Obesity is a major problem worldwide, and it is related to abnormalities in glucose and lipid metabolism. The purpose of this study is to investigate the effect of a dietary supplement containing probiotic (Bifidobacterium animalis ssp. lactis 420) and/or prebiotic (Litesse) on change in body fat mass in a double-blind, randomized, placebo-controlled intervention trial. The supplement is ingested once per day for the duration of six months, and participants will attend a follow-up visit one month after the end of the intervention. The study will enroll 232 participants (58 per study arm) in four research centers in southern Finland.