The reviewed article is ‘Depression: An important comorbidity with metabolic syndrome in a general population’ by Dunbar and colleagues from 2008 and freely available here. In the abstract the authors conclude that
‘Metabolic syndrome was associated with depression but not psychological distress or anxiety‘
So is it possible on the basis of this study to draw the above conclusions. The researchers completed three cross-sectional studies in rural Australia. The first thing to say is that as the study was conducted in rural Australia the findings might be specific for this population. It seems more likely that this should be generalisable. However if we consider urban versus rural settings for instance a number of other factors come into play ranging from the structure of health service provision through to social networks and lifestyle which might influence either metabolic syndrome or depression or both. Men and women ‘aged 25 to 84 years were selected from the electoral roll’ by a random sampling method. I couldn’t find a reference to the type of random sampling method that was used but it is reasonable to suppose that there was no obvious selection bias as a result other than the self-selection that results from participation. The authors do note however that the study did not include those that had left the region and this is relevant in the context of the above point about urban versus rural settings. The researchers identify a number of outcome measures including psychosocial factors and factors relevant to the diagnosis of metabolic syndrome such as fasting glucose and waist and hip circumference. The criteria for metabolic syndrome were clearly identified. The researchers also used the Hospital Anxiety and Depression Scale, a commonly used scale which aids the assessment of depression in the hospital population. However the authors also used the ‘Kessler 10 measure’ which I wasn’t familiar with and describe it as a five-point likert scale where cumulative scores result in categorisation into low and moderate-high levels of ‘psychological distress’ in the last 4-weeks. The internal consistency values for K10 and the HADS subcomponents were provided ranging from 0.79 to 0.87. The primary outcome measures (or at least I presume they were the primary outcome measures) were clearly stated and the author examined the relationship between anxiety, depression and the metabolic syndrome. The statistical analysis of other relationships between the many variables that were used in the study was also clearly stated. 409 subjects met the criteria for the metabolic syndrome and a comparison was then made between those with and without the metabolic syndrome. I didn’t particularly understand the following within the results section
‘Participants with the metabolic syndrome were more likely to have moderate to severe depression (10 vs 6.9%, p = 0.069)‘
In the remainder of the sentence the authors write that with regards to another measure there was no statistical significance. However the p value above would not be significant either (at the 5% level). However the researchers did find a significant difference between metabolic syndrome and the diagnosis of depression (i.e. without reference to specific subtypes such as moderate or severe) and this time the difference reached a significance of 0.013 and depression scores were 3.41 versus 2.95. Thus there is a mean difference of roughly 1.5 points on the HADS depression subscale (I presume that it was this subscale although not stated explicitly in the sentence) at the group level.
So the relationship appears significant but the direction of this relationship is unclear as it was a cross-sectional study. A longitudinal study might be able to shed light on the directionality of the relationship. The authors speculate that an inflammatory pathway might mediate the link between depression and the metabolic syndrome.
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