A systematic review and meta-analysis was conducted to determine the effects of prebiotics or synbiotics on blood biomarkers of obesity, depression, and anxiety (including: ACTH [adrenocorticotropic hormone], cortisol, leptin, ghrelin, TSH [thyroid-stimulating hormone], PTH [parathyroid hormone], vitamin D, BDNF [brain-derived neurotrophic factor], and PCR [polymerase chain reaction]) in individuals with overweight or obesity.