I did see that, and maybe Japanese regional cuisine varies a lot more than I'm imagining. However, I'm still not convinced that something higher-level is going

I did see that, and maybe Japanese regional cuisine varies a lot more than I'm imagining. However, I'm still not convinced that something higher-level is going on. For example, I think you could find a similar correlation between (say) soul food restaurants and diabetes prevalence in the US. Soul food tends to be starchy and fatty itself; you could also make a similar argument that the number of soul food restaurants is a proxy for food preferences in each region. However, soul food is especially popular in African-American communities. There are genetic factors that predispose African-Americans to diabetes. Soul food restaurants are probably more common in poorer areas. Diabetes is also linked to poverty: partly through diet, but also through things like access to preventative healthcare and management of pre-diabetes. Perhaps there still is some residual effect of the restaurants themselves, but it would be much weaker after adjusting for other factors.