Soylent Green is
people!
Or is it?
Actually, in the original Harry Harrison novel (Make Room! Make Room!) upon which it was based, Soylent was not made of people, but rather it was made of soy and lentils. Hence the name.
Apparently Richard Fleischer and crew didn't think forcing the entire population of the world to eat soy products was scary enough of a proposition, so they decided to throw in a little cannibalism for good measure.