Just eat regular food. Hardly anything in my diet would be mysterious to a time traveler from 100 years ago, or indeed 200 years ago. Occasionally I enjoy chocolate or instant noodles but mostly it's just fruit, veg, meat, dairy, and grains. If you don't know what something is made out of or how, then don't eat it.
Not a panacea. I stay fat no problem without junk food. Processed foods make me ill, so I make almost all my own meals from whole ingredients. Too many calories is too many calories no matter how you get it.
Obviously one should not eat too much, but I was responding specifically to the comment about the industrial revolution leading to engineered foods full of empty calories in the grandparent comment. I would personally like to see such food regulated mostly out of existence, but in the meantime I just encourage people not to eat it.
