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.