Defining Metabolic Syndrome: what kind of causality, if any, is required?

The definition of metabolic syndrome has been, and still is, extremely controversial. My purpose is not to give a solution, but to argue that the controversy is at least partially due to the different ‘causal content’ of the various definitions: their theoretical validity and practical utility can be evaluated reconstructing or making explicit the underlying causal structure. I will propose therefore to distinguish kinds of definitions according to kinds of causal content: definitions grounded on associations, definitions presupposing a causal model build upon statistical associations and definitions grounded on underlying mechanisms. I suggest that analyzing definitions according to their causal content can be of help in evaluating alternative definitions of some diseases. I want to show how the controversy on MetS suggests a distinction among three kinds of definitions based on how explicitly they characterize the syndrome in causal terms, and on the type of causality involved. I will call ‘Type 1 definitions’ those definitions that are purely associative, ‘Type 2 definitions’ the definitions based on statistical associations, plus generic medical and causal knowledge, and ‘Type 3 definitions’ are definitions based on (hypotheses about) mechanisms. These kinds of definitions, although different, can be related to each other. A definition with more specific causal content may be useful in the evaluation of definitions characterized by a lower degree of causal specificity. Moreover, the identification of the type of causality involved is of help to constitute a good criterion for choosing among different definitions of a pathological entity. In section (1) I will introduce the controversy about the metabolic syndrome; in section (2) I will propose some remarks about medical definitions and their ‘causal import’ and in section (3) I will suggest that the different attitudes towards the definition of metabolic syndrome are relevant to evaluate their explicative power.