C-reactive protein levels and depression in older and younger adults - A study of 19,947 individuals. The Tromsø study

In recent years, a connection between depression and inflammation has been established, with a range of immunological changes, both cellular and humoral, presenting during depressive states (Beydoun et al., 2016; Haapakoski et al., 2015; Wium-Andersen et al., 2013). Furthermore, there seems to be a dose-response relationship between depression and inflammation, in the sense that the more severe the depression, the higher the level of systemic inflammation markers, most notably expressed as elevated levels of C-reactive protein (CRP) in peripheral blood (Kohler-Forsberg et al., 2017). Accordingly, CRP has been suggested as a marker of depression severity and depression subtypes, as well as an indicator of specific symptom profiles (Jokela et al., 2016). Furthermore, inflammation has been suggested as a target for treatment with immunomodulatory drugs (Alexopoulos and Morimoto, 2011; Kohler et al., 2014). However, the research populations are predominantly younger adults, mainly in clinical settings, and there are few community-based studies providing comparative analyses of age-groups, or focusing specifically on the older population. For those that do, the results are inconsistent, as some demonstrate an association between CRP and depression (Bondy et al., 2021; Sonsin-Diaz et al., 2020; White et al., 2017), while others do not (Baune et al., 2012; Bremmer et al., 2008; Eurelings et al., 2015; Penninx et al., 2003). Thus, it is still unclear whether the inflammation in depression unfolds to the same extent in depressed older adults as in younger adults, and how the severity of the depression relates to inflammation in different age groups.