Obesity rates rise sharply circa 1980, when the deregulatory climate became entrenched in the US and UK and there is a proliferation of new chemicals. You might say corn syrup became popular then but cane sugar was around long before obesity became epidemic.
Dietary and lifestyle interventions notoriously don’t work, or rather they work temporarily. There seems to be a set-point for weight and you just won’t eat enough to go over it and have a very hard time to go under it. That set point seems to have gone up.
This is explained directly in the video I linked. Cane sugar is more expensive and less consistent commodity while corn syrup is cheap and consistent. Fat became demonized from bad science and sugar was added to almost everything.
Dietary and lifestyle interventions notoriously don’t work
They absolutely work individually. If someone changes their diet to cut out sugar they lose weight very consistently. Expecting that of people is not a systemic solution however. Watch the video I linked instead of trying to explain everything with looser correlations.
https://www.youtube.com/watch?v=dBnniua6-oM