There's so much culturally different here that blaming just the differences in the system of health care is effectively meaningless.

Yearly physical exams are much more thorough in Japan. Unless you are optimally fit, you will be prescribed lifestyle changes to make and there is a strong expectation that you will work hard on these. Your employer will be involved. There is _tremendous_ social pressure if you are overweight.

Healthy food options are ubiquitous there with healthy and cheap meals available 24/7. Combini food certainly has bad options but nothing compared to American fast food or the American diet generally.

There are other health problems that are significantly overrepresented in Japan compared to the western world. Alcohol, smoking and stress-related illnesses. Liver & Kidney diseases. Peptic ulcers, GI problems in general.