Depression and body mass index, a u-shaped association

Background: Results of studies concerning the association between obesity and depression are conflicting. Some find a positive association, some a negative association and some find no association at all. Most studies, however, examine a linear association between Body Mass Index (BMI) and depression. The present study investigates if a nonlinear (U-shaped) trend is preferable over a linear trend to describe the relationship between BMI and depression, which means that both underweight and obesity are associated with depression. Methods: We investigated the existence of such a U-curve in a sample of 43,534 individuals, aged between 18-90 years, who participated in a cross-sectional study (Continuous Survey of Living Conditions) of physical and mental health in the general population of the Netherlands. We calculated linear and nonlinear (quadratic) ANOVA with polynomial contrast and curve fit regression statistics to investigate whether there was a U-shaped trend in the association between BMI and depression. Results: We find a very significant U-shaped association between BMI categories (underweight, normal, overweight and obesity) and depression (p=0.001). There is a trend indicating a significant difference in the association between males and females (p=0.05). We find a very significant U-shaped (quadratic) association between BMI (BMI2) and depression (p <= 0.001), continuous BMI is not linearly associated with depression (p=0.514). Conclusion: The results of this study give evidence for a significant U-shaped trend in the association between BMI and depression.