That would be tough. On the off chance they were already familiar with both circuses and peanuts, you’d still have to explain why someone invented a nearly inedible food that has nothing to do with either one.
Imitation Crab.
Military Tropical Chocolate Bars (D Rations).
Even if you know what they are, they aren’t food.
Froot Loops!
Also, Bugles.
Canned surströmming
NOTE: Video is safe for work, but contains some bad language and scenes of authentic retching. Viewer discretion is advised.Pringles
Instant mashed potato.
To be fair, while this was pretty prevalent when I was growing up, there wouldn’t be much difference in reaction between your person of 5000 years ago and a current teenager:
You eat that shit?
j
Something like this cake.
I’m not 100% convinced it’s food myself.
Bubble tea
Well, those Texans did it all wrong. To begin with, you should open the can under water to suppress the emission of noxious fumes. Thoroughly rinsed, the surströmmings make an excellent, although a bit salty dish together with potatoes, butter, onions and flatbread. Though I would advice you to skin them well, since the skin in my own experience has a tendecy to cause flatulence.
I imagine this kind of fermented food would have been a staple in most temperate countries some few hundred years ago already!
Wikipedia says Tofu may be 2000 years old or at least 1000? I mean, nowhere near the the time frame we’re talking about here, but not that new/unnatural. I’ve read about people making it at home in North Korea. Though I’m sure we have more an less natural versions (as we do with ice cream, and many other products).
Fruit rollups seems quite unnatural to me, but I’m not sure what’s in them besides sugar. Twizzlers (which I like) are a bit waxy. Wax lips candy never seemed like candy at all, just wax. Not sure how “unfood” that would be to people then, though.
Non-dairy creamer (aka coffee whitener). Not sure it has any food value at all.
But is it actually* food *(any more than all artificial flavors and colors that we encounter on a daily basis)? To me, it seems more like an additive. Hey, *salt *doesn’t have much of a nutritional value, does it?
Of course it does. Sodium is an essential nutrient.
Jello
Coca-Cola.
Of course it does. It’s got carbs, fat, and significant potassium and phosphorus in it. It’s not particularly unhealthy, in fact. It’s low in calories, and low in fat. Though many types have trans fat in it, which isn’t considered good. But the amount it contains is low enough to make little difference for most people.
Why would you think it doesn’t have food value? 
Nutritional label here: https://www.pinterest.com/pin/245938829627622210/?lp=true
for the powder as well as for other forms of it.
“Ice cream” that doesn’t melt. Perhaps candy corn. Maybe astronaut food.
This has to be the worst answer of the thread thus far.
Isn’t that like saying imitation sugar substitute? Margarine is imitation butter. What is imitation margarine?? Sorry if this is common knowledge.
But most animals seem to do quite well without deliberately* adding* it to their food!