I mean you can visit Tokyo and barely notice anyone being fat while in New York there’s about 30% people being obese, even though both countries have fast food. Although Japan has less of it tending to burn off that by walking or exercise (it also reflects their lifestyle) and portion sizes regarding food & calorie intake while in US there are literally multiple fast food joints next to each other along with huge portion sizes.
I don’t think many people eat like that most days anymore. Breakfast, for those who eat it, is toast for like 60% of people, almost any bento sold at supermarkets and konbini have pasta in them (along with multiple other carbs, sometimes including potatoes), etc. Lots of additives and such in cup noodles which a lot of people also eat way too regularly. Japanese do tend to eat more fresh fruit than Americans, anecdotally.