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

Editorial / commentary on the paper by Peet 2004. No abstract is available, but the first and last sections are presented here.

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?

.....

Peet’s study meets only the strong association criterion for assessing causation (Hill, 1965). Whether or not the results are biologically plausible is a value judgement. Dietary explanations for just about everything are the spirit of the age and popular in lay circles, at least in developed countries; whereas those, like us, who believe in evidence-based everything prefer their data served up from rigorous prospective cohort studies and randomised controlled trials. Arguably, however, dietary factors deserve just as much study as the other ill-defined influences on international differences in psychiatric epidemiology for which they may be a proxy. Ultimately, if Peet’s study serves to rekindle interest in the cultural influences on major psychiatric disorders, it will have been a valuable contribution.