why can't we eat aliens?

[Murmur the Fallen]

I was reading something a while back (and I have a crap memory so please bear with me) on wikipedia a little while back. It was on a fiction book or series and how an alien species is unable to eat earth biological stuff because of some incompability. It then took me to a link to a science article on how molecules or cells or something took one direction on Earth but it doesn't neccessarily have to have taken that direction elsewhere.
Anyone know what I'm talking about?
-murmur
bothering me, man. niggling at the mind.

[Kokuten]

probably thinking of handedness of proteins..
en.wikipedia.org/wiki/Protein
en.wikipedia.org/wiki/Protein_folding

Not exactly what you're looking for, but its some framework for the basic concept.
Alternatively - if 'alien' life is based off of, say, silicon over ammonia instead of carbon over oxygen..
Mr Alien would get the same nutritional value out of a delicious steak as we would get off of a tasty chunk of granite, to totally molest the science involved.. Wire Geek - Burning the weak and trampling the dead since 1979Wire Geek - Burning the weak and trampling the dead since 1979

[Valles]

Yup. DNA isn't the only vital organic chemical that builds itself in spirals. Proteins and sugars (all of them, I think, though I may be wrong about that part) also do so, IIRC both in a left-handed direction. In terms of energy release and general chemical behavior, the direction of the twist is completely irrelevant - but enzymes are shape keyed, and twist does matter to them. The body just wouldn't have the tools it'd need to usefully break down a dextro-rotating carbohydrate, say.
I don't think that the different structure would be toxic, and symmetrical molecules like, oh, say, alcohol would still work, but there'd be no nutrition to be had.
I can recall the topic coming up in a series of books by someone named Swann and in a short story by Spider Robinson.
So, short version: there's a one in four chance that a given alien biome would have happened to have both types of molecule break the same way Earth's did.

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"Reseeestunce ees fiutil. Yoo weeel bee Useemooletud. Borg Borg Borg."
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"V, did you do something foolish?"
"Yes, and it was glorious."

[Kokuten]

some of the artificial sweeteners take advantage of this - there's at least one that's simply cane sugar, selected for left-handedness. Ends up being similar enough that we get the 'taste benefits', and with identical physical properties (melting point, behavior under heat, behavior in a mixture), but.. we don't get the nutritional 'values' of it.

It's in the stack of Wired, which means it's probably a beta of a good idea being presented as a product, but still, it's a fairly good idea of what you got.

Also, William Sleator did some work with it in The Boy Who Reversed Himself, which ties into the delightfully wacky Interstellar Pig
In William Sleator's version of handedness - ketchup, when reversed, tastes like ambrosia, addictively so, in fact, but has zero nutritional value..
Intriguing lines of thought indeed, and a pair of wonderful books. Wire Geek - Burning the weak and trampling the dead since 1979Wire Geek - Burning the weak and trampling the dead since 1979

[VladimirTherin]

Also, keep in mind that alien proteins that are completely innocuous in their biology might be very toxic to humans. Even among earth species, there are animals that eat things that kill others quite quickly.
Humans are very omnivorous however (We eat things that make most animals go blind), so I am sure that at some point someone will try anyways.

[Morganite]

Isacc Asimov's book "The Left Hand of the Electron" looks at this issue from several different angles. '.'
-Morgan, also some of the most amusing science I've ever read..."Mikuru-chan molested me! I'm... so happy!"
-Haruhi, "The Ecchi of Haruhi Suzumiya"
---(Not really)

[ECSNorway]

It's also quite simply possible that alien life forms will have evolved to use chemical processes that are so totally different from ours, or processed in such different ways, that their animal and plant matter will be indigestible to us.
As for left-handed sugar, isn't that what Sucralose is all about?--
"I give you the beautiful... the talented... the tirelessly atomic-powered...
R!
DOROTHY!
WAYNERIGHT!
--
Sucrose Octanitrate.
Proof positive that with sufficient motivation, you can make anything explode.

[CattyNebulart]

The handedness of molecules is very important, and I am quite surprised that wrong-handed sugar is edible. I know that for many medicines which must be kept cold it's so that the molecules don't switch handedness (there is some process when dissolving and recrystalizing that can switch the handedness of many organic molecules.) becuase the effect that wrong handed molecules have on the body is usually completely different and unrelated to what it is supposed to do. It's also the source of many sideeffects in medicines, and for a lot of medicine having to many of the wrong handed molecules tends to be lethal.
E: "Did they... did they just endorse the combination of the JSDF and US Army by showing them as two lesbian lolicons moving in together and holding hands and talking about how 'intimate' they were?"
B: "Have you forgotten so soon? They're phasing out Don't Ask, Don't Tell."

[jpub]

Spider Robinson also touched on wrong-handed molecules, where mirrors can be gateways to another universe where everythign is reversed.

Another point is that a lot of the food we eat is known for having trace chemicals in them *we* need to survive. By the same token, an alien life form could be filled with trace elements.--
Christopher Angel, aka JPublic
The Works of Christopher Angel
"Camaraderie, adventure, and steel on steel. The stuff of legend! Right, Boo?"

[robkelk]

Different trace elements... like potassium cyanide, for example. Wouldn't want to eat that.

-Rob Kelk
"Read Or Die: not so much a title as a way of life." - Justin Palmer, 6 June 2007
--
Rob Kelk
"Governments have no right to question the loyalty of those who oppose
them. Adversaries remain citizens of the same state, common subjects of
the same sovereign, servants of the same law."

- Michael Ignatieff, addressing Stanford University in 2012