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.