In case humans discover some planet populated by alien species, how likely is it that we could eat and would find the alien species nutritious enough to live off?
Heck, some humans can’t even handle Mexico.
Unlikely. Our digestive system is shaped to break down foods into certain sugars, amino acids, and fats that our bodies can use. It is quite unlikely that an alien life form would be made up of the same amino acids, sugars, and fats, and even less likely that they’d be chained together in ways that our digestive system can break down. Then add in the fact that alien life forms might have traces of any number of substances that are poisonous to our bodies.
Extremely unlikely, in my opinion, though I’ll point out that until we have at least one more data point to work with, it’s all just more-or-less-educated speculation.
Agreed its unlikely, but if we do find anything of value there (food, natural resources, etc) those aliens better have defenses. Otherwise the Terrans are moving in.
Plants, fungi and even species as closely related as fish can be deadly for humans to eat. Aliens? No way.
I don’t have the book around, but I seem to recall in the novel of 2010: (Space Odyssey Two), one of the “Europan” (moon of Jupiter) sea creatures eats a human space explorer’s corpse (from the crashed Chinese spacecraft maybe?), and gets sick and dies because of incompatible chemistries between the two species - so it’s not like this concept hasn’t been thought of before.
If we assume we have the technology to get to this planet then we can assume that we would also have the technology to genetically engineer both plants that would grow in the alien soil and that would be safe for us to eat and also animals that would eat native vegetation but be safe for us to eat.
I doubt it. Terran multicellular metabolism isn’t very diverse We can eat many animals, plants, and fungi because their metabolisms are very similar to ours. Our digestive systems break their bodies down into the same building blocks that our bodies use. If we encounter some alien grasses that don’t use glucose (and by extension cellulose) or the same amino acids we do, or that have left-handed DNA, engineering a grazing animal that can subsist on said grass and then convert everything into nutrients suitable for a Terran eukaryote would be either impossible or require such an enormous biochemistry overhaul that it would never be worth it.
Even if we assume that the amino acids we’re familiar with are a natural choice for all life to use, and even if we’re only trying to eat some alien species or another (not all of them) so we can pick and choose ones that happen to not have any toxins (if there are any), the chances are still only 50% at best. Almost all biological molecules have a handedness, and only one handedness is actually used by terrestrial life. Life would work just as well if all of those molecules were of the opposite handedness, so you’d have to assume that every planet where life developed just flipped a coin to choose one.
Unless it turns out that panspermia is correct, and all life in our galaxy has a common source.
Why would that be, given that we are finding examples of all 3 of these components in extraterrestrial sources?
Even that might not help. The living things that we enjoy as food are the end results of evolution (so far) on this planet - even starting with the same origins, the end result could probably have been quite wildly different.
Because even though there are amino acids, fats and sugars out there, we can only use the small subset of amino acids, fats and sugars utilized by life on Earth. Even if alien life uses amino acids, fats and sugars it’s unlikely it uses the exact same set, and even when it does use the “same” molecule it might be the wrong handedness.
It was just in the news that simple sugars were found around another star. If recognizable organic molecules occur in space I wouldn’t be surprised if chemistry similar to what supports life here happens on other planets.
Given the hugeness of space I would expect there to be something to eat out there made of proteins, carbohydrates and fats. No telling how much luck we’ll ever have of finding it or how much cooking it would take to be edible for us.
There could be a Twinkie planet out there infested with Twinkies, but for all we really know about other planets such a place only exist to us as a dream for now.
Let’s just hope we aren’t someone else’s Twinkie Planet. 
Just spread out and look for Roddenberries. 
Like Chronos said, biochemicals have handedness. For example, lipids in terrestrial eukaryotes are right-handed, with few exceptions. Here’s a story about a lipid that’s left-handed, specifically to make it harder to digest:
http://www.rikenresearch.riken.jp/eng/research/6906
Lipids have a handedness, so do sugars, and so do proteins. I suppose getting them all to match would be a 1:8 chance.
First point: It’s incredibly unlikely that alien life would even use amino acids or sugars. Sure, amino acids and sugars can occur by accident. But so do literally hundreds of thousands of equivalent chemicals. We might find it interesting to discover those compounds in nebulae because they are what we are made of, but this just human selection bias at work. We are ignoring the plethora of other organic molecules that are found in precisely the same environments.
Even if alien life did use amino acids and sugars, it is ridiculously unlikely that they combine them in a form that we could digest. Hell, even ubiquitous carbohydrates like lactose and cellulose are utterly indigestible to 99% of life on this planet, as are proteins like keratin or Conchiolin. And we evolved alongside those things.
If alien life did make use of amino acids and sugars, the odds of being able to digest the proteins would be much more likely than the odds of being able to digest carbohydrate. Proteins are fairly basic condensation polymers. Any water and aa based lifeform will probably come up with proteins. But carbohydrates? Hell no. Put two common sugars together the wrong way and they become instantly indigestible. The odds of utterly unrelated life combining precisely the same sugars in precisely the same ways that Earth life does is astronomical.
Even if the proteins and carbohydrates were technically digestible, the odds of them being toxic seems very high. There is no reason to assume that alien proteins would use sulfur and aromatic side chains the way that Earth life does. That seems to be largely the result of random chance. If alien life uses Arsenic, fluorine, cyanide or other side chains we would very likely find them to be digestible *and *universally poisonous.
So that leaves fats. Lipids are actually the one food stuff that is likely to be universal for carbon and water based life. The use carbon chains in carbon based life seems unavoidable, and the attachment of a carboxyl group to a carbon chain to make it partially water soluble would then seem inevitable for water based life.
That seems like a non sequitur. Biotechnolgy and space flight are utterly unrelated technologies. Given that we have absolutely no idea how to achieve either goal at this point, I can’t see any possible way to justify a claim that if we can do one impossible thing we can do two. It’s like a caveman claiming that if people can fly to the moon then they can overcome old age. Both achievements are equally impossible, but they are so unrelated that an ability to do one doesn’t go any way top addressing the other.
Once again, there’s no reason to assume this is true.
What would seem more plausible is the use of GM bacteria to innoculate ruminants. Within limits, whatever a rumen bacterium can digest, the ruminant can get food out of. If the alien plant life wasn’t too dissimilar, for example if it used proteins and sugars that were indigestible to Earth enzymes, then it should be possible, even using current technology, to engineer rumen bacteria that could convert those into digestible material.
I see two assumptions here that need more justification: 1) that the set of aminos we use are completely accidental and not subject to any evolutionary selection at all, hence without a greater than random chance of being selected for
2) that there exists no natural process for selection of chirality for organic molecules, which again might not be the case
ETA that I agree with everyone saying it’s highly unlikely that alien life will be digestible, BTW, I just disagree about the lack of the same basic building blocks off Earth.