as asked as well, which should help prevent unnecessary downvotes. Feel free to roll back if you disagree, but be aware that answers to different questions are often considered "not useful" and thus downvoted so in that case you may want to add a TL;DR at the top instead. $\endgroup$
    – user
    Jul 20, 2016 at 9:01
  • $\begingroup$ @MichaelKjörling - thank you. The edit is good. :) $\endgroup$
    – Simba
    Jul 20, 2016 at 10:26
  • $\begingroup$ There were cycads in the Triassic, and their seeds are sort of edible (with processing) see: plantnet.rbgsyd.nsw.gov.au/PlantNet/cycad/ethn.html $\endgroup$
    – user42528
    Jan 11, 2019 at 15:02
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The short answer is, "it depends":

  • If you subscribe to any Panspermia ideas, then the answer is a maybe to probably. If you go with the more aggressive Panspermia approach where "all these worlds were once settled by humans in the previous galactic empire that crumbled into dust" then the answer moves up to yes. Terraforming has been done and random evolutionary chemistry is toast. Everything, right down to the microbes, is compatible.

  • If you view a new world where life evolved from scratch, with random chemistry, cellular structures, and biome then you are not long for the world. You would be lucky to survive a breath of air or touch of a plant, much less trying to stick something into mortal combat with the colony creature that is our stomach.

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  • $\begingroup$ This sounds overly dramatic. Why would a plant on an alien planet have developed a poison that is deadly to humans - a species it has never before encounterd? I would expect touching an alien plant to be approximately as dangerous as touching rock. $\endgroup$
    – Poseidaan
    Nov 26, 2020 at 14:11
  • $\begingroup$ It wouldn't be a poison; just horribly incompatible. If it's not a CHON base, then it might uses cyanide, or something radioactive, or, hopefully, silicon. Injesting such a substance is more like drinking a few things out of the chemical cabient than out of the quarry. $\endgroup$
    – Charles Merriam
    Dec 13, 2020 at 11:45
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Glucose is a simple molecule and a good energy store. I wouldn't have any trouble with believing in alien biochemistries that used it, and the various saccharides built up from it. Going up the complexity scale to fats and proteins, things start getting progressively less plausible.

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  • $\begingroup$ I believe that combining glucose with oxygen provides the highest energy release of any biochemical oxidation/reduction reaction in Earth biology. Any card carrying biochemists able to confirm this? If it is true then the first alien biochemistry to hit on synthesis and use of glucose will rapidly outcompete everything else. $\endgroup$
    – DrBob
    Jul 20, 2016 at 16:39
  • $\begingroup$ @Luaan. Nothing to do with human body temperature. It's how far apart glucose and oxygen are on the redox tower - the further apart the two chemicals, the more energy released when they combine in an oxidation reaction. There's a redox tower with glucose on it on this webpage (scroll about half way down): cnx.org/contents/oetasFbh@1/Bis2A-51-REDOX-Chemistry-and-t All oxygen 'breathing' creatures from aerobic bacteria to whales use glucose in their metabolism. $\endgroup$
    – DrBob
    Jul 23, 2016 at 17:57
  • $\begingroup$ @DrBob Duh, I just realized you said "in Earth biology". I was thinking about a bit more extreme conditions, e.g. atmosphere so oxygenated or hot that glucose spontaneously combusts, too cold/hot for our metabolism etc. $\endgroup$
    – Luaan
    Jul 23, 2016 at 19:37
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Assuming that it is a colony ship, it should already be extensively using Hydroponics. As elements are universal, you should still be able to use the alien plant life as a composting agent in order to grow Earth plants on the ground, and use the existing Hydroponics farms on the ship to supplement the food source until a proper crop rotation can be established. Why settle for alien food that is yet to be proven safe to eat when you can just eat Earth stuff for many generations to come. Once you've done the science to determine the nutrition or use of alien plant life then you can start cultivating those plants.

If you're worried about the alien compost making the food toxic, then it's not a huge leap that a society that can make an FTL drive can have a relatively inexpensive way of dividing up matter into it's base elements.

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  • $\begingroup$ This reminds me of Nonstop, where the whole population of a generation ship was poisoned because of an alien amino acid all their (Earth) plants took on. It made the plants grow much faster, but happened to kill most humans. It might be safer to break everything down to bare elements before using it as fertilizer :D $\endgroup$
    – Luaan
    Jul 21, 2016 at 13:47
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There are only so many amino acids, and they can only be combined in so many different ways. It is possible that humans could digest foodstuffs found on an alien planet. It is not probable though.

