An intriguing and hitherto unexplained co-occurrence: Depression and chronic fatigue syndrome are manifestations of shared inflammatory, oxidative and nitrosative (IO&NS) pathways.
Maes, Michael·Progress in neuro-psychopharmacology & biological psychiatry·2011
Depression and ME/CFS often occur together, and this study suggests they share common biological root causes involving inflammation and oxidative stress (damage to cells). While depression and ME/CFS look different clinically, they appear to involve similar underlying problems with the immune system, energy production, and antioxidant defenses. The authors argue these should be considered connected disorders rather than two separate conditions that happen to co-occur.