IL6 and major depressive disorder: Interestingly, prior studies have postulated that the association between depression and being overweight/obese is bidirectional.6,27 One explanation for this finding is based on the inflammatory theory of depression in which proinflammatory cytokines, such as IL-6 and tumor necrosis factor α, have been reportedly observed in patients with depression, and concurrently, inflammation is correlated with metabolic abnormalities (e.g., leptin).28,29