Around 25% of people with depression have low-grade inflammation, which is unfavourable for treatment, but its’ cause is unknown. Short chain fatty acids (SCFA) may play a role in this: They reduce inflammation, but in depression, SCFAs and the bacteria that produce them are often low. New animal and human data suggest that especially one SCFA, called butyrate, is linked to depression severity and inflammation. In this project the scientists will assess the link of plasma SCFAs and various inflammatory mechanisms using a large existing database (400 patients and controls). Subsequently, they use a dietary intervention to enhance butyrate in depression with signs of inflammation. The research could yield an affordable, freely available add-on treatment for depression with signs of inflammation.