2.1. Study Design and Participants

This study is part of a large, double-blinded study and a parallel group design that was registered with ClinicalTrial.gov ({"type":"clinical-trial","attrs":{"text":"NCT02642289","term_id":"NCT02642289"}}NCT02642289) and approved by the Human Research Ethics Committee of the University of Almeria (Spain). The study protocol and recruitment procedure have been previously described [30]. Fibromyalgia patients were recruited from the Almeria Fibromyalgia Association (AFIAL—Spain) or from El Ejido Fibromyalgia Association (AFIEL—Spain) and were diagnosed at least 1 year before entering the study according to the criteria of the American College of Rheumatology [1,32]. Exclusion criteria involved: (1) use of antibiotics and nutritional supplements, (2) allergies, (3) current participation in other psychological or medical studies, (4) being pregnant or breastfeeding, (5) severe intestinal disease and (6) meeting the criteria for psychiatric disorders other than depression and/or anxiety. More information about the participants’ characteristics can be found in the first part of the study [31].