Digestive enzymes would be the limiting factor. There are foodstuffs here on earth that humans cannot eat. Like high-cellulose vegetation. Humans are only designed to break down specific compounds. And even among humans there are those who cannot break down commonly consumed compounds like lactose.

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    $\begingroup$ I would doubt that alien life would even use the same amino acids as Earth life. $\endgroup$
    – zeta
    Jul 20, 2016 at 5:59
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    $\begingroup$ @sumelic: So what amino acids would they use? $\endgroup$
    – Williham Totland
    Jul 20, 2016 at 7:50
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    $\begingroup$ There are only so many amino acids on Earth. Is that really a fixed limit, or just a quirk of how life developed over time? Is there something preventing the existence of hundreds of other amino acids? Some perhaps digestible (and convertible) by humans, but others useless or outright poisonous? Not to mention that proteins made out of those amino acids may be much more of a problem than the acids themselves - after all, that's how plenty of toxins work. And yes, plants are tricky even on earth - but what about alien animals? Alien fats might be useful even to us, for example. $\endgroup$
    – Luaan
    Jul 20, 2016 at 8:40
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    $\begingroup$ @WillihamTotland Take your pick. There's 70+ amino acids which scientists have coaxed existing organisms to use, and there's there's specialty suppliers who have thick product catalogs listing various amino acids you can use with purely synthetic techniques. There's some reasons behind the set of amino acids we have, but to some degree it's a historical accident: The first successful organism used that set, and everything since has (mostly) kept it because it's too hard to change. $\endgroup$
    – R.M.
    Jul 20, 2016 at 16:46
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    $\begingroup$ @WillihamTotland I think it's a bit debated. Francis Crick said "... the actual amino acids in the code are at least in part due to historical accident." but Stanley Miller (yes, that Miller) later wrote in an extensive analysis "If life were to arise on another planet, we would expect that the catalysts would be poly-alpha-amino acids and that about 75% of the amino acids would be the same as on the earth." $\endgroup$
    – R.M.
    Jul 20, 2016 at 19:24
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Assuming humans would survive the trillions of new viruses long enough to worry about grabbing dinner then I would say maybe, with a lot of work.

The physical elements of life on an alien world would pretty much be the same (Hydrogen, Carbon, Oxygen, etc) so there is always the possibility of taking alien life and breaking it down into its raw elements and regrowing it into something edible. However the constraints of doing this are numerous. Even if we assume that we have abundant energy, complex machines, a willing robot work force, a willing alien world that is happy for us to eat it, etc, the process would be time consuming and probably not very scale-able. The infrastructure needed to support and feed a city of a million could take years to develop.

Also if we assume we land on a world as complex and diverse as our own, there would probably be something we could eat--or find genetically interesting--but the problem would be finding it. Think about the Amazon. There are millions of different plants and animals hidden in there but finding them is a problem. The logistics of putting together an proper research time to find, catalogue, test and taste every species on a new planet would be staggering. In the end it might be cheaper just to fly back to Earth for lunch (supplies.)

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    $\begingroup$ Hi Robert, and welcome to Worldbuilding. What makes you think that alien viruses would be able to affect humans at all? (What's to say they use DNA or RNA? Both are awfully complex, and I don't see how either would be needed even to construct proteins and amino acids like those we are used to, it's just the approach that was taken by Earth life.) And while the physical elements of a human-habitable alien world would probably be quite similar to those of Earth, saying that this holds for the life of any alien world seems a stretch. $\endgroup$
    – user
    Jul 20, 2016 at 8:00
  • $\begingroup$ Not to science nerd out on you or anything (hell, if you can't do it here, where can you), but DNA and RNA aren't really that complex. The chemical formula for Cytosine--one of the based in DNA-- is just C4H5N3O. The right conditions and just random probability alone could generate DNA. From there its just a matter of stacking and re-stacking the DNA in one of the trillions upon trillions of space virus out there until you find some deadly combination that causes our intrepid space adventures to grow a second head, turn green, etc.... $\endgroup$
    – Robert Ben Parkinson
    Jul 21, 2016 at 6:15
  • $\begingroup$ @RobertBenParkinson Consider the viruses that actually affect humans on earth. Almost all of them originate from animals extremely close to us (simians, domestic animals). Unlike bacteria, viruses are extremely sensitive to their hosts - they exploit that your body's cells to do all the work for them. It's like a computer virus - sure, a bug in the WMF parser might allow you to run arbitrary code, but if you can't read WMF at all, you're safe. Not to mention reading it on an IBM/360, for example. Even if a lot of things end up being the same as on Earth, there's a lot of wiggle room left. $\endgroup$
    – Luaan
    Jul 21, 2016 at 13:57
  • $\begingroup$ Given that we barely even have any shared illnesses with plants, which are at least distantly related to us, I doubt anything evolved for an alien ecosystem could infect us. $\endgroup$
    – Ettina
    May 30, 2017 at 13:25
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Humans are good at dealing with substances which occur in nature, we are tolerant of them. In comparison we tend to be very intolerant of things we haven't encountered much on our evolutionary journey to the point that minuscule amounts tend to be very harmful or even fatal. Consider as examples of this simple elements like lead or mercury which are extremely toxic to us.

