El Goonish Shive VII: The Comic is 20 Years Old, but the Cast is Still 18

[DavidSh]

Protein is one thing. There are just multiple ways to acquire it.

I think Maat Mons was talking about the nine essential amino acids, none of which can be substituted for each other. Strictly speaking, nine equivalence classes of amino acids, as there are a few groups within which the acids can be inter-converted, but cannot be synthesized from raw materials outside the group.