Depression is a severe and heterogeneous disorder resulting from interacting 
genetic, environmental, and epigenetic factors. Nutrient deficiency resulting 
from bariatric bypass surgery has been involved in the pathophysiological 
mechanisms of depression and treatment response.We report the case of a patient 
who developed, after a bariatric bypass surgery, a severe depressive episode, 
refractory to both pharmacological treatment and electroconvulsivotherapy (ECT). 
Folate deficiency was evidenced. A dramatic response to ECT was observed after 
folate supplementation. We focused on the involvement of folate in the 
pathophysiological mechanisms of depression and response to both pharmacological 
treatment and ECT. We emphasize on the need for close monitoring of patients 
experiencing psychiatric disorder (in particular, depressive unipolar disorder) 
after bariatric surgery.