Given this, it seems likely that unless the alien life is pretty much identical to us at a biochemical level, the best case scenario is probably just surviving ingesting any of it, never mind deriving any sustenance.

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  • $\begingroup$ Yes but the point is the reason they are so harmful is probably that we (i.e. life on earth) didn't encounter them in the wild very often and so haven't adapted to their presence. If Lead was more common in the environment I bet earth organisms would probably have evolved to be more robust to their presence (right down to the 'protein' level). I think it's a perfect example. $\endgroup$
    – crobar
    Jul 21, 2016 at 14:21
  • $\begingroup$ If the entire surface of the earth had always been covered with lead particles I bet there might still be life of some kind and it would be tolerant of lead (which includes being able to detect and avoid it). We only avoid eating it if we know about it. Can you detect lead in water by taste? We are virtually the only organism on the planet which can avoid lead by detecting it using science. $\endgroup$
    – crobar
    Jul 22, 2016 at 9:04
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Given that we are only able to digest a limited number of compounds it's unlikely that humans would be able to digest the compounds of life on another planet.

Also, considering that complex molecules that we aren't evolved to digest tend to be toxic, an alien life form, even if it was not poisonous to most other life on its planet, might be poisonous for humans to eat.

If one were to kill and eat an alien "animal" for instance one might be in a worse position than if he/she hadn't eaten the alien "meat", as not only would the alien "meat" be unlikely to provide nutritional value, but it's likely it would cause the person to get sick or even to die from ingesting something that is poisonous to humans.

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I'd suggest that humans would have been modified by this point to be capable of 'ignoring.' eating normally or reconstituting on the fly any given foodstuff they cam into contact with.

Growing and carrying food around is immensely wasteful, no interplanetary colony ship would be carrying hydroponics or any such thing. Rather they'd be taking one of two approaches:

Firstly: To make use of communal devices that breakdown and reconstitute any given material into it's component parts and reconstitute in any given desired format. That isn't star trek science, it's a given that this is possible as we already do it.

Secondly: To do essentially the same but with individual internalized devices (biological or otherwise.)

The second approach would clearly require more effort, but I propose that it would be a device/modification that every and all interstellar travellers would make use of, being as it with one shot renders starvation and gastric poisoning implausible.

Eating for the sake of taste would therefore probably become a niche interest/industry, and the 'compatibility' is then a given as you're not caring about the availability or providence of a given complex molecule.

Other than that, the human body is designed to take in lots of mass and make use of what it can, on the assumption that it cannot or (in any particular instance) does not use most of it.

We could suppose that in an alien ecology a person might have to eat several times the volume to gain anything like the same nutritional benefit, or the compounds might take longer to break down in the stomach and not actually be absorbed by the time bowel movements pass them out (same with many terrestrial foods.) It's a mistake to imagine that everything we eat is absorbed by the body, or that everything we pass out cannot be used. It's a very very very far from perfect process even given foodstuffs we're relatively well adjusted to absorbing.

The opposite can also be true and certain chemicals might be more abundant in alien foodstuffs than in terrestrial options, even should those chains perform a quite different function in the alien organism. A given protein does not do x or stop y in every conceivable organic architecture, rather it's inclusion and operation is dependent upon the rest of the organism's constituent parts & it's environment.

The likelihood of one or t'other or any alternative is impossible to calculate of course, as any question of "would non-terran life be or do z or f" is.

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  • $\begingroup$ Not sure what the -1 is for, does somebody disagree that humans can breakdown or synthesize any given molecule? Or that by the time we're star-hopping the re-engineering of our gastric system will be ancient history? Well.. $\endgroup$
    – mensenisevirem
    Apr 5, 2017 at 7:25
  • $\begingroup$ "... breakdown and reconstitute any given material into it's component parts and reconstitute in any given desired format. That isn't star trek science, it's a given that this is possible as we already do it." - I have epically failed to keep up with the literature. Link please? $\endgroup$
    – Emilio M Bumachar
    Jul 29, 2019 at 11:43

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