Association between C-reactive protein and depression: modulated by gender and mediated by body weight.

Literature on the relationship between depression and C-reactive protein (CRP), a biomarker of systematic inflammation, remains inconsistent. Insufficient adjustment for confounders and effect modifiers might be one explanation. We used the data of 6396 men and 6610 women aged 18 or older, who completed a depression screening and had blood collected as a part of the National Health and Nutrition Examination Survey, 2005-2010. Depression was measured using the 9-item depression scale of the Patient Health Questionnaire (PHQ-9). The odds ratios (ORs) of depression were 1.00 (reference), 1.89 (95% CI=0.77-4.67) and 3.41(1.25-9.25) respectively for men with low, intermediate and upper quartile of CRP. Adjustment for covariates, mainly body mass index, diminished the association among women, from 1.65(1.00-2.74) to 1.08(0.57-2.03) for intermediate, from 2.44 (1.43-4.16) to 1.05 (0.56-1.98) for upper quartile of CRP. Adjustment for the history of major medical illnesses changed ORs neither among men nor among women. The study concluded that CRP remained significantly associated with depression in a dose-response fashion among men but women after being adjusted for body weight. Abnormal body weight, both under and overweight, explained a substantial part of the relationship between CRP and depression among women.