Cross-national differences in diet, the outcome of schizophrenia and the prevalence of depression: You are (associated with) what you eat

Malcolm Peet (2004, this issue) presents some evidence that the international variations in the outcome of schizophrenia are associated with higher amounts of refined sugar and dairy products in the average national diet data published by the Food and Agriculture Organization of the United Nations. An association is also found between the prevalence of depression and a low dietary intake of fish and seafood. It is a simple correlational study with a multivariate statistical analysis that finds very strong associations. The regression coefficients reported suggest that these foodstuff consumption levels account for almost all the variance in the outcome of schizophrenia and the prevalence of depression. Is Peet correct in stating that previous epidemiological studies of these issues are missing crucial information, or is the reported association between diet and psychiatric disorder at a population level just a particularly dramatic example of the ecological fallacy?