"Food" is a social construct in the same way as every label we put on a thing is a social construct. "Chair" is a social construct. (The universe didn't know what a "chair" was before humans started making and naming chairs.) "Tree" is a social construct. (Any physical thing you pick apart enough is particles (and I'm definitely oversimplifying here) and by giving it a human-made label like "tree", we're imposing something that wouldn't otherwise be there.) "Particles" are a social construct! (They're very much an abstraction of what's actually going on. Even the math we use to understand things like quantum mechanics is just our way of thinking about something that may or may not "exist" but if it does, definitely isn't the same as our "thoughts" about it.)

All words are social constructs, but I think there's at least one more layer at which "everything is a human construct." Even before we give something a name, we've already made the decision to distinguish it from a "background" as a distinct "thing." (A sufficiently alien mind might, if it encountered earth, consider all of earth "atomic" and "indivisible" to the point that the idea of "a human" wouldn't make sense to it. It's not like there's any empty space between our skin and the soup of amosphere we constantly live in, so in what sense am I a separate thing from the rest of earth?)

So, yeah, "food" is a social construct, but humans are very much removed from "reality" by an opaque ocean of social constructs.

All that said, I wouldn't say that "food" is a social construct in any way that, say, a "planet" or a "fork" or a "rock" or a "human" isn't.