I'd need to read the article again but I think the study was done on civil employees. I don't know how broad the pay range is for civil employees nor if the authors sampled the full range.

The thing to keep in mind is that Western science made a dichotomy between the mind and body for religious reasons. In reality, they are completely interrelated. Eating unhealthy food will eventually affect your mind: right now it's just not predictable how or to what extent.