Inflamed Depression: The link between Depression and Inflammation expressed by CRP and IL-6 markers in the elderly population
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Even though various interventions for the treatment of depression have been developed, around 30% of depressed individuals do not respond to the treatment which indicates that there is a gap in understanding the underlying mechanisms of this disorder (Leighton et al., 2018). One potential biological mechanism may be inflammation, as depressed individuals often have heightened inflammatory responses such as heightened levels of Interleukin-6 (IL-6) and C-reactive protein (CRP). The current paper, therefore, aims at investigating to what extent heightened inflammatory responses can be explained by depression. The hypotheses that depression will be associated with higher levels of CRP, as well as higher levels of IL-6, were determined through the review of existing literature. Two longitudinal data sets from The Swedish Adoption/Twin Study of Aging and OCTO-Twin study were used (N=474, Mage=79, 63% female). Two sets of linear regression models were run for each inflammatory marker: CRP and IL-6. The analyses yielded no significant results regarding the prediction of CRP and IL-6 from depression. However, the exploratory analyses revealed significant effects of age, smoking, and BMI on the inflammatory markers, suggesting that inflammation can be better explained by these factors than depressive symptoms as observed in the elderly population. Causal conclusions are extremely challenging to draw due to unknown confounding variables as well as the possibility of reversed directionality with inflammation posing greater risks for the development of depression rather than the other way around. Keeping in mind the rise of disabilities following depression, as well as the crisis of Sars-CoV-2 which imposes great risks for the older population as well as potential risks for depressed individuals, future research should continue exploring the topic of the bi-directional link of depression and inflammation in order to develop novel approaches for the prevention, diagnosis, and treatment of the issues.