Though obsessive-compulsive disorder (OCD) and anorexia nervosa share many traits and are often comorbid, OCD is often treatable with serotonin-modulators such as SSRIS, while SSRIs are often insufficient to treat anorexia nervosa despite the likely similar pathogenesis mechanisms like serotonergic dysfunction. Food restriction likely leads to low typtophan levels, impairing metabolism of serotonin. Upstream manipulation of serotonin availability was shown to be possible in mice by colonizing germ-free mice with Clostridium microbes from healthy human controls, restoring serum serotonin levels. However, colonization with Bacteroides species did not impact peripheral serotonin.