The association of physical illness and low-grade inflammatory markers with depressive symptoms in a large NHANES community sample: Dissecting mediating and moderating effects.

Background: Both low-grade elevation in peripheral inflammatory markers (e.g., white blood count (WBC) and C-reactive protein (CRP)) and physical illness (both chronic and acute) have been associated with depressive symptomology. However, it is unclear if low-grade elevation in inflammatory markers mediates relationships between physical illness and depression or if physical illness positively moderates relationships between inflammatory markers and depression.

Methods: In a well-powered, racially diverse cohort (n = 21,525) from NHANES datasets, we examined if inflammatory markers (CRP and WBC) and physical illnesses (acute and chronic) were independently associated with depression severity. We also examined if associations between physical illness and depression severity were mediated by inflammatory markers and if physical illness moderated associations between inflammatory markers and depression.

Results: We found that both inflammatory markers and physical illness were associated with depression severity, even after considering a wide range of potential confounders (e.g., age, gender, body mass index). Inflammatory markers mediated a marginal portion (<5%; p < 0.001) of potential effects of physical illness on depression severity. In moderation analyses, associations between inflammatory markers and depression severity were significantly stronger in participants with chronic physical illness than those without. This moderating effect was not present for acute physical illness.

Conclusions: Inflammatory markers and physical illness appear independently linked to depression severity and, in individuals with chronic physical illness, inflammatory markers are more tightly connected to depressive symptomology. Such findings could help guide future individualized treatment research for depression based on both inflammatory marker level and physical illness burden.