Methodological and conceptual complexities of assessing relationships between single-occasion CRP inflammation and daily affect

Inflammation is commonly implicated in sustained levels of depressed mood, chiefly with concurrent measures. There is a dearth of research on understanding how mood-inflammation relationships change on a day-to-day timescale. Determining how inflammation and mood may fluctuate and interact with each other is imperative to determining which pathways may lead to a depressed mood due to inflammation, and, more broadly, which factors induce inflammation in the first place. Therefore, we explored a means of elucidating the nature of mood-inflammation relationships using daily measures of mood and a single time-point measure of inflammation, C-Reactive Protein (CRP). We predicted that the relationship between affect and this measure of inflammation would be time-invariant because of evidence suggesting factors contributing to inflammation are persistent over time, such as obesity or poor gut-microbiome health. Our sample consisted of 1397 young adult participants who completed daily surveys for thirteen days and provided a blood sample for CRP measurement once at the conclusion of the study. A Bayesian multivariate regression model was performed to determine how daily levels of positive and negative mood could be predicted by this single time-point measure of inflammation. As part of our analysis, we sought to control for two key moderators, BMI and physical activity. Results indicated that moderate levels of inflammation were not associated with poor mood when the individual exercised. We also determined that high BMI participants exhibited a greater impact of inflammation on their mood relative to low BMI participants. However, contrary to our primary prediction that this mood-inflammation relationship would be time-invariant, we did indeed find that the relationship was time-variant. This result indicated that research examining associations involving inflammation daily will be required to understand which causative factors may contribute to fluctuations of a mood-inflammation relationship on a daily basis.