High-fructose corn syrup is probably the worst: high-calories, produces insulin spikes and carries estrogen-like chemicals from corn. Studied independently under current guidelines it would probably be banned.
Portion size: there was plenty junk food in the 60s like soda, chips, etc. But no ginormous sodas and bags of chips. Restaurant portions were also much smaller. You were full easting an ordinary meal, but now we train ourselves to overeat big portions and still feel hungry after eating an adequate meal.
Most importantly is exercise: kids were encouraged to go out and play and there were no video games, computers, and smart phones. I am often in Europe and you can see European kids are catching up with obesity now, too, despite the fact that food quality in Europe is much better than here.
Indeed, there was plenty of junk food in the 1950s. Cereal companies emphasized the sugar content of their products—Sugar Smacks, Sugar Pops, Rice Honeys, etc. And as the voice of Tony the Tiger, Thurl Ravenscroft, whom my mother knew in his later years, admonished America that “Sugar Frosted Flakes are great!”
“High-fructose corn syrup”
fructose - fruit sugar
Fruit juice probably should be removed from the federal nutrition programs.
Fruit often is produced for a short time each year.
Human biochemistry probably favors storing up its once annual bounty in some manner.