Medically unexplained physical symptoms, anxiety, and depression: a meta-analytic review.
Henningsen, Peter, Zimmermann, Thomas, Sattel, Heribert·Psychosomatic medicine·2003
This large review of 244 studies examined whether people with ME/CFS, fibromyalgia, IBS, and similar conditions experience more anxiety and depression than healthy people or those with other illnesses. The researchers found moderate but consistent links between these conditions and depression/anxiety, but importantly, the conditions cannot be fully explained by mental health alone. This suggests these syndromes are distinct medical conditions rather than simply psychiatric problems.