You're tilting at windmills if you think Japanese foods approach the level of caloric density and unhealthiness of burgers, burritos, french fries, Doritos, and other shit Americans eat.
The stereotype of Japanese food abroad is it's all just grilled fish and sushi but there is a LOT of fried food involved.
I'll say US portions are absolutely out of control though, I think that's the biggest thing. When my Swedish friends went to visit the US they ended up just ordering one thing at every restaurant and sharing it, and the waiters would just laugh at them.
It clearly wasn't the mere existence of unhealthy foods, the Japanese have tons of unhealthy food as well. You can get a burger there without any issue.
If you saw a population of people injecting heroin into their veins and getting frail, would you say you have a frail ness epidemic? No. What you have now is a food addiction epidemic. You should treat it as you treat any other addiction.
>Combining that with an individualistic culture
So that is the only change since the 50s? Seems like a very weak explanation.
I agree that overeating should be shamed, but was that needed 70 years ago? To be honest I don't believe the cultural issue. Which percentage of US citizens believe that they should loose weight? Likely a very large proportion.