Obese individuals are a particularly vulnerable population for mental health problems, especially depression and anxiety. The aim of this study is to evaluate whether the intake of a synbiotic, composed of prebiotics and beneficial intestinal bacterial strains, is capable of producing changes in the gut microbiota and its functionality, improving metabolic and inflammatory parameters, intestinal function and appetite control in patients with obesity and psychological disorders. In addition, the production of neurotransmitters at the level of the gut-brain axis will be studied, as well as mood and quality of life. For this purpose, a prospective, randomized, doubleblind, placebo-controlled intervention study will be carried out in patients with obesity (BMI=30-40 kg/m2) and symptoms of anxiety and/or depression, or patients with obesity but without these psychological disorders (n=120). The groups will be randomly divided into two groups (n=60) according to the intake of a synbiotic (1 capsule/day composed of bifidobacterium, Lactobacillus and tannin-based phytocomplexes) or its corresponding placebo for 12 weeks. Individualized psychological and nutritional follow-up will be carried out, demographic, lifestyle and mental health variables will be collected, and biological samples will be collected before and after the intervention. In addition, all patients will undergo an assessment of body composition and nutritional status, together with cardiovascular risk factors and comorbidities (hypertension, dyslipidemia, DM2, insulin resistance). Inflammatory parameters (IL6, TNF , IL1b, adiponectin, PAI-1, IL10, resistin, adipsin), antioxidant capacity, intestinal function (zonulin, LPS, occludin, LBP, FABP2/I-FABP, -glucan, Reg3A), satiety, appetite control (Leptin, GLP1, GIP, Ghrelin, PP) and neurotransmitter production (cortisol, dopamine, serotonin, oxytocin) in plasma/serum, urine or saliva using ELISA Kits and Luminex XMAP technology will be analyzed. In addiition, the investigators will perform analysis of genetic markers of inflammatory and metabolic pathways (Nanostring technology), metabolomic profiling (NMR spectroscopy and PLS-DA) in plasma, and both content and diversity of the intestinal microbiota (16S rRNA amplicons, and direct metagenomic sequencing, with Illumina MiSeq technology) in faeces will be evaluated. Finally, the investigators will study in vitro the mechanism of action of colonic digest on complex cellular models that simulate the gut-brain axis (organ-on-chip model, OoC).